Synthesis and photoluminescent properties of red phosphorescent random copolymers with photopatternable moiety

Article abstract of DOI:10.1080/15421400701497597

Two types of photopatternable blue emitting random copolymers, poly(VK-ran-CEMA) (1) and poly(VK-ran-CEMA-ran-MAE) (2), containing UV curable cinnamoyl moieties capable of generating negative type patterned images were synthesized and characterized. In addition, their potential possibility to be used as a photopatternable polymeric host matrix for a red phosphorescent dopant was studied by using UV-Visible absorption and PL emission spectroscopy. The λmax,PL value of poly(VK-ran-CEMA) (1) doped with a red phosphorescent dopant, (bsn)2Ir(acac), was shifted from 343 nm of poly(VK-ran-CEMA) (1) to 610 nm, with increasing the content of (bsn)2Ir(acac). Poly(VK-ran-CEMA-ran-(bsn) 2IrMAE) (3), prepared by the reaction of [(bsn)2IrCl]2 with poly(VK-ran-CEMA- ran-MAE) (2), exhibited max,PL at 610 nm in the range of red emission. These results implied efficient energy transfer from photopatternable blue emitting random copolymers to a red phosphorescent dopant. It was also found upon UV exposure on the synthesized copolymers that negative patterned images with high resolution were produced by standard photolithographic process.

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Synthesis and
Photoluminescent Properties
of Red Phosphorescent
Random Copolymers with
Photopatternable Moiety
a
a
Minhyeon Song , Kyung-Hoe Kim , Younghwan
a
b
Kwon & Yoon Soo Han
a
Department of Chemical Engineering , Daegu
University , Gyeongsan, Korea
b
Display and Nano Devices Laboratory, DGIST ,
Daegu, Korea
Published online: 22 Sep 2010.
To cite this article: Minhyeon Song , Kyung-Hoe Kim , Younghwan Kwon & Yoon Soo
Han (2007) Synthesis and Photoluminescent Properties of Red Phosphorescent Random
Copolymers with Photopatternable Moiety, Molecular Crystals and Liquid Crystals,
470:1, 315-328, DOI: 10.1080/15421400701497597
To link to this article: http://dx.doi.org/10.1080/15421400701497597
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Mol. Cryst. Liq. Cryst., Vol. 470, pp. 315–328, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701497597
Synthesis and Photoluminescent Properties
of Red Phosphorescent Random Copolymers
with Photopatternable Moiety
Minhyeon Song
Kyung-Hoe Kim
Younghwan Kwon
Department of Chemical Engineering, Daegu University, Gyeongsan,
Korea
Yoon Soo Han
Display and Nano Devices Laboratory, DGIST, Daegu, Korea
Two types of photopatternable blue emitting random copolymers, poly(VK-ran-
CEMA) (1) and poly(VK-ran-CEMA-ran-MAE) (2), containing UV curable
cinnamoyl moieties capable of generating negative type patterned images were
synthesized and characterized. In addition, their potential possibility to be used
as a photopatternable polymeric host matrix for a red phosphorescent dopant
was studied by using UV-Visible absorption and PL emission spectroscopy. The
k
max,PL value of poly(VK-ran-CEMA) (1) doped with a red phosphorescent dopant,
bsn) Ir(acac), was shifted from 343 nm of poly(VK-ran-CEMA) (1) to 610 nm, with
increasing the content of (bsn) Ir(acac). Poly(VK-ran-CEMA-ran-(bsn) IrMAE) (3),
prepared by the reaction of [(bsn) IrCl] with poly(VK-ran-CEMA-ran-MAE) (2),
(
2
2
2
2
2
exhibited kmax,PL at 610 nm in the range of red emission. These results implied
efficient energy transfer from photopatternable blue emitting random copolymers
to a red phosphorescent dopant. It was also found upon UV exposure on the synthe-
sized copolymers that negative patterned images with high resolution were
produced by standard photolithographic process.
Keywords: phosphorescence;
photolithographic
process;
photoluminescence,
photopatternable polymer
This research was supported in part by Daegu University Research Grant, 2006.
Address correspondence to Younghwan Kwon, Department of Chemical Engineering,
Daegu University, Gyeongsan, Gyeongbuk 712-714, Korea. E-mail: y_kwon@daegu.ac.kr
315

