Highly efficient color filter using fluorescent materials, (Z)-3-(4'-(dimethylamino)biphenyl-4-yl)-2-phenylacrylonitrile

Article abstract of DOI:10.1080/15421400701544786

The transmittance and color characteristics of color filters are directly related to the energy efficiency and image quality of LCD panels. We tried to investigate the effect of fluorescent dyes, (Z)-3-(4'-(dimethylamino)biphenyl-4- yl)-2-phenylacrylonitr

Full text of DOI:10.1080/15421400701544786

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Highly Efficient Color
Filter Using Fluorescent
Materials, (Z)-3-(4′-
(dimethylamino)biphenyl-4-
yl)-2-phenylacrylonitrile
a
a
Yong-Jae Hwang & Dong-Myung Shin
a
Department of Chemical Engineering , Hong-Ik
University, Sangsu-Dong , Mapo-Gu, Seoul, Korea
Published online: 22 Sep 2010.
To cite this article: Yong-Jae Hwang & Dong-Myung Shin (2007) Highly Efficient
Color Filter Using Fluorescent Materials, (Z)-3-(4′-(dimethylamino)biphenyl-4-yl)-2-
phenylacrylonitrile, Molecular Crystals and Liquid Crystals, 472:1, 25/[415]-32/[422],
DOI: 10.1080/15421400701544786
To link to this article: http://dx.doi.org/10.1080/15421400701544786
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Mol. Cryst. Liq. Cryst., Vol. 472, pp. 25=[415]–32=[422], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701544786
Highly Efficient Color Filter Using Fluorescent
Materials, (Z)-3-(4 -(dimethylamino)biphenyl-4-yl)-
0
-phenylacrylonitrile
2
Yong-Jae Hwang
Dong-Myung Shin
Department of Chemical Engineering, Hong-Ik University,
Sangsu-Dong, Mapo-Gu, Seoul, Korea
The transmittance and color characteristics of color filters are directly related to
the energy efficiency and image quality of LCD panels. We tried to investigate
0
the effect of fluorescent dyes, (Z)-3-(4 -(dimethylamino)biphenyl-4-yl)-2-phenyla-
crylonitrile, on the transmittance and the color gamut of LCD panel. The fluor-
escent dye was synthesized and dispersed in polymer matrix and implemented
at the lamp side before the color filters. Energy transfer from blue to green using
fluorescent dye 1 can be applied to increase energy efficiency of green color resist.
The fluorescent films using dye 1 can improve the transmittance about 4–5% and
the color gamut and color purity that can be inferred from the color coordinate
change from (X ¼ 0.309, Y ¼ 0.467) to (X ¼ 0.325, Y ¼ 0.521).
Keywords: color filter; energy transfer; fluorescent dye; transmittance
INTRODUCTION
The transmittance and color characteristics of color filters have been
improved recently. The physical characteristics are critical to improve
image quality. Color filters are composed of glass substrate, black matrix,
RGB color resist, overcoat layer, ITO layer. The structure of color filters
varies with the LCD types and liquid crystal mode [1–3]. The manufac-
turing techniques are very diverse which include dyeing method, print-
ing method, electro-deposition method, and pigment-dispersed method.
Recently, large panel size requires many physical characteristics which
This work was supported by Seoul Research and Business Development Program
(2005) and 2004 Hong-Ik University Research Fund.
Address correspondence to Dong-Myung Shin, Department of Chemical Engineering,
Hong-Ik University, 72-1, Sangsu-Dong, Mapo-Gu, Seoul 121-791, Korea. E-mail:
shindm@wow.hongik.ac.kr
25=[415]

