Synthesis and characterization of thermally stable dyes with improved optical properties for dye-based LCD color filters

Article abstract of DOI:10.1039/c2nj20938a

We have designed and synthesized six phthalocyanine dyes and one benzoperylene dye for dye-based green Liquid Crystal Display (LCD) color filters. The solubility, spectral properties and thermal stability of the prepared six phthalocyanine dyes varied dep

Full text of DOI:10.1039/c2nj20938a

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PAPER
Synthesis and characterization of thermally stable dyes with improved
optical properties for dye-based LCD color filtersw
a
a
Jun Choi,z Se Hun Kim,z Woosung Lee,a Chun Yoonb and Jae Pil Kim*a
Received (in Montpellier, France) 6th November 2011, Accepted 21st December 2011
DOI: 10.1039/c2nj20938a
We have designed and synthesized six phthalocyanine dyes and one benzoperylene dye for
dye-based green Liquid Crystal Display (LCD) color filters. The solubility, spectral properties
and thermal stability of the prepared six phthalocyanine dyes varied depending on their isomeric
structures and core metals. They were dissolved in industrial solvent–binder composites and spin
coated with and without yellow compensating benzoperylene dye into dye-based LCD color
filters. The prepared color filters exhibited superior optical properties compared to conventional
pigment-based ones due to the fluorescence of benzoperylene dye and less light scattering resulting
from the smaller particle size of the dyes. The phthalocyanine dyes substituted at the peripheral
(b) positions showed enough thermal stability for color filters and the yellow benzoperylene dye
effectively compensated the spectra for vivid green colors.
can be achieved.3 Although color filters produced by this method
Introduction
have good thermal and photo-chemical stability, they have low
chromatic properties due to the aggregation behavior of pigment
particles used as colorants.4 Dyes can be attractive alternatives to
overcome this limitation due to the reduced light scattering
resulting from the fact that they are dissolved in the media and
exist in molecular form. However, in order for the dyes to be
applied successfully to the LCD manufacturing process, their low
thermal stability needs to be improved.5 Moreover, they should
be highly soluble in industrial solvents and have sharp absorption
peaks for superior optical properties.
Displays are the most important media in the information age.
Since the late 20th century, flat-panel displays have replaced
the market share of CRTs (Cathode Ray Tubes).1,2 Among the
flat-panel displays, LCDs, electronically modulated optical
devices made up of many segments filled with liquid crystals
and arrayed in front of a light source (backlight) to produce
images in color, hold the greatest market share due to the
many advantages such as low cost, low power consumption,
and flexibility. The color filter, which converts the white
backlight into red (R), green (G), and blue (B) colored lights,
is the key component in the growth of display technology since
it has the greatest potential to enhance picture quality.1,2 Only
6.3% of the backlight can pass through the whole panel, which
includes a polarizing filter, TFT-array, LC cell, and color
filter. In particular, the transmittance through the color filter
is the lowest (30%). Therefore, developing high transmittance
color filters is essential for saving energy consumption as well
as improving high-quality displays.
Therefore, to replace pigments with dyes in LCD manu-
facturing, the remarkably stable molecule, phthalocyanine
(PC), has been modified into six highly soluble dyes and
blended with a yellow benzoperylene dye to exhibit appropriate
green colors. To the best of the authors’ knowledge, this is the
first report of applying 100% dyes to the green color resist for
photolithography process. The green color, which transmits light
within the range of 520–550 nm, has the greatest contribution to
the optical property of color filters since it is the most sensitive to
human eyes.2 For this purpose, the thermal stability, optical
properties, and solubility of PC and benzoperylene dyes as the
colorant materials for color filters were investigated according to
their characteristic structures. In addition, the transmittance and
color gamut of the spin coated color filters with these dyes were
examined and compared with the pigment-based color filter.
Currently, the pigment dispersion method to form RGB
patterned pixels using photolithography has been widely adopted
to produce color filters because color filters with superior stability
a Department of Materials Science and Engineering,
Seoul National University, Seoul 151-744, Korea.