3
16
M. Song et al.
INTRODUCTION
Polymer light-emitting diodes (PLEDs) have the advantage of facile
processability, high flexibility and thermal stability. Polymers also
have lower production costs and can afford uniform films with large
flat panel areas due to inexpensive solution processing, such as spin
coating process [1–4]. Since one of recent interests in the field of
PLEDs has been directed to the realization of full color flat displays,
patterned integration of red, green, and blue microstructures with fea-
ture sizes less than 100 mm on the same substrate is required for these
practical applications. Current trends in the spatial deposition of these
materials focus on area-selected electropolymerization [5], photo-
chemical patterning [6], and nonreactive techniques such as screen
printing [7], inkjet printing [8] and microcontact printing [9].
Recently, photochemical cross-linking to produce insoluble polymer
networks [10] and laser-induced thermal imaging [11] were success-
fully used in manufacturing devices. Lithographic patterning by
photochemical cross-linking of EL polymers has been still considered
as a simple and reliable chemical technique, which is compatible with
conventional semiconductor photolithography [12–14]. A drawback of
PLEDs was their limited quantum efficiency. It has been demon-
strated that electrophosphorescent PLEDs are capable of overcoming
the limitation. In such PLEDs, a phosphorescent heavy metal complex
is doped into a polymeric host material. Energy transfer can be
achieved by selectively doping phosphorescent dopant, which is favor-
able for full color display application.
In this study, PVK-based blue-emitting photopatternable random
copolymers functionalized with cinnamoyl groups were synthesized
and characterized. By photoinduced cross-linking and insolubilization
upon exposure to UV-Visible light, the pattern with high resolution
was achieved by dissolution of unexposed regions, resulting in nega-
tive patterned images. In addition, their potential possibility to be
used as a photopatternable polymeric host matrix for a red phosphor-
escent dopant was also studied.
EXPERIMENTAL
Materials
N-Vinyl carbazole (98%), cinnamoyl chloride (98%), 2-(methacryloy-
loxy)ethyl acetoacetate (MAE, 95%) and hydroxyethyl methacrylate
(
HEMA, 98%) were purchased from Aldrich Chemical Co. Triethyl-
amine (TEA, 99%) and 2,2-azobisisobutyronitrile (AIBN, 98%) were
obtained from Junsei Co. Phosphorescent dopant, (bsn) Ir(acac), was
2

Synthesis and Photoluminescent Properties
317
synthesized by following the literature previously reported [15].
Solvents such as tetrahydrofuran (THF) and dichloromethane were
purified prior to use. All other chemicals were reagent-grades and
were used as received.
Synthesis
The structure of photopatternable poly(VK-ran-CEMA) (1), used as a
host matrix for a red phosphorescent dopant , (bsn) I(acac), is shown
2
in Scheme 1 [16].
The synthetic route to poly(VK-ran-CEMA-ran-MAE) (2) is pre-
sented in Scheme 2.
Cinnamoyl ethyl methacrylate (CEMA). Cinnamoyl chloride
ꢁ
ꢁ2
2
ꢁ2
(
(
2.468 g, 1.54 ꢀ 10 mol), HEMA (2.000 g, 1.54 ꢀ 10 mol) and TEA
1.552 g, 1.54 ꢀ 10 mol) were dissolved in 40 mL of dichloromethane
under nitrogen atmosphere with an ice bath. After removing an ice
bath, the reaction was carried out at room temperature for 12 hrs.
ꢁ2
CEMA (3.00 g, 1.18 ꢀ 10 mol) was obtained through silicagel column
chromatography (n-hexane=ethyl acetate ¼ 9=1 v=v). (Yield ¼ 77%).
1
H NMR (CDCl ), d: 7.71 (d, 1H, ꢁCH ¼), 7.54 (s, 2H, Ar CH), 7.39
3
SCHEME 1 Chemical structures of (a) poly(VK-ran-CEMA) and (b)
(
2
bsn) Ir(acac).

3
18
M. Song et al.
SCHEME 2 Synthetic route to poly(VK-ran-CEMA-ran-MAE) (2).
(
ꢁ
s, 3H, Ar CH), 6.46 (d, 1H, =CHꢁAr), 6.16 (s, 1H, ꢁCHꢁ), 5.60 (s, 1H,
CHꢁ), 4.46 (t, 4H, ꢁCH ꢁCH ꢁ), 1.96 (s, 3H, ꢁCH ).
2
2
3
Poly(VK-ran-CEMA-ran-MAE) (2). N-Vinyl carbazole (2.00 g, 1.04 ꢀ
ꢁ
2
ꢁ2
1
1
0
mol), CEMA (0.33 g, 0.13 ꢀ 10 mol) and MAE (0.28 g, 0.13 ꢀ
ꢁ
2
0
mol) was dissolved by stirring in 45 mL of THF under nitrogen
ꢂ
ꢂ
atmosphere, and then temperature was set to 60 C. At 60 C of reaction
ꢁ4
temperature, AIBN (0.128 g, 7.80 ꢀ 10 mol) was added. The polymer-
ization was carried out with stirring for 12 h. The resulting mixture
was poured into methanol, filtered, and then dried in vacuum for

