Synthesis and characterization of novel triazatetrabenzcorrole dyes for LCD color filter and black matrix

Article abstract of DOI:10.1016/j.dyepig.2013.05.026

We have synthesized three novel triazatetrabenzcorrole dyes via a ring-contraction reaction to be applied as colorants for dye-based liquid crystal display color filter and black matrix. The absorption peaks of the synthesized dyes showed a red-shifted So

Full text of DOI:10.1016/j.dyepig.2013.05.026

Dyes and Pigments 99 (2013) 357e365
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and characterization of novel triazatetrabenzcorrole dyes for
LCD color filter and black matrix
*
Jun Choi, Woosung Lee, Jin Woong Namgoong, Tae-Min Kim, Jae Pil Kim
Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We have synthesized three novel triazatetrabenzcorrole dyes via a ring-contraction reaction to be
applied as colorants for dye-based liquid crystal display color filter and black matrix. The absorption
peaks of the synthesized dyes showed a red-shifted Soret band and a blue-shifted Q-band compared to
phthalocyanine dyes, and this can be extremely beneficial when applied for liquid crystal display color
filter and black matrix. These dyes exhibited enhanced solubility in the industrial solvents by the
introduction of bulky axial substituents. The dyes were successfully spin-coated with binders on the
substrate showing a lowered aggregating tendency as illustrated in the field emission scanning electron
microscopy images. The spin-coated films prepared with these dyes exhibited superior optical and
chromatic properties compared to the phthalocyanine-based or pigment-based ones for green color
filter, even without the addition of a yellow compensating dye. In addition, they showed satisfactory
dielectric properties for the black matrix of color filter on array mode.
Received 18 April 2013
Received in revised form
24 May 2013
Accepted 24 May 2013
Available online 3 June 2013
Keywords:
Dyes
Triazatetrabenzcorrole
Ring-contraction reaction
Liquid crystal display
Color filter
Ó 2013 Elsevier Ltd. All rights reserved.
Black matrix
1. Introduction
with the electrical signals of TFT to be applied for color filter on
array (COA) mode that CF is patterned right onto the TFT array.
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 CF, which converts the
white backlight into various colored lights, consists of RGB (red,
green, and 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 cells [1e4].
Currently, various dye molecules are under extensive investi-
gation for the manufacturing of RGB pixels and BM to improve the
performance of LCD. The pigments, being used for the main col-
orants of RGB pixels, show superior thermal and photo-chemical
stabilities, but have low optical and chromatic properties due to
their aggregation behavior [5,6]. 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 sta-
bilities because the LCD manufacturing process temperature has
been decreasing [7]. The colorants for LCD BMs should not interfere
Therefore, extensive investigation is currently being performed on
colorants with low dielectric constants such as dyes, so that LCD
BMs can replace the carbon black with high dielectric constant
[8,9]. Recently, our group applied the phthalocyanine (PC) de-
rivatives with high color strength and thermal stability to LCD CF
and BM by improving their solubility [10,11]. In this study, the novel
triazatetrabenzcorrole (TBC) dyes were designed and synthesized
through the ring-contraction process of PC dyes. They exhibited
more advantageous absorption peaks for LCD CF and BM, and
enhanced solubility to the industrial solvents by the introduction of
bulky axial substituents. For the above purpose, the thermal sta-
bility, optical properties, and solubility of the prepared dyes as the
colorant materials for LCD CF and BM were investigated according
to their characteristic structures. In addition, the transmittance,
color gamut, and dielectric constants of the spin-coated films with
these dyes were examined.
2. Experimental
2.1. Materials and instrumentation
1,8-Diazabicyclo-7-undecene (DBU), 3(4)-nitrophthalonitrile,
2,5-bis-(1,1-dimethylbutyl)-methoxyphenol purchased from TCI,
* Corresponding author. Tel.: þ82 2 880 7187; fax: þ82 2 880 7238.
E-mail address: jaepil@snu.ac.kr (J.P. Kim).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.05.026

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J. Choi et al. / Dyes and Pigments 99 (2013) 357e365
and 4-methylphenol, PBr3, CuCl2 purchased from SigmaeAldrich
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 CF and acrylic binder
DW1 were supplied by NDM Inc.
filtered to remove excess pyridinium bromide as a white crystalline
solid. The filtrate was reduced in volume under reduced pressure
and loaded onto a silica gel column for purification with 1/7 ethyl
acetate/hexane as the eluent. The bright green band to elute was
collected and dried under vacuum to give RC2 as a bright green
solid (0.75 g, 58%). MALDI-TOF MS: m/z 1752.4 (100%, [Mþ]); Found:
C, 75.17; H, 8.20; N, 5.65. Calc. for C110H142N7O10P: C, 75.35; H, 8.16;
N, 5.59.