E-mail: jaepil@snu.ac.kr; Fax: +82 2 880 7238;
Tel: +82 2 880 7238
b Department of Chemistry, Sejong University, 98 Gunja-dong,
Gwangjin-gu, Seoul 143-747, Korea. E-mail: chuny@sejong.ac.kr;
Fax: +82 2 3408 3218; Tel: +82 2 3408 3218
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for new compounds. See DOI:
10.1039/c2nj20938a
Experimental
Materials and instrumentation
1,8-Diazabicyclo-7-undecene (DBU), 3(4)-nitrophthalonitrile,
2,5-bis-(1,1-dimethylbutyl)-methoxyphenol purchased from TCI,
z Both authors contributed equally to the research.
c
812 New J. Chem., 2012, 36, 812–818
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and ZnCl2, CuCl2, CoCl2 purchased from Sigma-Aldrich were
used as-received. All the other reagents and solvents were of
reagent-grade quality and obtained from commercial suppliers.
Transparent glass substrates were provided by Paul Marienfeld
GmbH & Co. KG. Commercial pigment-based color filters and
acrylic binders were supplied by NDM Inc.
solvents. The dyes were then heated to 220 1C and held at that
temperature for 30 min to simulate the processing thermal
conditions of color filter manufacturing. The dyes were finally
heated to 400 1C to determine their degradation temperature.
The temperature was raised at a rate of 10 1C minꢀ1 under a
nitrogen atmosphere.
1H NMR spectra were recorded on a Bruker Avance 500
spectrometer at 500 MHz using chloroform-d and TMS as the
solvent and internal standard, respectively. Matrix Assisted
Laser Desorption/Ionization Time Of Flight (MALDI-TOF)
mass spectra were collected on a Voyager-DE STR Biospectro-
To check the thermal stability of the dyes in color filters,
the fabricated color filters were heated to 230 1C for 1 h in a
forced convection oven (OF-02GW Jeiotech Co., Ltd.). The
color difference values (DEab) before and after heating were
measured on a color spectrophotometer (Scinco colormate) in
CIE L’a’b’ mode.
metry Workstation with
a cyano-4-hydroxycinnamic acid
(CHCA) as the matrix. Absorption and transmittance spectra
were measured using a HP 8452A spectrophotometer. Fluores-
cence spectra were measured using a Shimadzu RF-5301PC
spectrofluorometer. Chromatic characteristics of the color filters
were analyzed on a Scinco color spectrophotometer. Thermo-
gravimetric analysis (TGA) was conducted under nitrogen at a
heating rate of 10 1C minꢀ1 using a TA Instruments Thermo-
gravimetric Analyzer 2050. The thickness of the color filters was
measured using a Nano System Nanoview E-1000.
Results and discussion
Synthesis (see ESIw)
The solubility enhanced six metal PCs tetra-substituted at the (b)
peripheral and (a) non-peripheral positions were designed and
synthesized as shown in Scheme 1. Each of their precursors (1, 2)
was synthesized through a nucleophilic aromatic substitution
Preparation of dye-based inks and color filters
Green ink for a color filter was composed of propylene glycol
methyl ether acetate (PGMEA) (3.2 g), acrylic binder (1.4 g),
and phthalocyanine dye (0.1 g) or phthalocyanine dye (0.07 g)
with benzoperylene dye (0.03 g).
The prepared dye-based inks were coated on a transparent
glass substrate using a MIDAS System SPIN-1200D spin
coater. The coating speed was initially 100 rpm for 5 s, which
was then increased to 500 rpm and kept constant for 20 s. The
wet dye-coated color filters were then dried at 80 1C for
20 min, prebaked at 150 1C for 10 min, and postbaked at
230 1C for 1 h. After each step, the coordinate values of the
color filters were measured. All spin coated dye-based color
filters were 1.6 mm thick.
Investigation of solubility
Solubility of the synthesized dyes in PGMEA was examined to
determine the effects of substituents at the a and b positions.
The prepared dyes were added to the solvents at various
concentrations, and the solutions were sonicated for 5 min
using an ultrasonic cleaner ME6500E. The solutions were left
to stand for 48 h at room temperature, and checked for
precipitation to determine the solubility of the dyes.
