Synthesis and characterization of di-hexyl-fluorene and triphenylamine based copolymers containing 1,3,4-oxadiazole pendants

Article abstract of DOI:10.1080/15421406.2011.563673

π-Conjugated polymers, poly(N-(4-(5-(4-tert-butylphenyl)-1,3,4- oxadiazol-2-yl)phenyl)-4-(9,9-dihexyl-9H-fluoren-2-yl)-N-phenyl-benzenamine (TPAOXDPF) and poly(N-(4-butylphenyl)-4-(9,9-dihexyl-9H-fluoren-2-yl)-N- phenylbenzenamine) (TPAPF), were synthesiz

Full text of DOI:10.1080/15421406.2011.563673

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Synthesis and Characterization of Di-
hexyl-fluorene and Triphenylamine
Based Copolymers Containing 1,3,4-
Oxadiazole Pendants
a
a
a
a
Young Sue Park , Sang Jun Park , Woong Shin , Bong Lee ,
b
a
Bumsuk Jung & Joo Hyun Kim
a
Department of Polymer Engineering , Pukyong National University,
Yongdang-Dong , Nam-Gu, Busan, Korea
b
Department of Environmental Engineering and Biotechnology ,
Myongji University, San Nam-dong , Yongin, Kyunggi-do, Korea
Published online: 16 May 2011.
To cite this article: Young Sue Park , Sang Jun Park , Woong Shin , Bong Lee , Bumsuk Jung & Joo
Hyun Kim (2011) Synthesis and Characterization of Di-hexyl-fluorene and Triphenylamine Based
Copolymers Containing 1,3,4-Oxadiazole Pendants, Molecular Crystals and Liquid Crystals, 538:1,
118-126, DOI: 10.1080/15421406.2011.563673
To link to this article: http://dx.doi.org/10.1080/15421406.2011.563673
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Mol. Cryst. Liq. Cryst., Vol. 538: pp. 118–126, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.563673
Synthesis and Characterization of Di-hexyl-
fluorene and Triphenylamine Based Copolymers
Containing 1,3,4-Oxadiazole Pendants
1
YOUNG SUE PARK, SANG JUN PARK, WOONG
1
1
SHIN, BONG LEE, BUMSUK JUNG, AND JOO
1
2
1
HYUN KIM
1
Department of Polymer Engineering, Pukyong National University,
Yongdang-Dong, Nam-Gu, Busan, Korea
2
Department of Environmental Engineering and Biotechnology, Myongji
University, San Nam-dong, Yongin, Kyunggi-do, Korea
p-Conjugated polymers, poly(N-(4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)-
phenyl)-4-(9,9-dihexyl-9H-fluoren-2-yl)-N-phenyl-benzenamine (TPAOXDPF)
and
poly(N-(4-butylphenyl)-4-(9,9-dihexyl-9H-fluoren-2-yl)-N-phenylbenzena-
mine) (TPAPF), were synthesized for light emitting diode application. The electron
transporting unit, conjugated 1,3,4-oxadiazole, is attached on the main chain as a
ꢀ
pendant. The copolymers were thermally stable up to 393 C. The electroluminescent
(EL) device based on TPAOXDPF (ITO=TPAOXDPF=Al) has an efficiency of
0.018 cd=A, which is significantly higher than that of the device based on TPAPF
(ITO=TPAPF=Al) (0.008 cd=A). This is due to that 1,3,4-oxadiazole pendants
improve the electron transporting properties in the emissive layer.
Keywords Dihexylfluorene; 1,3,4-oxadiazole; polymer light emitting diode;
triphenylamine
Introduction
Polymer light emitting diodes (PLEDs) have made dramatic progress in comparison
to LEDs based on inorganic materials over the past years [1]. The electroluminescent
(EL) device has the potential to provide an innovative low cost technology for back-
light, illumination, and display application [2–6]. For the device efficiency point of
view, balanced charge injection and transport are essential to get high external quan-
tum efficiency (EQE) in PLED. Small molecules or polymers containing triphenyl-
amine (TPA) units are one of the most widely used hole-transport materials
because they are easily oxidized to form stable radical cation and relatively high-
lying ionization potential [7]. 1,3,4-Oxadiazole (OXD) was aroused special interest
in recent years due to its high electron affinity and good thermal stability. It has
Address correspondence to Joo Hyun Kim, Department of Polymer Engineering,
Pukyong National University, Yongdang-Dong, Nam-Gu, Busan, Korea. Tel.: 82-51-629-
6452; Fax: 82-51-629-6429; E-mail: jkim@pknu.ac.kr
118