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 (Seoul National Uni-
versity National Center for inter-University Research Facilities).
Matrix Assisted Laser Desorption/Ionization Time Of Flight
(MALDI-TOF) mass spectra were collected on a Voyager-DE STR
Biospectrometry Workstation with a-cyano-4-hydroxy-cynamic
acid (CHCA) as the matrix. Absorption and transmittance spectra
were measured using an HP 8452A spectrophotometer. Fluores-
cence spectra were measured using a Shimadzu RF-5301PC
spectrofluorometer. Elemental analysis was carried out with a
Flash EA 1112 CNH analyzer. Cyclic voltammetry was performed
using a three-electrode cell and a VersaSTAT 3-100. Chromatic
characteristics of the spin-coated films were analyzed on a Scinco
color spectrophotometer. Dielectric constants were measured
using Edward E306 thermal evaporator and Agilent 4294A pre-
cision impedance analyzer. Thermogravimetric analysis (TGA)
was conducted under nitrogen at a heating rate of 10 ꢀC minꢁ1
using a TA Instruments Thermogravimetric Analyzer 2050. The
thickness of the spin-coated film was measured using a Nano
System Nanoview E-1000. Field emission scanning electron mi-
croscopy (FE-SEM) images of spin-coated films were taken by
JSM-840A microscope.
2.2.3. Di(4-methylcyclohexoxy)phosphorus(V)2(3)-Tetrakis(2,5-
Bis(1,1-dimethylbutyl)-4-methoxyphenoxy)-triazatetrabenzcorrole
(RC3)
2 (1.24 g, 0.74 mmol) was placed in a 100 mL flask equipped with
a condenser and gas inlet adapter and dissolved in 30 mL of pyri-
dine. An amount of PBr3 (9.0 g, 33.0 mmol) was added and the
resulting solution heated to 120 ꢀC and stirred for 2 h. After 2 h, the
volume of the reaction mixture was reduced by vacuum distillation
to 1e2 mL and then treated with a 50/50 solution of CH2Cl2/4-
methylcyclohexanol for 12 h. The resulting green solid was dis-
solved in CH2Cl2 and filtered to remove excess pyridinium bromide
as a white crystalline solid. The filtrate was reduced in volume
under reduced pressure and loaded onto a silica gel column for
purification with CH2Cl2 as the eluent. The second bright green
band to elute was collected and loaded onto a silica gel column for
purification again with 3/40 ethyl acetate/hexane as the eluent. The
first bright green band to elute was collected and dried under
vacuum to give RC3 as a shiny bright green solid (0.78 g, 55%).
MALDI-TOF MS: m/z 1917.8 (100%, [Mþ]); Found: C, 76.11; H, 8.56;
N, 5.13. Calc. for C122H162N7O10P: C, 76.41; H, 8.52; N, 5.11.
2.2. Synthesis
2.3. Preparation of dye-based inks and spin-coated films
2(3)-Tetrakis(2,5-Bis(1,1-dimethylbutyl)-4-methoxyphenoxy)-
phthalocyanine (2) was synthesized according to the previously
reported procedures [11], 2(3)-Tetrakis(2,5-Bis(1,1-dimethylbutyl)-
4-methoxyphenoxy)-phthalocyaninatocopper(II) (2b) and N,N0-
Bis(2,6-diisopropylphenyl)-5-phenylbenzoperylene-2,3,8,9-
tetracarboxdiimides (3) were synthesized according to the pro-
cedures reported elsewhere [10].
Dye-based ink for a CF and BM was composed of the propylene
glycol methyl ether acetate (PGMEA) (2.8 g), acrylic binder (2.0 g),
and dye (0.07 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 10 s, which was
then increased to 600 rpm and kept constant for 20 s. The wet dye-
coated glasses were then dried at 80 ꢀC for 20 min, prebaked at
150 ꢀC for 10 min, and postbaked at 230 ꢀC for 1 h. After each step,
the coordinate values of the dye-coated glasses were measured.