Synthesis and Photoluminescent Properties
319
2
SCHEME 3 Synthetic route to poly(VK-ran-CEMA-ran-(bsn) IrMAE) (3).
ꢂ
1
2 h at 50 C. Two compositions of poly(VK-ran-CEMA-ran-MAE) were
prepared with the same procedure mentioned above.
Photopatternable poly(VK-ran-CEMA-ran-(bsn) IrMAE) (3) com-
2
plexed with red phosphorescent Ir-containing side group was prepared
as can be seen in Scheme 3.
Poly(VK-ran-CEMA-ran-(bsn) IrMAE) (3). [(bsn) IrCl] was synthe-
2
2
2
sized by following the literature previously reported [15]. Poly(VK-
ran-CEMA-ran-MAE) (0.5 g, 0.036 mmol) and [(bsn) IrCl]2 (0.054 g,
2
0
.036 mmol) and sodium carbonate (2 mg, 0.018 mmole) was added

3
20
M. Song et al.
in dichloromethane under nitrogen atmosphere and refluxed for
2 hrs. After reaction, the reaction mixture was cooled down to room
7
temperature and precipitated into methanol. After filtration, washing
with methanol several times and vacuum drying, poly(VK-ran-
CEMA-ran-(bsn) IrMAE) (3) was obtained.
2
Instruments
1
H NMR spectra of the synthesized copolymers were measured by
using Varian 300 MHz spectroscopy (Mercury 300) using CDCl or
3
DMSO-d (Merck Co.) as a solvent. The number- and weight-average
6
molecular weights were measured with a Waters gel permeation chro-
matography (GPC) system equipped with a 410 differential refrac-
tometer, a 510 HPLC pump and a Styragel HR 5E column with THF
as an eluent in 1 mL=min of flow rate against polystyrene standards
at room temperature. Ultraviolet-visible (UV-Vis) absorption spectra
of the copolymers were obtained with Shimadzu UV-2100. The photo-
luminescence (PL) spectra of the copolymers excited by He-Cd laser at
3
15 nm were monitored with an optical multichannel analyzer (Laser
Photonics, OMA system).
RESULTS AND DISCUSSION
Synthesis and Characterization of Photopatternable
Random Copolymers
A series of PVK-based photopatternable random copolymers, poly(VK-
ran-CEMA) (1) and poly(VK-ran-CEMA-ran-MAE) (2), containing
cinnamoyl moieties as side chains were prepared. In the case of
poly(VK-ran-CEMA) (1) [16], poly(VK-ran-HEMA) was first prepared
by radical polymerization of VK and HEMA, and then reacted with
cinnamoyl chloride in the presence of TEA for 12 h at room tempera-
ture. On the other hand, poly(VK-ran-CEMA-ran-MAE) (2) was pre-
pared by radical polymerization of VK, CEMA and MAE, in which
CEMA monomer was synthesized by the reaction of HEMA and cinna-
moyl chloride before the polymerization.
Results for molecular weights, polydispersity indices [weight-aver-
age molecular weight=number-average molecular weight (M =M )],
w
n
and compositions of patternable random copolymers are summarized
in Table 1. The number-average molecular weights (M ’s) of random
n
copolymers determined by GPC with THF as an eluent were in the
range of 2,800–4,170 g=mol with a polydispersity index of 1.49–2.05.
1
Compositions of poly(VK-ran-CEMA) (1) were determined from
H
NMR spectra using the integration values of the aromatic hydrogens