Measurement of spectral and chromatic properties
Absorption spectra of the synthesized dyes and transmittance
spectra of pigment-based and dye-based color filters were
measured using a UV-vis spectrophotometer. The fluorescence
spectrum of yellow compensating dye was measured using a
Shimadzu RF-5301PC spectrofluorometer. Chromatic values
were recorded on a color spectrophotometer (Scinco colormate).
Measurement of thermal stability
Thermal stability of the synthesized dyes was evaluated by
TGA. The prepared dyes were heated to 110 1C and held at
that temperature for 10 min to remove residual water and
Scheme 1 Synthesis route of a-position and b-position substituted
phthalocyanines.
c
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Table 1 Solubility of the synthesized dyes at 20 1C in PGMEA
Synthesized dyes
Solubility (g per 100 ml)
a-Position
Zn (1a)
Cu (1b)
Co (1c)
Zn (2a)
Cu (2b)
Co (2c)
8.5
8
8
7
6
b-Position
Yellow (3)
6
8
dyes exceeded 5 wt% in industrial solvents, PGMEA, as shown
in Table 1. When it comes to PC dyes, the bulky functional
groups substituted at the a,b-positions cause steric hindrance to
reduce intermolecular aggregation and the ether linkages in their
structures enhance their miscibility with PGMEA. They were
tetra-substituted to have higher solubility than octa-substituted.
Their higher solubility was induced from lower crystallinity
owing to the isomers.10,11 Dyes 1a–c substituted at the more
sterically crowded a positions showed higher solubility and in
each group (1a–c, 2a–c), zinc PCs 1a and 2a were more soluble in
PGMEA.12,13 Benzoperylene dye 3 also showed superb solubility
in PGMEA since the bulky aryl groups substituted at the bay
and terminal positions are twisted from the core body of the dye
to prevent intermolecular p–p stacking.14
Scheme 2 Synthesis route of monophenylbenzoperylene.
reaction between nitro phthalonitriles and phenols with func-
tional groups having affinity to industrial solvent, PGMEA, and
Spectral properties of dyes. The green dyes should strongly
absorb in the range above 630 nm to be applied for dye-based
LCD color filters effectively. In this aspect, the PCs have very
good traits of their spectra for green color filters as well as their
excellent stability. The absorption peaks of the six synthesized
PC dyes started from 600 nm and showed extremely strong and
sharp maxima in the range of 680–710 nm as shown in Fig. 1.
The Q-bands of a-position substituted PCs (1a–c) were
bathochromically shifted approximately 25 nm compared to
b-position substituted ones (2a–c) as shown in Table 2. When
substituted at the a-position, because of the increased electron
donating effect of the substitutes, the linear combinations of
atomic orbital coefficients (LCAO) of HOMO levels are greater
than those at the b-position, which leads to greater destabili-
zation of HOMO levels and hence the energy gap between
HOMO and LUMO levels decreases.7,15 The cyclic voltammetry
curves and oxidation potential data of a,b-position substituted
1
their structures were confirmed by H NMR.6
The metal PCs (1a–c, 2a–c) were synthesized by a cyclo-
tetramerization reaction of the precursors and purified via
column chromatography.7 The PCs synthesized from mono-
substituted phthalonitriles can theoretically have constitutional
isomers, and their properties were observed without further
attempts to separate these isomers in this work.8 All metal PCs
were obtained in relatively high yields and those with less
sterically crowded b-position substituted had slightly higher
yields. The structures of synthesized PCs were confirmed by
MALDI-TOF spectroscopy and elemental analysis.
A
novel compound, monophenylbenzoperylene tetra-
carboxdiimide 3, that enlarges its aromatic system along the
short molecular axis of perylene asymmetrically was synthe-
sized by reaction between phenyl vinyl boronic acid and
bromo-substituted perylenediimides 5 under similar conditions
to Suzuki-coupling and spontaneous cyclization as shown in
Scheme 2. In order to obtain mono-brominated precursor
1-bromoperylene-3,4:9,10-tetracarboxdiimide 4, the bromination
reaction was conducted under milder conditions than standard
bromination of perylene, and the isomer 1,7-dibromoperylene-
3,4:9,10-tetracarboxdiimide was separated by column chromato-
graphy.9 The structure of monophenylbenzoperylene 3 was
confirmed via 1H NMR, 13C NMR, MALDI-TOF spectroscopy,
and elemental analysis.