2.2.1. Hydroxyphosphorus(V)2(3)-Tetrakis(2,5-Bis(1,1-
dimethylbutyl)-4-methoxyphenoxy)-triazatetrabenzcorrole
Hydroxide (RC1)
2.4. Investigation of solubility
2 (1.24 g, 0.74 mmol) was placed in a 100 mL flask equipped with
a condenser and gas inlet adapter and dissolved in 20 mL of pyri-
dine. An amount of PBr3 (9.0 g, 33.0 mmol) was added and the
resulting solution heated to 120 ꢀC and stirred for 2 h. After 2 h,
approximately 3/4 of the solvent was allowed to evaporate under a
flow of argon. The mixture was then poured into a water/ice bath
and the resulting suspension filtered to give a green solid. The solid
was purified on a silica gel column using 1/6 ethyl acetate/hexane
as the eluent. The bright green band was collected and concen-
trated producing RC1 as a bright green solid (0.45 g, 35%). MALDI-
TOF MS: m/z 1725.0 (100%, [M þ OH]þ); Found: C, 75.07; H, 8.15; N,
5.75. Calc. for C108H138N7O10P: C, 75.19; H, 8.06; N, 5.68.
Solubility of the synthesized dyes in propylene glycol methyl
ether acetate (PGMEA) was examined to determine the effects of
peripheral and axial substituents. 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 so-
lutions were left to stand for 48 h at room temperature, and
checked for precipitation to determine the solubility of the dyes.
2.5. Measurement of spectral and chromatic properties
Absorption spectra of the synthesized dyes and transmittance
spectra of pigment-based and dye-based CFs were measured using
2.2.2. Dimethoxyphosphorus(V)2(3)-Tetrakis(2,5-Bis(1,1-
a UVevis spectrophotometer. Fluorescence spectra of the
dimethylbutyl)-4-methoxyphenoxy)-triazatetrabenzcorrole (RC2)
2 (1.24 g, 0.74 mmol) was placed in a 100 mL flask equipped
with a condenser and gas inlet adapter and dissolved in 30 mL of
pyridine. An amount of PBr3 (9.0 g, 33.0 mmol) was added and the
resulting solution heated to 120 ꢀC and stirred for 2 h. After 2 h, the
volume of the reaction mixture was reduced by vacuum distillation
to 1e2 mL and then treated with a 50/50 solution of CH2Cl2/MeOH
for 2 h. The resulting green solid was dissolved in CH2Cl2 and
synthesized dyes were measured using a Shimadzu RF-5301PC
spectrofluorometer. Chromatic values were recorded on a color
spectrophotometer (Scinco colormate).
2.6. Measurement of dielectric properties
The prepared dye-based inks were coated on a aluminum sub-
strate using a MIDAS System SPIN-1200D spin coater. The coating

J. Choi et al. / Dyes and Pigments 99 (2013) 357e365
359
speed was initially 100 rpm for 10 s, which was then increased to
800 rpm and kept constant for 20 s. Aluminum electrodes were
deposited by Edward E306 thermal evaporator and capacitances
were measured by Agilent 4294A precision impedance analyzer.
temperature for 10 min to remove residual water and solvents. The
dyes were then, heated to 220 ꢀC and held at that temperature for
30 min to simulate the processing thermal conditions of CF
manufacturing. The dyes were finally heated to 400 ꢀC to determine
their degradation temperature. The temperature was raised at the
rate of 10 ꢀC minꢁ1 under nitrogen atmosphere.
2.7. Measurement of thermal stability
To check the thermal stability of the dyes in spin-coated films,
the fabricated spin-coated glasses were heated in a forced con-
vection oven (OF-02GW Jeiotech Co., Ltd.). The color difference
Thermal stability of the synthesized dyes was evaluated by TGA.
The prepared dyes were heated to 110 ꢀC and held at that
O
OH
O
CN
CN
O2N
CN
CN
O
K2CO3, DMF, 80'C, 12h
1
O
O
O
O
O
N
HN
Li
N
2
N
N
Xylene, EtOH
NH
N
N
O
O
O
PBr3,
H2O
/ice
PBr3,
pyridine
ROH,
pyridine
CH2Cl2
OH-
O
O
O
O
RO
N
O
HO
N
O
O
O
O
O
N
N
N
N
N
N
P
N
N
P
N
N
N
N
O
O
O
O
OR
O
O
RC1
RC2 (R=CH3)
RC3 (R=4-methylcyclohexyl)
Scheme 1. Synthesis of triazatetrabenzcorrole dyes.