Synthesis and Photoluminescent Properties
321
TABLE 1 Compositions and Molecular Weights of Patternable Random
Copolymers
Composition of VK : CEMA : MAE
(
mole ratio)
GPC
Polymers
Monomer
Polymer
M
n
w n
M =M
Poly(VK-ran-CEMA) 1
Poly(VK-ran-CEMA) 2
Poly(VK-ran-CEMA) 3
Poly(VK-ran-CEMA) 4
Poly(VK-ran-CEMA) 5
Poly(VK-ran-CEMA-ran-MAE) 1
Poly(VK-ran-CEMA-ran-MAE) 2
20 : 80 : 0
40 : 60 : 0
50 : 50 : 0
60 : 40 : 0
80 : 20 : 0
80 : 10 : 10
70 : 20 : 10
14.8 : 85.2 : 0
24.5 : 75.5 : 0
38.4 : 61.6 : 0
49.5 : 50.5 : 0
66.3 : 33.7 : 0
65 : 15 : 20
3,390
4,110
3,030
3,710
2,800
4,170
3,240
1.61
1.70
1.79
2.05
1.49
1.85
1.74
50 : 20 : 30
(
d ¼ 8.05–7.12 ppm) of PVK and the methylene peaks (ꢁOꢁCH ꢁ,
2
d ¼ 3.71–4.50 ppm) of poly(CEMA). Compositions of poly(VK-ran-
1
CEMA-ran-MAE) (2) were also calculated from H NMR spectra
(
Fig. 1) by comparing the integration values of the aromatic hydro-
gen’s (d ¼ 8.05–7.12 ppm) of PVK, the methylene peak (ꢁOꢁCH ꢁ,
2
d ¼ 3.71–4.50 ppm) of poly(CEMA) and the methylene peak
(
ꢁCOꢁCH ꢁCOꢁ, d ¼ 3.40–3.71 ppm) of poly(MAE), respectively. It
2
was observed from Table 1 that carbazole content in copolymers
1
FIGURE 1 H NMR spectrum of poly(VK-ran-CEMA-ran-MAE) (2).

3
22
M. Song et al.
was lower than that in monomer feed. The relatively lower carbazole
content in copolymers might be caused by lower reactivity of N-vinyl
carbazole than that of other methacrylate monomers.
Photoluminescent Properties of Photopatternable
Random Copolymers
UV-Visible absorption and PL spectra obtained from the solution of
poly(VK-ran-CEMA-ran-MAE) (2) in THF are shown in Figures 2 (a)
and (b), respectively. UV-Visible absorption maxima (kmax,UV) were
measured at 343 nm, and PL maxima (kmax,PL) was found at
3
70 nm in the range of blue emission, regardless of the compositions
of the copolymers. It was good in agreement with previous report
17] that kmax,UV of PVK were measured at 330 nm and 344 nm.
[
kmax,PL value of PVK was observed at 367 nm, when excited at its
own kmax,UV. As presented from previous study [16], kmax,UV and
kmax,PL of poly(VK-ran-CEMA) (1) were reported at 343 nm and
3
70 nm, respectively. Therefore, two types of photopatternable ran-
dom copolymers ((1) and (2)) with blue emission in PL spectra could
be used as polymeric host matrix for phosphorescent dopants in
PLEDs.
UV-Visible absorption and PL emission spectra of poly(VK-
ran-CEMA) (1) doped with a red phosphorescent dopant, (bsn) Ir(acac),
2
were measured as a function of the content of dopant, as can be
seen in Figure 3 (a) and (b). Poly(VK-ran-CEMA) 5 with the highest
content of VK, presented in Table 1, was used as the host matrix.
As the content of (bsn) Ir(acac) dopant was increased in the
2
(
bsn) Ir(acac)=poly(VK-ran-CEMA) 5 (1) mixture solution, PL inten-
2
sity of (bsn) Ir(acac) dopant at 610 nm steadily increased while PL
2
intensity of poly(VK-ran-CEMA) 5 (1) decreased, suggesting energy
transfer from poly(VK-ran-CEMA) (1) host matrix to (bsn) Ir(acac)
2
dopant.
UV-Visible absorption and PL emission spectra of poly(VK-
ran-CEMA-ran-(bsn) IrMAE) (3) complexed with red phosphorescent
2
Ir-containing side group were also measured, as presented in Figure 4
(
a) and (b). Two different compositions of poly(VK-ran-CEMA-
ran-(bsn) IrMAE) (3) were prepared by post reaction of poly(VK-
2
ran-CEMA-ran-MAE) 1 and poly(VK-ran-CEMA-ran-MAE) 2 (2),
respectively, with a red phosphorescent precursor, [(bsn) IrCl] ,
2
2
resulting in the incorporation of a red phosphorescent dopant in the
polymer side chain. kmax,UV values of two poly(VK-ran-CEMA-ran-
(
bsn) IrMAE) (3) were measured at 343 nm, identical to that of
2

Synthesis and Photoluminescent Properties
323
FIGURE 2 (a) UV-Visible absorption and (b) PL emission spectra of poly-
VK-ran-CEMA-ran-MAE) (2); (1) poly(VK-ran-CEMA-ran-MAE) 1 and (2)
(
poly(VK-ran-CEMA-ran-MAE) 2. (See COLOR PLATE XIV)
poly(VK-ran-CEMA-ran-MAE) (2). However, poly(VK-ran-CEMA-ran-
(
bsn) IrMAE) (3) exhibited k
at 610 nm in the range of red emis-
max,PL
2
sion from the phosphorescent dopant. This also implies efficient energy
transfer within poly(VK-ran-CEMA-ran-(bsn) IrMAE) (3) complexed
2
with red phosphorescent Ir-containing side group.