Characterization of dyes
Solubility of dyes. For dyes to be superior to pigments as
colorants for LCD color filters they should exhibit high
solubility in industrial solvents. Regardless of the compati-
bility with binders, dye molecules should dissolve more than
5 wt% in industrial solvents to have the desired optical
properties without aggregation. The solubility of all synthesized
Fig. 1 Absorption and fluorescence spectra of the synthesized dyes in
PGMEA (5 ꢁ 10ꢀ6 mol litreꢀ1 for dyes 1a–2c and 10ꢀ5 mol litreꢀ1 for
dye 3).
c
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Table 2 Absorption spectra of the synthesized dyes in PGMEA
Synthesized dyes
lmax/nm
e
max/L molꢀ1 cmꢀ1
1a
1b
1c
2a
2b
2c
3
702
706
702
680
680
678
468
236 390
200 708
86 958
276 404
226 736
104 236
47 806
dyes (1a, 2a) are provided in ESI.w Accordingly, b-position
substituted PCs (2a–c) absorbing relatively more hypsochromic
range showed hypsochromic-shifted transmittance compared to
a-position substituted PCs (1a–c), and hence their transmittance
peaks after yellow compensation appeared to be sharper as
depicted in the later section. As described in the preceding
section, a-position substituted PCs generally had higher solubility
due to the heavily twisted structures which led to the decreased
aggregation peak as shown in Fig. 1.16,17 The twisted structures
of a-position substituted PCs made their extinction coefficients
(e) smaller than those of b-position substituted PCs, and this
tendency was much more severe at the Soret band, as shown in
Fig. 1. According to the core metals of PCs, the color strength
decreased as Zn 4 Cu 4 Co.
In order to realize the ideal green colors for LCD displays,
the dyes should transmit only from 490 to 610 nm selectively.
Therefore, the yellow compensating dye should be added
because this condition cannot be achieved only with PC dyes.
The absorption peak of the synthesized benzoperylene diimide
dye 3 was optimal for a yellow compensating dye, since it had
a very strong absorption maximum at 468 nm, the absorption
decreased steeply and ended just before 500 nm. Moreover,
dye 3 showed exceptionally strong color strength compared to
other yellow chromophores. In general, stability and color
strength of dye molecules often increase with increasing size of
the aromatic system, which inevitably causes a bathochromic
shift in absorption.18 The fluorescence spectrum range of dye 3
was in the green region as shown in Fig. 1, which is advanta-
geous for high transmittance of green color filters.19 Therefore,
dye 3 could be combined with PC dyes to make ideal green
colors for LCD color filters.
Fig. 2 Thermogravimetric analysis (TGA) of the synthesized dyes.
advantageous for intermolecular interactions, such as van der
Waals force and dipole–dipole interaction.27
For dyes to be used as color filters, they should endure a
temperature of 220 1C, which is the current highest tempera-
ture in the LCD manufacturing process, without significant
weight loss.28,29 The b-position substituted PC dyes (2a–c)
showed less than 1% weight loss after 30 min at 220 1C and
were stable up to 400 1C, whereas the a-position substituted
PC dyes except Zn–PC (1b–c) showed insufficient thermal
stability as shown in TGA graph, Fig. 2. This gap in thermal
stability is owing to the difference in aggregation tendency
from the structural difference. Namely, the difference in
the aggregating tendency of the two isomers was attributed
to a steric constraint of the a-position isomer that restricts
conformational freedom and, consequently, hinders formation
of dimer and higher order complexes.12 In the previous report,
field emission scanning electron microscopy (FE-SEM) studies
of various dyes showed that the dye with less steric hindrance
had the bigger aggregate size that led to superior thermal
stability.30 The yellow compensating dye 3 showed less than
3% weight loss in the isothermal section as shown in Fig. 2,
which is also sufficient for the LCD manufacturing process.
The practical thermal stability of spin coated color filters needs
to be discussed with color difference (DEab) values as expressed
in the later section since it is affected by the compatibility with
industrial solvents and binders as well as the inherent thermal
stability of dye molecules.