360
J. Choi et al. / Dyes and Pigments 99 (2013) 357e365
values (
D
Eab) before and after heating were measured on a color
3.2. Characterization of dyes
spectrophotometer (Scinco colormate) in CIE L’a’b’ mode.
3.2.1. Solubility of dyes
For dyes to be successfully applied as colorants for LCD CF or BM,
they should exhibit high solubility in industrial solvents. Regardless
of the compatibility with binders, dye molecules should dissolve
more than 5 wt% in industrial solvents to have the desired optical
properties [10,11,16]. Especially, to prevent the reaggregation
behavior between the dye molecules after the post baking process,
bulky substituents should be introduced and indeed such mole-
cules are therefore normally also better soluble in organic solvents.
Such a reaggregation between molecules is believed to be the pri-
mary cause of the decline of transmittance of LCD CFs. The solu-
bility data of the synthesized dyes in industrial solvent, PGMEA, are
listed in Table 1. In the case of PC dye 2b (Scheme 2), as found in the
previous work, the bulky functional groups substituted at the pe-
ripheral positions caused steric hindrance to reduce intermolecular
aggregation and the ether linkages in its structures enhanced its
miscibility with PGMEA [10]. However, the introduction of bulky
axial ligands was considerably more effective in hindering the
3. Results and discussion
3.1. Synthesis (see Supporting Information y)
The novel three TBC phosphorus compounds with enhanced
solubility and broadened absorption peak were designed and syn-
thesized as shown in Scheme 1. Each precursor of metal-free PC (1)
was synthesized through a nucleophilic aromatic substitution re-
action between nitro phthalonitriles and phenols with the func-
tional groups having affinity with the industrial solvent, PGMEA,
and its structure was confirmed by 1H NMR [12]. The metal-free PC
(2) was synthesized by a cyclotetramerization reaction of the pre-
cursors and purified via column chromatography [13]. The PCs
synthesized from monosubstituted phthalonitriles can theoretically
have constitutional isomers, and their properties were observed
without further attempts to separate these isomers in this work [14].
The TBC phosphorus compounds were synthesized via a ring-
contraction reaction mediated by PBr3 in pyridine in which a
meso-nitrogen atom is extruded from an appropriate metal-free PC
precursor. In order to ensure a good yield of TBC, the pyridine must
be carefully distilled over CaH2, and the phthalocyanine starting
material must be dried in a heated (50 ꢀC) vacuum oven for several
hours immediately prior to use [15]. After the ring-contraction re-
action, the TBC compound RC1 was successfully precipitated by
quenching with an ice/water mixture. Purification by silica gel
chromatography was carried out by using ethyl acetate/hexane so-
lution as the eluent, rather than ROH/CH2Cl2 solution, to avoid any
axial ligand exchange with alcohols. The TBC compounds RC2 and
RC3 were synthesized by quenching with an appropriate ROH/
CH2Cl2 mixture. The structures of synthesized TBCs were confirmed
by MALDI-TOF spectroscopy and elemental analysis.
intermolecular
pep stacking between phthalocyanine hetero cy-
cles [17e22]. The RC1e3 dyes, with bulky axial ligands at the
phosphorus atom, showed considerably higher solubility than PC
dye 2b with only peripheral substituents in PGMEA. In particular,
RC3 exhibited the highest solubility contributed by the bulkiest
axial substituent, the methylcyclohexoxy group. The increased
solubility can be advantageous for the RC1e3 series to show
excellent optical properties as the colorant materials for LCD.
3.2.2. Spectral properties of dyes
The UVevis absorption spectra of PC dye 2b and TBC dyes RC1e
3 are shown in Fig. 1 and the corresponding data are listed in
Table 2. As reported elsewhere, the absorption peaks of TBC com-
pounds RC1e3 had a red-shifted Soret band and blue-shifted Q-
Scheme 2. Synthesis of phthalocyanine dye.

J. Choi et al. / Dyes and Pigments 99 (2013) 357e365
361
Table 1
Table 2
Solubility of the synthesized dyes at 20 ꢀC in PGMEA.
Absorption spectra of the synthesized dyes in PGMEA (5 ꢂ 10ꢁ6 mol Lꢁ1).
