Brightness enhanced blue to green light color conversion copolymer, alkylfluorene-benzothiadiazole-benzylidenylfluorene

Article abstract of DOI:10.1080/09273948.2017.1338900

A new photoluminescent (PL) polymer, alkylfluorene-benzothiadiazole-benzylidenylfluorene copolymer, was synthesized by a Suzuki coupling reaction, and its optical properties and applicability as a color conversion material were studied. The synthesized PL polymer showed good thermal stability and superior color conversion ability which converts blue light into green. A color conversion film was fabricated using a screen printing method and its optical properties were analyzed after placing the fabricated color-conversion film on a blue light emission device. The measured brightness was 1120 cd/m², which was approximately 346% higher than that measured for the device without a color-conversion film (251 cd/m²).

Full text of DOI:10.1080/09273948.2017.1338900

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Brightness enhanced blue to green light
color conversion copolymer, alkylfluorene-
benzothiadiazole-benzylidenylfluorene
Dong Won Oh, Woo Bung Lee, Heung Jin Choi, Hong Tak Kim, Hyeong-Rag
Lee & Sung-Youp Lee
To cite this article: Dong Won Oh, Woo Bung Lee, Heung Jin Choi, Hong Tak Kim, Hyeong-Rag
Lee & Sung-Youp Lee (2017) Brightness enhanced blue to green light color conversion copolymer,
alkylfluorene-benzothiadiazole-benzylidenylfluorene, Molecular Crystals and Liquid Crystals, 651:1,
91-98, DOI: 10.1080/09273948.2017.1338900
To link to this article: http://dx.doi.org/10.1080/09273948.2017.1338900
Published online: 12 Oct 2017.
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Date: 18 October 2017, At: 04:50

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Brightness enhanced blue to green light color conversion
copolymer, alkylfluorene-benzothiadiazole-
benzylidenylfluorene
Dong Won Oha, Woo Bung Leea, Heung Jin Choib, Hong Tak Kimc, Hyeong-Rag Leec,
and Sung-Youp Leec
aDepartment of Chemistry Education, Kyungpook National University, Daegu, Korea; bDepartment of Applied
Chemistry, Kyungpook National University, Daegu, Korea; cDepartment of Physics, Kyungpook National
University, Daegu, Korea
ABSTRACT
KEYWORDS
Conducting
photoluminescent polymer;
color-conversion material;
self-luminescence
A new photoluminescent (PL) polymer, alkylfluorene-benzothiadiazole-
benzylidenylfluorene copolymer, was synthesized by a Suzuki coupling
reaction, and its optical properties and applicability as a color conver-
sion material were studied. The synthesized PL polymer showed good
thermal stability and superior color conversion ability which converts
blue light into green. A color conversion film was fabricated using a
screen printing method and its optical properties were analyzed after
placing the fabricated color-conversion film on a blue light emission
device. The measured brightness was 1120 cd/m2, which was approxi-
mately 346% higher than that measured for the device without a color-
conversion film (251 cd/m2).
Introduction
An attractive issue in the display field is the low power consumption and stability of the device.
One method to reduce the power consumption and increase the stability of the device is to
increase the stability and efficiency of the material. High-efficiency materials with a good ther-
mal stability are desperately needed. Thus, we report here a color-conversion material with
a better stability and higher efficiency. We synthesized a copolymer and fabricated a color-
conversion film and investigated its physical and optical characteristics.
In recent years, color conversion materials have attracted interest in many fields. Recent
studies of these materials have focused on their applicability in devices, such as organic light-
emitting diode (OLED) and light-emitting diode (LED) [1–4]. The principle of the color con-
version process is that materials release new light through self-luminescence by absorbing
light at certain wavelengths.
Conductive luminescent polymers are very important compounds used in many fields.
The characteristics of electric conductive polymers were first discovered for polyacetylene
in 1977. Subsequently, studies for various types of conductive polymers, such as poly-p-
phenylenevinylene (PPV), polythiophene (PTh), polypyrrole, polyvinylcarbazole (PVK), and
polyalkylfluorene (PAF) derivatives [5–15] were conducted in many areas. PAF derivatives
CONTACT Sung-Youp Lee
Korea.
physylee@knu.ac.kr
Department of Physics, Kyungpook National University, Daegu ,
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