Thermal stability of dyes. The dye molecules should have
strong intermolecular interactions and form compact aggre-
gates to have high thermal stabilities.20–22 The metal PC dyes
as main colorants for green color filters are known to be
generally very stable. In particular, they have high thermal
stability since they consist of extensive aromatic planar and
symmetric systems which lead to one dimensional stacking like
rolls of coins. They do not melt; Cu–PC is sublimed in an inert
gas atmosphere at approximately 550 1C and ambient
pressure.23–25 The benzoperylene dye 3 as a yellow compensating
dye is also known to be very stable compared to other yellow
chromophores due to the strong p–p stacked interaction
resulting from an extensive aromatic system and a nearly flat
molecular structure.26 In particular, this dye can sustain the
characteristic high thermal stability of perylene since only one
side of the bay positions is substituted to keep the planarity of
the perylene main body.9 The high molecular weight and polar
substituents of the synthesized green and yellow dyes are
Characterization of dye-based color filters
Spectral and chromatic properties of dye-based color filters.
In order to make vivid and bright green color in LCD displays,
the transmittance graphs of the color filters should be sharp
and have transmission maxima between 520–550 nm. Fig. 3 and 4
show the transmittance graphs of the spin-coated color filters
with synthesized green dyes. Each figure includes the spectra of
green color filters before and after compensation with the
yellow dye 3, and a comparison is made between them and
pigment-based color filters. The dye-based color filters had
superior transmittance compared to pigment-based ones
because dyes dissolved in the media have a smaller particle
size leading to less light scattering.27,31 When the yellow
compensating dye 3 was added to color resists, clearer green
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Table 3 Transmittance of the pigment-based and dye-based color
filters
Spin-coated
dyes
Transmittance Transmittance
at 520 nm
at 540 nm
lmax
1a
1b
1c
2a
2b
2c
1a + 3
1b + 3
1c + 3
2a + 3
2b + 3
2c + 3
Pigment
91.56%
91.67%
90.76%
90.78%
91.49%
87.78%
78.52%
88.93%
87.50%
90.54%
87.42%
83.39%
86.78%
89.96%
85.23%
89.79%
88.81%
83.32%
87.81%
94.05%
94.37%
91.97%
94.13%
92.07%
88.01%
87.91%
520 nm (91.56%)
502 nm (93.12%)
518 nm (90.97%)
516 nm (91.51%)
498 nm (94.85%)
532 nm (88.35%)
538 nm (94.38%)
542 nm (94.47%)
542 nm (92.34%)
538 nm (94.14%)
538 nm (92.24%)
536 nm (88.69%)
536 nm (88.11%)
Fig. 3 Transmittance spectra of the pigment-based and dye-based
color filters.
Table 4 The coordinate values corresponding to the CIE 1931
chromaticity diagram and the color difference values of the pigment-
based and dye-based color filters
Spin-coated dyes
Y
x
y
DEab
1a
1b
1c
2a
2b
2c
3
1a + 3
1b + 3
1c + 3
2a + 3
2b + 3
2c + 3
Pigment
55.67
58.98
48.51
54.13
56.25
49.46
93.28
60.62
61.24
51.37
59.56
60.14
55.70
57.86
0.206
0.215
0.190
0.194
0.203
0.181
0.433
0.309
0.303
0.236
0.308
0.299
0.260
0.244
0.55
1.88
6.95
13.03
1.45
0.93
2.15
1.04
2.61
8.08
16.73
2.20
1.87
3.46
0.85
0.546
0.589
0.545
0.532
0.607
0.515
0.537
0.539
0.564
0.526
0.519
0.585
0.504
Fig. 4 Transmittance spectra of the pigment-based and dye-based
color filters.
colors were obtained since the undesirable transmittance at
500 nm was cut off and the wavelengths of maximum trans-
mittance were red-shifted approximately 15–20 nm. Therefore,
the transmittances without yellow compensation had their
maxima near 520 nm, and the ones with yellow compensation
had their maxima near 540 nm as shown in Table 3. Before the
yellow compensation, dye 2b showed the highest maximum
transmittance (94.85%) and after the yellow compensation,
dye 1b showed the highest maximum transmittance (94.47%).