Synthesized dyes
Solubility (g/100 ml)
Synthesized dyes
lmax (nm)
3
(L molꢁ1 cmꢁ1
)
max
2b
6
9
9
RC1
RC2
RC3
2b
450, 662
450, 662
450, 662
340, 680
183 594, 91 996
158 640, 81 918
244 538, 126 696
104 630, 226 736
RC1
RC2
RC3
10
band compared to the corresponding PC dye 2b [23e25]. These
traits can be extremely beneficial for dyes RC1e3 to be applied as
colorants of green CF and BM of an LCD.
prepared dyes RC1e3 was about 110 nm, which was more than that
of the general TBC compounds [23e25]. Such a bathochromic shift
of the Soret band is likely to be the result of the destabilization of
the HOMO-1 (a2u) orbital caused by the removal of a meso-
nitrogen atom [15]. The Q-bands of the synthesized dyes RC1e3
were hipsochromic shifted about 18 nm compared to PC dye 2b,
comparable to those of common TBC compounds [23e25]. The
molar extinction coefficients of dyes RC1e3 increased in the order
of RC2 < RC1 < RC3, whereas the absorption maxima (lmax) of the
dyes were identical as expressed in Table 2. In particular, the color
strength of RC3 was very strong since its molar extinction coeffi-
cient was even higher than that of PC dye 2b.
For dye-based LCD CF, the green dyes should strongly absorb the
range above 630 nm [10]. The absorption peaks of the three syn-
thesized TBC dyes started from 600 nm and showed extremely
strong and sharp maxima in the range of 650e680 nm as shown in
Fig. 1. In addition, the growth and bathochromic shift of the Soret
bands induced from the conversion from PC compounds to TBC
compounds is extremely crucial for these TBC compounds to
exhibit vivid green colors. This very strong and sharp absorption at
450 nm cut off the transmittance before 480 nm, replacing the role
of the yellow compensating dyes, as shown in Fig. 4 [10,26]. As
reported in our previous work, the synthesis of stable yellow dye
with strong absorption is very difficult because, in general, the
stability and color strength of dye molecules often increase with
increasing size of the aromatic system, which inevitably causes a
bathochromic shift in their absorption [27]. However, the synthe-
sized TBC dyes RC1e3 can eliminate the addition of yellow
compensating dye, which can simplify the manufacturing process,
omitting the need to consider the content ratio, compatibility, and
thermal deterioration accompanied with the blending of a green
main dye and yellow compensating dye.
For dye-based LCD BM, the dyes should have absorptions that
are as strong and broad as possible in the entire visible region [11].
The synthesized TBC dyes exhibited additional strong absorption in
the visible range induced from the growth and bathochromic shift
of the Soret band, which made their absorption peaks closer to
those of the panchromatic dyes. The drawback of PC dyes, as
possible candidates for dye-based LCD BM, is their absorption
deficiency from 450 nm to 600 nm due to the wide gap between the
Soret band and the Q-band [11]. On the other hand, the synthesized
TBC dyes RC1e3 can easily prevent light leakage in the visible range
since they have a narrower gap between the Soret band and the Q-
band. In particular, the bathochromic shift of the Soret band of the
3.2.3. Thermal stability of dyes
The dye molecules should have strong intermolecular in-
teractions and form compact aggregates in order to have high
thermal stabilities [28e30]. The PC dyes, as the precursor of the
TBC dyes as the main colorants for green CFs and BM, 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 stacking like rolls of coins [31e
33]. The synthesized TBC dyes RC1e3 mostly maintained the
superior stability of the PC dyes. However, their stabilities were
somewhat deteriorated due to the axial substituents at phos-
phorus inducing the decreased
pep stacking interaction. While
the peripheral substituents of RC1e3 are also bulky enough to
cause steric hindrance, having the terminal alkoxy group was
found to relieve the deterioration of thermal stability [11,34].
The high molecular weight and polar substituents of the syn-
thesized TBC dyes are advantageous for intermolecular in-
teractions, such as Van der Waals Force and dipoleedipole
interaction [35]. In order to use dyes as CFs, they should be able
to endure a temperature of 220 ꢀC (which is the conventional
highest temperature in the LCD manufacturing process) without
significant weight loss [36,37]. As shown in Fig. 2, dyes RC2 and
1.4
100
90
RC1
RC2
RC3
2b
1.2
1.0
0.8
0.6
0.4
0.2
0.0
80
70
RC1
RC2
60
RC3
50
300
400
500
600
700
800
50
100
150
200
250
300
350
400
Wavelength(nm)
Temperature('C)
Fig. 1. Absorption spectra of the synthesized dyes in PGMEA (5 ꢂ 10ꢁ6 mol Lꢁ1).