3
24
M. Song et al.
FIGURE 3 (a) UV-Visible absorption and (b) PL emission spectra of
(
2 2
bsn) Ir(acac)=poly(VK-ran-CEMA) 5 (2); contents of (bsn) Ir(acac) (1) 0 wt%,
(
2) 1 wt%, (3) 3 wt%, (4) 5 wt%, and (5) 10 wt%. (See COLOR PLATE XV)
Formation of Patterns by Photolithographic Process
Solution (1 wt%) of photopatternable random copolymers ((1) and (2))
was prepared by dissolving in THF. The solution was filtered through
a 0.5 mm PTFE membrane filter (Advantec Mfs, Inc.), and then spin-
coated with a spin-coating device (SC-300, E. H. Co., Japan) on a

Synthesis and Photoluminescent Properties
325
FIGURE 4 (a) UV-Visible absorption and (b) PL emission spectra of (1)
poly(VK-ran-CEMA-ran-(bsn) IrMAE) 1 and (2) poly(VK-ran-CEMA-ran-
2
(
2
bsn) IrMAE) 2, prepared from the corresponding poly(VK-ran-CEMA-ran-
MAE) 1 and poly(VK-ran-CEMA-ran-MAE) 2, respectively. (See COLOR
PLATE XVI)
cleaned glass substrate at a series of rotation speeds, 500 rpm for 5 s
and then 2500 rpm for 25 s. The resulting film was exposed to UV
2
at 365 nm (intensity of 2,000 mJ=cm ) through a photomask in a
contact mode.

3
26
M. Song et al.
FIGURE 5 Optical microscopic photographs of (a) poly(VK-ran-CEMA) and
(
b) poly(VK-ran-CEMA-ran-MAE). (line width ¼ 40 mm)
The optical photographs of poly(VK-ran-CEMA) (1) containing
3
3.7 mol% of cinnamoyl photosensitive group and poly(VK-
ran-CEMA-ran-MAE) (2) were presented in Figure 5 (a) and (b),
respectively, as examples of the patterned images formed by conven-
tional lithographic process. Since the UV exposed area became insolu-
ble, the patterned image was obtained upon dissolution of unexposed
area with acetone. Therefore, it was found that negative stripe
patterns were obtained and the fine lines with 40 mm of width were

Synthesis and Photoluminescent Properties
327
uniform with smooth surfaces. This observation may provide the possi-
bility that photopatternable random copolymers can be used as blue
emitting host matrix materials in conjunction with a photolitho-
graphic process.
CONCLUSIONS
We have synthesized and characterized two different types of blue
emitting PVK-based random copolymers, poly(VK-ran-CEMA) (1) and
poly(VK-ran-CEMA-ran-MAE) (2), functionalized with photopattern-
able cinnamoyl groups. From the study on photoluminescent proper-
ties, it was found that these copolymers emitted kmax,PL at 343 nm in
the range of blue emission. The kmax,PL value of poly(VK-ran-CEMA)
(
1) doped with a red phosphorescent dopant, (bsn) Ir(acac), was shifted
2
from 343 nm of poly(VK-ran-CEMA) (1) to 610 nm, with increasing the
content of (bsn) Ir(acac). Poly(VK-ran-CEMA-ran-(bsn) IrMAE) (3),
2
2
prepared by the reaction of [(bsn) IrCl] with poly(VK-ran-CEMA-
2
2
ran-MAE) (2), exhibited kmax,PL at 610 nm in the range of red emission.
This energy transfer from photopatternable blue emitting random
copolymers to a red phosphorescent dopant showed their potential
possibility to be used as a photopatternable polymeric host matrix for
phosphorescent PLED devices. By photoinduced cross-linking and
insolubilization upon UV exposure, the stripe patterns with 40 mm of
line width were achieved by developing of unexposed regions, resulting
in negative patterned images. The combination of this photolitho-
graphic method and the photoluminescent results could offer possi-
bility for full color electrophosphorescence PLEDs.
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