92
D. W. OH ET AL.
have high thermal and chemical stabilities, and a variety of substituents can be introduced at
the C-9 position. These properties provide many advantages, such as increased solubility, easy
processing, and diversity and flexibility of molecular design. Thus, there are currently many
active studies for PAF.
In the previous paper, we have reported PAF derivative as color conversion material [16].
In this study, we have synthesized a new PAF derivative with higher efficiency than previously
reported material. The physical properties of the synthesized polymer were investigated. The
applicability of the synthesized polymer as a color conversion material was also investigated.
Experimental
(1) Synthesis of polymer
Synthetic precursors (2,7-dibromofluorene, benzaldehyde, and 2,1,3-benzothiadiazol),
the monomers (9,9-dioctylfluorene-2,7-bis(trimethyleneborate (1) and 2,7-dibromo-9,9-
dioctylfluorene (2)), and the two end capping reagents (2-thiopheneboronic acid and 2-(4-
bromophenyl)-5-phenyl-1,3,4-oxadiazole) were purchased from Aldrich Chemical Co. and
were used without further purification.
2,7-dibromo-9-benzylidenylfluorene (3): Monomer 3 was synthesized according to a lit-
1
erature procedure [7]. Yield: 90%. mp: 100∼102°C. mass: m/z 412. H NMR (CDCl3, 500
MHz): δ 7.0∼7.8 (m). 13C NMR (CDCl3, 125 MHz): δ 120.7, 120.9, 120.9, 121.2, 123.6, 127.4,
128.7, 128.8, 129.1, 129.9, 131.1, 131.4, 134.4, 135.6, 136.9, 138.0, 139.0, 141.0.
4,7-dibromo-2,1,3-benzothiadiazole (4): Monomer 4 was prepared following the proce-
1
dure described in ref 6. Yield: 89%. mp: 186-187°C. mass: m/z 294. H NMR (CDCl3, 500
MHz): δ 7.73 (s, 2H). 13C NMR (CDCl3, 125 MHz): δ 152.9, 132.4, 113.9.
Alkylfluorene-benzothiadiazole-benzylidenylfluorene copolymer (5): Copolymer (5) was
synthesized by Suzuki cross-coupling reaction. The phase transfer catalyst, Aliquat 336 (0.70
g), tetrakis(triphenylphosphine)palladium (0.02 g) and aqueous sodium carbonate (2 M, 25
mL) were added sequentially to a mixture of monomer 1 (2.50 g, 4.477 mmol), 2 (1.719 g,
3.134 mmol), 3 (0.2768 g, 0.6715 mmol), and 4 (0.1974 g, 0.6715 mmol) in toluene (35 mL)
under nitrogen. The solution was stirred vigorously and heated under gentle reflux for 1 hour
until a viscous reaction mixture was obtained. An additional 10 mL of toluene was added, and
the reaction mixture was stirred at 90°C for 48 hours. Subsequently, the polymer was capped
by adding 2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole (0.05g) and heating for additional
6 hours, followed by the addition of 2-thiophene boronic acid (0.15g) and heating for 6 hours.
The reaction mixture was cooled to about 50°C and poured slowly to a stirred solution con-
taining 500 mL of methanol and 50 mL of deionized water. The polymeric product was col-
lected by filtration and washed with methanol. The crude product was washed with acetone
for 12 hours in a Soxhlet extractor. The polymer was further purified by passing a toluene
solution of the polymer through a column of silica gel and re-precipitated from methanol-
acetone (1:1). The product was obtained as a yellow solid (3.17 g, approximately 93% yield).
1H NMR (CDCl3, 500 MHz): δ 8.02∼7.40 (m), 2.13 (4H, br), 1.25∼1.14 (20H, br), 0.83∼0.76
(10H, br). 13C NMR (CDCl3, 125 MHz): δ 151.8, 140.5, 140.0, 129.5∼127.2, 121.7, 121.5,
120.0, 55.3, 40.4, 31.8, 31.48, 30.0, 29.3, 22.6, 14.1, 14.0.
(2) Fabrication of color-conversion film
The applicability of the synthesized polymer as a color-conversion material was examined
using the following procedure. The solution for color-conversion film was prepared by mixing