Fig. 2. TGA analysis of the synthesized dyes (isothermal 30 min at 220 ꢀC).

362
J. Choi et al. / Dyes and Pigments 99 (2013) 357e365
RC3 were stable at the TGA analysis, showing less than 5%
weight loss after 30 min at 220 ꢀC. However, the weight loss of
RC1 was relatively larger since the distortion of the RC1 mole-
cule was induced by the axial substituent introduced asym-
metrically as illustrated in Fig. 3. In addition, the hydroxyl axial
group of RC1 is believed to be chemically less stable than the
alkoxy axial group of the other dyes. The RC3 dye showed
greater weight loss than RC2 at the isothermal period, but
appeared more stable as the temperature increased.
100
80
60
40
20
0
RC1
RC2
RC3
2b+3
Pig
3.2.4. Geometry optimization and electrochemistry of dyes
Geometry optimizations of the synthesized dyes were carried
out using Gaussian 09 software. They were calculated with DFT on a
B3LYP/6-31 þ G (d,p) level. The calculated results were in accord
with the solubility and thermal stability data of the prepared dyes.
The synthesized TBC dyes RC1e3 have axial substituents, intro-
duced perpendicular to the main body as illustrated in Fig. 3,
300
400
500
600
700
Wavelength(nm)
believed to be effective to prevent their
pep stacking. This anti-
aggregation effect was expressed well as their solubilities in
Table 1. The influence of the axial substituent gave RC3 maximum
solubility since this methyl cyclohexyl axial group was geometri-
cally even bulkier than other groups. The main body of RC1 was
distorted more than that of RC2 and RC3, as illustrated in Fig. 3,
leading to a lower thermal stability.
Fig. 4. Transmittance spectra of the pigment-based and dye-based color filters. (For
interpretation of the references to colour in this figure legend, the reader is referred to
the web version of this article.)
3.3. Characterization of spin-coated films
The oxidation potential data of TBC dyes RC1e3 are listed in
Table 1S (Supporting Information) and are deduced from normal-
ized absorption, fluorescence spectra, and cyclic voltammetry
curves shown in Figs. 1S and 2S. The overall electrochemical
behavior of the three TBC dyes RC1e3 appeared similar, as shown in
Fig. 2S, suggesting that the influence over their redox potential
induced from the axial ligand difference is negligible. The negative
values for Eox and Eox ꢁ E0-0 of RC1e3 were greater than those of
common PC dyes, impliying that RC1e3 are easier to oxidize and
more difficult to reduce than PC dyes. This is consistent with the
notion that the corroles, in general, help stabilize a higher oxidation
state [15].
3.3.1. Spectral and chromatic properties of spin-coated films for LCD
CFs
The transmittance spectra of spin-coated TBC dyes RC1e3 are
shown in Fig. 4, and the corresponding data are listed in Table 3.
These transmittance spectra were compared to the spectrum of the
mixture of PC dye 2b and the yellow compensating dye 3 (see
Supporting Information y), and the spectrum of the pigment-based
CF. The spin-coated films with TBC dyes RC1e3 exhibited
outstanding transmittance from 490 nm to 580 nm, advantageous
to be used as LCD green CFs. In order to achieve a high brightness
(Y) for the green color (the most important index for high quality
Fig. 3. Geometry-optimized structures of the synthesized dyes.

J. Choi et al. / Dyes and Pigments 99 (2013) 357e365
363
Table 3
Transmittance of the pigment-based and dye-based color filters.
Spin-coated dyes
lmax (nm)
Maximum transmittance (%)
RC1
RC2
RC3
514
510
520
538
522
93.18
93.25
92.59
92.24
91.30
2b þ 3
Pigment
LCD CF), the square shaped high transmittance spectrum is ideal.