Except dye 2b, the maximum transmittances of the dyes
increased after yellow compensation due to the fluorescence
of yellow dye 3 and the lowered contents of the green dyes.19
In general, the color filters coated with a-position substituted
PC dyes (1a–c) had slightly higher transmittance than those
with b-position substituted ones (2a–c) due to the lower
aggregating tendency. The a-position substituted PC dyes will
show reduced light scattering after being spin-coated onto the
substrate which leads to the higher transmittance. Co–PC dyes
(1c, 2c) showed lower transmittance than other metal PC dyes
on average.
Fig. 5 CIE 1931 chromaticity diagram of the pigment-based and
dye-based color filters.
economical that they showed similar or larger color gamuts
in spite of the much lower colorant contents in the inks due
to the much higher tinctorial strength and the fluorescence
of the compensating dye.9,19 The color matching function of
The coordinate values of the spin coated dye-based color
filters are expressed in Table 4 and Fig. 5 in comparison to a
pigment-based color filter. All dye-based color filters are so
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The various properties of PC dyes for the color filters were
influenced by the positions of the bulky substituents. The
b-position substituted PC dyes were superior in the absorption
range, color strength and thermal stability, but inferior in
solubility to the a-position substituted ones. The prepared dye-
based color filters exhibited excellent transmittance due to the
smaller particle size of the dye molecules suppressing light
scattering. In addition, they expressed similar or larger color
gamuts in spite of the much lower colorant contents in the
inks, due to a much higher tinctorial strength compared to the
pigment-based color filter. When the yellow compensating
dye 3 was mixed in the color filters, the half-band width of
the transmittance peak decreased due to its absorption. In
addition, the maximum transmittance and the brightness (Y)
of the coordinate values increased due to its fluorescence,
which led to a vivid green color.
Fig. 6 The CIE 1931 XYZ color matching functions.
the variable y, determining the brightness (Y) value of green
color, has the highest level of contribution near 540 nm as
shown in Fig. 6. It is noted that the Y values after compensation
with yellow dye 3 increased because in the vicinity of 540 nm the
transmittance after compensation was higher as shown in
Table 3. The Y values of the color filters with Co–PC dyes
(1c, 2c) were lower than those with other metals as in the case of
transmittance, but their color gamut expanded, thanks to the
increased y values. They exhibited more bluish green colors
because of the decreased x values.
In conclusion, the synthesized dyes were successfully applied
to the LCD color filters and showed superior optical properties
than conventional pigment-based color filters. Some dyes were
also sufficient in thermal stability, and hence, showed potential
for use in high-definition displays.
Acknowledgements
Thermal stability of dye-based color filters. In order to
estimate the thermal stability of color filters, the DEab values
resulting from post-baking were measured as shown in
Table 4. The DEab values of the color filters should be less
than 3 after heating for one hour at 230 1C for commercial
applications.9 The DEab values of the dye-based color filters
except 1b and 1c were less than 3, which means that their
thermal stability is sufficient to be applied for LCD color
filters. The difference of thermal stability resulting from
structural difference was reflected so that the color filters
with b-position substituted PC dyes (2a–c) showed smaller
DEab than those of color filters with a-position substituted PC
dyes (1a–c). In general, the thermal stabilities of the color
filters with Co–PC dyes, deduced from the DEab values, were
lower than those with other metal PC dyes, which accords with
the results of TGA analysis. The yellow compensating dye,
benzoperylene diimide 3, showed a good DEab value of 1.04
when solely spin coated, but exhibited larger DEab values to
some degree when blended with green PC dyes. These results
are considered to mean that although the thermal stabilities of
the individual dyes are superb, their color difference values
can increase due to the enhanced migration of dye molecules
when blended with each other and added to an industrial
solvent and binder. Nevertheless, dyes 1a, 2a, and 2b showed
sufficient thermal stability when spin coated onto color filters
with or without dye 3, although the DEab values of the
dye-based color filters with these were slightly higher than
that of the pigment-based color filter.
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.
Notes and references
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The six PC dyes and a benzoperylene dye were synthesized and
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