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
93
a polyethylene (PE) binder with a 10% toluene solution of copolymer 5 in a weight ratio of 1:1.
The solution for the color-conversion film was printed on a polyethylene terephthalate (PET)
film by screen printing and dried for 3 hours at 60°C. The optical and physical properties of
the color-conversion film were measured using a luminance meter after placing the produced
color filter on a blue light emission device.
(3) Measurements
The polymer molecular weight was determined using a Waters Alliance 2000 gel permeation
chromatography (GPC) instrument calibrated with polystyrene standards. 1H and 13C NMR
spectra of the polymer dissolved in CDCl3 were recorded at 500 MHz on a Bruker Avance
Digital 500 spectrometer. Gas chromatography-mass spectra were obtained using a Shimadzu
Corporation QP-1000A spectrometer. Absorption and PL emission spectra of the polymer
were measured using an Optizen 3220 UV-Vis spectrophotometer (Mecasys) and a Spec-
traPro 2150i spectrometer (Princeton Instruments), respectively. Thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) of the polymer were performed under a
nitrogen atmosphere at a heating rate of 10°C/min using a SETSYS 24 TG-DTA instrument
(SETARAM) and SSC5200H thermal analyzer (Seiko), respectively. Fluorescence lifetime of
copolymer 5 thin film was measured using time-resolved photoluminescence system (KBSI
Daegu Center). Exponential fitting for the obtained fluorescence decays was performed using
the Symphotime software (version 5.3). The CIE coordinates and brightness of the color-
conversion film were determined using a luminance meter (MINOLTA, LS-100).
Results and Discussion
Monomer 3 was synthesized in high yield by the condensation reaction of 2,7-
dibromofluorene with benzaldehyde in a tetrabutylammonium hydroxide solution. The
results of the structural analysis are consistent to the literature data [5]. Monomer 4 was
synthesized by electrophilic aromatic bromination reaction of 2,1,3-benzothiadiazol with
bromine. The structural analysis data are well agreed with literature data [6].
Copolymer 5 was synthesized by Suzuki cross-coupling polymerization from monomer 1,
2, 3, and 4. The synthetic route for copolymer 5 is shown in Fig. 1. Copolymer 5 was char-
1
1
acterized and verified by H NMR and 13C NMR spectroscopy as shown in Fig 2. The H
NMR spectrum showed 8.02∼7.40 ppm for the aromatic phenyl and benzothiadiazole pro-
tons and 2.13∼0.76 ppm for the aliphatic octyl protons. From the 13C NMR spectra, signals
were observed at 152∼119 ppm for the aromatic phenyl and benzothiadiazole carbon atoms,
40∼14 ppm for the aliphatic octyl groups, and 55 ppm for the C-9 atoms of the fluorene ring.
The molecular weight of copolymer 5 was determined by gel permeation chromatography
(GPC) using a polystyrene standards. Copolymer 5 exhibited the weight-average molecular
weight (Mw) of 65,227, number-average molecular weight (Mn) of 28,229, and polydispersity
index (PDI) of 2.31. The thermal properties of copolymer 5 were measured by thermogravi-
metric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmo-
sphere at a heating rate of 10°C/min. The glass transition temperature (Tg) of copolymer 5 was
approximately 111°C, which is higher than that of the poly(9,9-disubstitutedfluorene) deriva-
tives (∼55°C) [8]. The excellent thermal stability of the resulting polymer was determined by
TGA (Fig. 3 (a)). The decomposition temperature of polymer 5 was higher than 400°C. The
initial weight loss (4.6%) was observed around 417°C, with the main decomposition occur-
ring around 574°C (45.1%).