The reason why the transmittance spectra of RC1e3 are closer to
this ideal transmittance is mainly attributed to their absorption
spectra. Firstly, the Soret bands of the absorption peaks of RC1e3,
compared to the spectrum of yellow compensating dye 3 [26], were
sharper and slightly hipsochromic shifted, leading to the steep-
shaped transmittance peak and the increase of transmittance in
the 490e510 nm region. In addition, the undesirable shoulder
before the main transmittance disappeared because of the higher
molar extinction coefficients of their Soret bands. Secondly, TBC
dyes RC1e3 showed a declined aggregation peak in front of the Q-
band, resulting from higher solubility, and their Q-bands were
slightly less hipsochromic shifted compared to common TBC
compounds [17,38]. These factors led to the increased trans-
mittance in the 560e580 nm region. Lastly, RC1e3 with decreased
aggregating tendency had smaller particle sizes in the media
leading to less light scattering, resulting in an overall rising trend of
transmittance [35,39]. In particular, RC2 exhibited 93.25% of the
highest maximum transmittance at 510 nm. RC3 cut off the unde-
sirable transmittance most clearly because of the highest molar
extinction coefficients.
The coordinate values of the spin-coated dye-based CFs are
compared with a pigment-based CF as shown in Table 4 and Fig. 5.
The all dye-based CFs are so efficient 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 [10,16]. The TBC
dyes RC1e3 showed a superior brightness (Y) value compared to PC
dye 2b and the pigment-based CF owing to the higher trans-
mittance in the green region. Especially, RC2 exhibited 62.84 of the
highest brightness value. RC3 showed the widest color gamut with
the highest y value. Judging from this result, the transmittance and
brightness of RC3 can increase further since the amount of RC3 in
the color ink can be reduced for obtaining the same color strength.
Fig. 5. CIE 1931 chromaticity diagram of the pigment-based and dye-based color
filters.
demanded dielectric constant with the current commercialized
thickness of BM (1e1.5 mm) is smaller than 7, whereas the dielectric
constant of the carbon black-based BM is larger than 20 [40]. The
dielectric constants of the TBC dyes RC1e3 spin-coated on the
aluminum substrate are listed in Table 5. Their dielectric constants
were calculated according to the following equation. The symbol C
refers to capacitance (F),
3
refers to dielectric constant under
0
vacuum (8.854 ꢂ 10ꢁ14 Fcmꢁ1), t refers to thickness (cm), and A
refers to the area of the electrode (cm2).
3
rðdielectric constantÞ ¼ C= 0 ꢂ t=A
3
The dielectric constants of the spin-coated films with the TBC
dyes of RC1e3 were less than 7 and satisfactory for industrial
application, owing to the low dielectric character of the dye mol-
ecules. In particular, RC3 showed the lowest dielectric constant of
3.33 at 10 kHz frequency. The dielectric constants of the spin-
coated films decreased as the applied frequency increased due to
the lag in the charge transfer of the dye molecule caused by the
rapid change in the external electronic field [39].
3.3.2. Dielectric properties of spin-coated films for LCD BMs
The synthesized TBC dyes RC1e3 have excellent optical prop-
erties to be applied for LCD BM since the gap between the Soret
band and Q-band is narrow. In recent years, the dielectric proper-
ties of the colorants have been considered very important for
satisfactory application to COA mode, in which CF is patterned right
onto the TFT array. Accordingly, colorants with low dielectric con-
stants such as dyes are under extensive investigation for LCD BMs
since the conventional carbon black-based BM with high dielectric
constant interferes with the electrical signals of TFT [8,9]. The
3.3.3. Thermal stability and FE-SEM images of spin-coated films
The practical thermal stability of spin-coated CFs and BMs needs
to be discussed with color difference (DEab) values as expressed in
Tables 4 and 6 since it is affected by the compatibility with indus-
trial solvents and binders as well as the inherent thermal stability
of dye molecules [10]. The criterion of sufficient thermal stability
for commercial applications was that the
DEab values of the spin-
coated films should be less than 3 after heating for one hour at
230 ꢀC [16]. However, the baking temperature decrease in the LCD
Table 4
Table 5
The coordinate values corresponding to the CIE 1931 chromaticity diagram and the
color difference values of the pigment-based and dye-based color filters.
Dielectric constants of the dye-based black matrices.