2
6=[416]
Y.-J. Hwang and D.-M. Shin
are printability, transmittance, clarity, and manufacturing feasibility.
The pigment-dispersed method, which has been applied to the manu-
facturing a-Si TFT-LCD, uses the color resist that consists of pigments
dispersed photo-responsive resin. The color patterns are made using
photo-lithography process [4–6].
When dye contents are high, the color characteristic can be improved.
However the transparency of the films can be drastically deteriorated.
Transparency of the films directly affects the energy efficiency of
the LCD devices. Low dye contents can improve transparency and
many film characteristic and manufacturing feasibility losing color
characteristics such as brightness, hue and colorfulness [7–9].
We tried to investigate the effect of fluorescent dyes on the color
gamut of LCD panel. Improvement of color gamut without losing
other optical characteristics of color filters is of current interest.
LCD industries do not tend to use fluorescent dyes for the color filters,
because many fluorescent dyes are apt to fade color. To improve the
transmittance of color filter, we tried to use energy transfer from the
light which has to be removed to generate color to the region fitted
to the color we want. Using adequate fluorescent dyes, we can improve
the hue and the transmittance of color filter by cutting the unnecess-
ary light and dumping that light energy to the color filter region.
EXPERIMENTAL
Commercial color resists were obtained from the company which sup-
ply color resist to LCD manufacturing company. The color resists were
coated on 2cm ꢀ 2 cm glass substrate using spin and split coating
method. The transmittance and absorbance of the films were measured
by using UV-visible spectrophotometer (HP model 8453). The film was
coated by two steps, 800 rpm followed by 1500, 2000 or 2500 rpm by
using spin coater (home made). The solvent for color resist was PGMEA
(
Propylene glycol monomethyl ether acetate), which was dried after
spin coating. A fluorescent color film was fabricated as the same
method. The fluorescence spectrum was obtained from spectrofluorom-
eter (Perkin Elmer LS50). The fluorescent biphenyl derivative was
obtained from 4-bromobenzaldehyde and 4-dimethyl aminoboronic acid
0
which generated 4 -(Dimethylamino)biphenyl-4-carbaldehyde. The
carbaldehyde was reacted with benzyl cyanide under basic condition
0
using sodium ethoxide in ethanol. The (Z)-3-(4 -(dimethylamino)biphe-
nyl-4-yl)-2-phenylacrylonitrile 1 (Scheme 1) was yellow color and very
fluorescent at solid state.
The fluorescent dye 1 dissolved in chloroform and PGMEA was
blended with the commercial color resist solution. The ratio of the

Highly Efficient Color Filter Using Fluorescent Materials
27=[417]
0
SCHEME 1 Synthesis of (Z)-3-(4 -(dimethylamino)biphenyl-4-yl)-2-phenyl-
acrylonitrile.
solution was varied and the resultant mixture solution was spin-
coated to form a single layer of color resist. We tried to maintain
dye concentration for all photoresist solutions. The spin-coated film
ꢁ
was thermal-cured at 150 C for 10min. Two layers, fluorescent layer
on top of color resist layer, of the fluorescent films were spin-coated
with polymethylemthacrylate (PMMA) chloroform solution contain-
ing 1. The film characteristics were investigated as a function of
fluorescent dye concentration and coating conditions, such as spin
speed and curing temperature. Also, The color coordinate was
obtained with MINOLTA Chroma meter CS-100A. The back light
was fluorescence light covered with white copy paper, which passed
through a polarizer.
RESULTS AND DISCUSSION
The maximum absorption wavelength of 1 was 402 nm and maximum
emission wavelength was 515 nm which is exactly on the center of

2
8=[418]
Y.-J. Hwang and D.-M. Shin
green region. Figure 1 illustrates that the fluorescence band of 1
resides under transmission band of green color resist, and absorption
band removes transmission band of color resist at 300–420nm. The
quantum efficiency of 1 was low in polar solvent and very high at solid
state and rigid matrixes include polymer film.
The transmittance at 540 nm increases slightly, from 92% to 95%
ꢂ2
with addition of 10% dye solution (3 ꢀ 10 M), Figure 2. As the con-
centration of 1 increases, the light absorption of 1 clearly block the
light transmittance at around 400 nm.
However, for concentrated solution the whole transmittance was
lower, due to light scattering of the film and the front face self absorp-
tion of 1 reduced the energy transfer from blue light to green. The
similar trend was observed from 5:9 (v=v) mixture films with various
ꢂ1
concentration of 1. Dye 1 (10 M, 0.2ml) was dissolved in PMMA
solution (2g of PMMA and 10 mL of chloroform) and was coated on
glass substrate. A lower dye concentration films were obtained by
dilution from the concentrated dye solution. Color resist was coated
on top of dye-PMMA films by spin-coating method in order to utilized
energy transfer by green emission. Figure 3 shows that the transmit-
tance is lowered over the whole spectral range when the concentration
ꢂ1
of 1 in PMMA is 10 M. At the lower concentration range, the green
FIGURE 1 The absorption and emission properties of 1 solubilized in PMMA
and transmission bands of color resist (CR).