94
D. W. OH ET AL.
Figure . Synthetic route of copolymer 5.
The optical band gap energy of thin film was calculated by Tauc’s equation as follow [17];
(αhν)2 = hν − Eg
(1)
where α is the absorption coefficient, hν is the photon energy and Eg is the band gap energy.
As shown in Fig. 3 (b), the optical band gap could be estimated by plot of (αhν)2 vs. photon
energy using optical absorption results. From extrapolating the straight part of this plot, the
optical band gap could be determined. As a result, the optical band gap of copolymer 5 was
approximately 2.49 eV.
The optical properties of copolymer 5 were analyzed by UV-Vis and PL spectroscopy.
Fig. 3 (c) shows the absorbance and emission spectra of copolymer 5, which was measured
in methylene chloride at room temperature. The main absorption peak of copolymer 5 was
Figure . NMR spectra of the copolymer 5; (a) C NMR, (b) H NMR.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
95
Figure . Physical properties of copolymer 5; (a) TGA thermogram, (b) Optical band gap, (c) UV-Vis absorp-
tion and photoluminescence spectra, (d) Normalized photoluminescence decay.
observed at 368 nm, with a shoulder at 391 nm. Those are attributed to π-π∗ transition of
polymer backbone. The PL spectrum of copolymer 5 in the thin film was recorded by excita-
tion of the 325 nm line of a He-Cd laser. The maximum emission band at 547 nm indicated
that the synthesized polymer emitted yellowish-green light.
The time resolved photoluminescence of the copolymer 5 film is shown in Fig. 3 (d). The
gray line was the experimental data and red line was the theoretical fitting curve based on
exponential function. The copolymer 5 showed single exponential decay with fluorescence
lifetime as 1.86 ns. In PL results, it seemed that the peak could be classified some peaks. How-
ever, the single decay behavior was measured in time-resolved PL. It is conjectured that the
single main peak effects on measurement of decay time very strongly.
The applicability of the synthesized polymer as a color-conversion material was also exam-
ined. A color-conversion film solution was prepared by mixing the PE binder with a 10%
toluene solution of copolymer 5 in a weight of 1:1. The PE binder was employed to improve
the stability of the color-conversion film and the adhesion with the PET film. Additionally,
it could improve the color purity of the color-conversion film by cutting-off the blue light of
backlight. The color-conversion film was fabricated by printing a color-conversion film solu-
tion onto a PET film using a screen printing method. A light emission device was fabricated
by installing a diffuser plate on top of a LED lamp of the six types for emitting blue light from
380 nm to 430 nm. The optical properties of the color-conversion film were measured using
a luminance meter after placing the fabricated color-conversion film on top of the blue light
emission device.
The general color filter absorbs certain wavelengths of light and allows only a portion of
the visible light to pass. Therefore, the color filter blocks a considerable amount of light to
present a much lower brightness than the original brightness of the backlight. However, the
color-conversion film fabricated in this study presented a higher brightness than the original

96
D. W. OH ET AL.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
97
brightness of the backlight. Table 1 lists the optical properties of the color conversion film
during color conversion.
In the CIE coordinate analysis, the x values are about 0.33 and y values are about 0.60.
This result shows that the material synthesized in this study emits yellowish-green light by
absorbing the blue light released from the device. The synthesized PL polymer showed supe-
rior color-conversion ability. The brightness increased in all six types of the blue light emission
devices. The rate of the increase in brightness was about 18% to 346%.
The light transmittance of the color conversion film was approximately 8% above 500 nm.
Transmittance refers to the value of color-conversion film measured by a UV-Vis spectropho-
tometer. This measured value shows that the brightness must have decreased by approximately
92% compared to that of the backlight. However, the color-conversion film fabricated in this
study presented higher brightness values than the original brightness of the backlight. In par-
ticular, the luminance using a color-conversion film fabricated in this study was approximately
346% higher than that measured for the device without a color-conversion film for the device
that emits blue light in the range of 420-430 nm.
Summary
A new PL polymer was synthesized by a Suzuki coupling reaction, and its optical properties
and applicability as a color-conversion material were studied. The synthesized PL polymer
showed a good thermal stability and superior color-conversion ability. Also, it had optical
bandgap of 2.49 eV, and fluorescence decay time of 1.86 ns. The brightness of the blue light
emission device without the color-conversion film was 251 cd/m2 at 420-430 nm. This was
significantly lower than 1120 cd/m2, which was observed when the color-conversion film was
placed on top of the device. This 346% increase was attributed to the ability of the organic
polymer on top of the blue light emission device to absorb the blue light released from the
device and emit yellowish-green light through color conversion. If the material is applied to
the display field, it can provide a high luminance with low power consumption. Thus, the
synthesized polymer is expected to be used as a material to increase the efficiency of general
color filters and displays.
Funding
This research was supported by Basic Science Research Program through the National Research Foun-
dation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1D1A1A01060483).
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