Spin-coated dyes t/A Frequency (kHz) Capacitance (pF) Dielectric constant
Spin-coated dyes
Y
x
y
DEab
RC1
RC2
RC3
0.46
0.63
0.44
1
10
1
10
1
0.94
0.88
0.55
0.51
0.72
0.67
4.88
4.57
3.91
3.63
3.58
3.33
RC1
RC2
RC3
60.33
62.84
62.29
60.14
57.86
0.279
0.298
0.275
0.299
0.244
0.516
0.509
0.523
0.519
0.504
4.90
2.95
3.03
1.87
0.85
2b þ 3
Pigment
10

364
J. Choi et al. / Dyes and Pigments 99 (2013) 357e365
Table 6
appeared to be the largest, probably due to the distortion of the
main body and the weak stability of the hydroxyl group. Therefore,
the replacement of such hydroxyl group into the alkoxy group is
believed to make the TBC dye more stable.
The coordinate values corresponding to the CIE 1931 chromaticity diagram and the
color difference values of the spin-coated films in reformed conditions.
Spin-coated dyes
Y
x
y
DEab
RC1a
RC2a
RC3a
RC1b
RC2b
RC3b
60.13
62.79
62.21
60.05
62.27
61.56
0.282
0.294
0.277
0.278
0.293
0.275
0.521
0.510
0.534
0.526
0.521
0.554
3.16
1.75
1.98
3.83
2.66
2.60
Under the most recent LCD manufacturing process baking
condition, the films spin-coated with TBC dyes RC1e3 exhibited
improved thermal stability as shown in condition a of Table 6.
Especially, RC2 and RC3 exhibited sufficient thermal stability for
commercial application. In this condition, the coordinate value y
was somewhat increased and the brightness (Y) was somewhat
decreased due to the reduced dye degradation. The thermal sta-
bility of spin-coated film can also be improved by slowing down the
spin-coating speed. The films spin-coated with RC1e3 exhibited
improved thermal stability under the condition b which had the
reduced rpm (100 rpm for 5 s, 300 rpm for 20 s) of spin-coating as
shown in Table 6. The improvement of thermal stability, namely the
a
Prebaked for 10 min at 90 ꢀC and postbaked for 30 min at 200 ꢀC.
Spin-coated with reduced rpm (100 rpm for 5 s, 300 rpm for 20 s).
b
manufacturing process is easing this criterion for sufficient thermal
stability. Under this criterion, the spin-coated TBC dyes RC1e3
exhibited somewhat less thermal stability compared to the PC dye
or pigment-based CF as shown in Table 4. This is because of the
relatively reduced aggregating tendency of axially substituted dyes
RC1e3, which coincides with the result of solubility discussion.
Such a reduced aggregating tendency of RC1e3 made their particle
size smaller than that of PC dye 2b on the spin-coated films as
illustrated in FE-SEM images in Fig. 6. This smaller dye aggregate
size leads to superior solubility and optical property which are in
a trade-off relationship with thermal stability to some degree. As
reported in a previous paper, the substituents hindering intermo-
decrease of the DEab value, in this condition was contributed to by
the growth of the film thickness resulting from the reduced spin-
coating speed. Such a growth of film thickness raised the coordi-
nate value y, but resulted in reduced brightness (Y).
4. Conclusions
The three novel TBC phosphorus dyes were successfully syn-
thesized and applied for dye-based LCD CF and BM. They were
synthesized via a ring-contraction reaction in which a meso-
nitrogen atom is extruded from an appropriate metal-free PC pre-
cursor. On account of this meso-nitrogen extrusion, the absorption
peaks of the synthesized dyes had a red-shifted Soret band and
a blue-shifted Q-band compared to the PC dyes, which can be
extremely beneficial for application as colorants of green CF and BM
lecular
pep stacked interaction induced smaller dye aggregate
sizes in the FE-SEM images, which in turn reduced the drop of
transmittance when the dye contents were increased [41]. How-
ever, the thermal stability of dyes improves as the dye aggregate
size increases in general [28e30]. As illustrated in Fig. 6, the dye
aggregate size decreased as the axial substituents became bulkier
in the order of RC1 > RC2 > RC3. However, the
DEab value of RC1
Fig. 6. The FE-SEM images of the spin-coated films.

J. Choi et al. / Dyes and Pigments 99 (2013) 357e365
365
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Acknowledgments
This work was supported by the Industrial Strategic Technology
Development Program (No. 10043048, Development of Hybrid Mill
Base and Dyes for Color Filter with High Contrast Ratio and Bright-
ness)funded bythe MinistryofKnowledgeEconomy(MKE) ofKorea.
Appendix A. Supplementary data
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