Highly Efficient Color Filter Using Fluorescent Materials
29=[419]
FIGURE 2 Transmittance spectra of dye solution of 1 and green color resist
ꢂ2
(
1:91 = vv ). The concentrations1 wo ef re 1, 2, 3 ꢀa 1n 0d 7M .
FIGURE 3 Transmission spectra of green color resist spin coated on top of
PMMA containing 1 compared with that with containing 1 (straight line).

3
0=[420]
Y.-J. Hwang and D.-M. Shin
FIGURE 4 Transmittance spectra of color resist film and that with stack
1–4 layers) fluorescent films containing 1 .
(
transmittance unchanged with significant loss in transmittance at
blue region, which affects hue generated from LCD panel.
The interface form between PMMA and color resist film was very
unstable and the optical quality was not reproducible due to the
polarity difference of PMMA and the PGMEA containing color dye
stuff. At the higher dye content in PMMA, the interfacial mismatch
pronouncedly deteriorates optical quality of color resist. The interfa-
cial scattering was significantly reduced after using separate fluor-
escent PMMA film containing 1, Figure 4. The transmittance at
5
40 nm increases gradually with the number of fluorescent films.
Accordingly, the absorbance gradually decrease around 400 nm region.
The color coordinate obtained from the color resist film and the
mixed stack of color filter and fluorescent film. The color coordinate
can be influenced by the light illuminated from backside. The color
coordinate also gradually shift to pure color region, Figure 5. The color
coordinate (X ¼ 0.309, Y ¼ 0.467) was obtained from the color resist
only, and with one fluorescence layer of 1 in PMMA, it shifted to
(
X ¼ 0.315, Y ¼ 0.490). For two layers, it became (X ¼ 0.319,
Y ¼ 0.502) and (X ¼ 0.322, Y ¼ 0.513) and (X ¼ 0.325, Y ¼ 0.521) for
3
and 4 layers, respectively [10–13].
The color coordinates represent quite same color in X coordinate,
but the increase in Y coordinate clearly shows that the color becomes
more pure and removes yellowish taint.

Highly Efficient Color Filter Using Fluorescent Materials
31=[421]
FIGURE 5 The CIE (x, y) chromaticity diagram and color coordinate obtained
from the color resist only, and with 1 to 4 fluorescence layers of 1 in PMMA.
It was very difficult to increase transmittance of color resist which
already has more than 90% transmittance. Energy transfer from blue
to green using fluorescent dye 1 can be applied to increase energy
efficiency of green color resist. Accordingly, it can be applied to the
color efficiency of red color filter, too. The fluorescent films using
dye 1 can improve 4–5% transmittance and the color gamut and color
purity that can be inferred from the color coordinate change from
(
X ¼ 0.309, Y ¼ 0.467) to (X ¼ 0.325, Y ¼ 0.521).
CONCLUSIONS
0
The fluorescent dye, (Z)-3-(4 -(dimethylamino)biphenyl-4-yl)-2-phenyl-
acrylonitrile, was synthesized and showed high fluorescence
efficiency. The dye 1 was dispersed in polymer matrix and implemen-
ted at the lamp side before the color filters. Energy transfer from blue
to green using fluorescent dye 1 occurred by absorption of light at
the blue region (350–450nm) and emission at the green region
(
480–620 nm). The fluorescent films using dye 1 can improve the
transmittance about 4–5% and the color gamut and color purity that
can be inferred from the color coordinate change from (X ¼ 0.309,

3
2=[422]
Y.-J. Hwang and D.-M. Shin
Y ¼ 0.467) to (X ¼ 0.325, Y ¼ 0.521). We think that these results go
with the capacity of fluorescent filter and the compatibility with com-
mercial color resist. Therefore, this study is meaningful at the side
that the light transmittance of color resist was increased with main-
taining the color purity of the existing color resist.
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