Synthesis and Characterization of Di-hexyl-fluorene and Triphenylamine 119
been successfully incorporated into polymers to apply as the electron-transport
materials in multilayer PLEDs. The attempts to simultaneously incorporate TPA
and OXD into small molecules have been proven to be an effective way to obtain
materials with high luminescent efficiency [8–13]. This study reports the preparation
and characterization of a series of polyfluorene (PF) derivatives with TPA segment
in polymers backbones and OXD group as a pendant to polymer main chain. The
OXD moieties were linked to the phenyl rings of TPA so that the electron affinity
of the polymer would be improved. Therefore, improvement of electron affinity
helps recombination in the emissive layer. We also synthesized poly[(N-(4-
butylphenyl)-4-(9,9-dihexyl-9H-fluoren-2-yl)-N-phenylbenzen-amine)] (TPAPF) to
compare the optical, electrochemical, and electroluminescent properties with
TPAOXDPF. We investigated the fundamental optical, electrochemical and electro-
luminescent properties as well as the role of OXD in the improvement of EL
efficiency of the polymer.
Experimental
Materials
All chemicals were purchased from Aldrich Chemical Co. or Tokyo Chemical Indus-
try (TCI) Chemical and used as received. Tetrahydrofuran (THF) and dichloro-
methane (DCM) were distilled from sodium hydride.
Synthesis of 4-Diphenylamino-benzonitrile (1)
A portion of 1.99 g (82.8 mmol) of NaH in 80 mL of anhydrous N,N-dimethylfor-
ꢀ
mamide (DMF) was stirred for 30 min at 0 C. A portion of 12.74 g (75.3 mmol) of
diphenylamine and 10.03 g (82.8 mmol) of 4-fluorobenzonitrile was slowly added
into a slurry of NaH in DMF under nitrogen gas atmosphere. After completion
of addition of diphenylamine and 4-fluorobenzonitrile, the reaction mixture was
ꢀ
stirred at 170 C for 15 hours. The reaction mixture cooled down to room tempera-
ture, 100 mL of water was added into the reaction mixture then extracted with
1
00 mL of DCM three times. The combined organic layer was dried over anhy-
drous magnesium sulfate then the solvent was removed using a rotary evaporator
under reduced pressure. The crude product was purified by flash chromatography
using n-hexane:ethyl actate (EA) (4:1). The white solid product was 14.5 g (64.1%).
þ
ꢀ
1
MS [M ], m=z 300. mp: 157ꢁ159 C. H-NMR (400 MHz, CDCl , ppm): d 7.91
3
1
3
d, 2H), 7.42 (d, 4H), 7.15 (m, 4H), 7.12 (d, 4H). C-NMR (100 MHz, CDCl ,
(
3
ppm): d 151.81, 146.18, 133.35, 129.97, 126.36, 125.33, 119.91, 119.81, 102,77.
Anal. Calcd. for C H N : C, 84.42; H, 5.22; N, 10.36. Found: C, 84.76; H,
.02; N, 10.28.
1
5
14
2
5
Synthesis of Diphenyl-[4-(2H-tetrazol-5-yl)-phenyl]-amine (2)
A portion of 10.0 g (37.0 mmol) of compound 1, 2.89 g (44.4 mmol) of NaN and
3
2
.37 g (44.4 mmol) of NH Cl was added into a 50 mL of DMF. The reaction mix-
4
ꢀ
ture was stirred at 120 C for 24 hours. The reaction mixture cooled down to room
temperature then neutralized with 5% HCl. The reaction mixture poured to
1
000 mL of water then the precipitate was filtered. The crude precipitate was

1
20
Y. S. Park et al.
purified by recrystallization from methanol. The white solid product yield was
þ
ꢀ
1
8
.12 g (76.3%). MS: [M ], m=z 319. mp: 221ꢁ224 C. H-NMR (400 MHz,
DMSO-d , ppm): d 7.36 (d, 2H), 7.14 (t, 4H), 6.86 (m, 2H), 6.73 (d, 4H), 6.64
d, 2H). C-NMR (100 MHz, DMSO-d , ppm): d 161.75, 146.25, 129.79, 128.15,
6
1
3
(
6
1
2
25.25, 124.33, 121.04. Anal. Calcd. for C H N5 S: C, 72.83; H, 4.82; N,
19 15
2.35; S. Found: C, 72.65; H, 4.55; N, 22.23.
Synthesis of f4-[5-(4-tert-Butyl-benzyl)-[1,3,4]oxadiazol-2-ylmethyl]-phenylg-
diphenylamine (3)
A mixture of 10.00 g (31.91 mmol) and compound 2, 7.53 g (38.29 mmol) of
4
-tert-butylbenzoyl chloride of pyridine was stirred for 5 hours at room temperature.
A portion of 100 mL of water was added into the reaction mixture then the precipi-
tate was filtered. The crude precipitate was purified by recrystallization from meth-
þ
anol. The white product yield was 14.0 g (82.1%). MS: [M ], m=z 493. mp:
ꢀ
13–315 C. H-NMR (400 MHz, DMSO-d , ppm): d 7.27 (d, 2H), 7.03 (m, 6H),
6
1
3
6
1
3
.91 (d, 2H), 6.82 (t, 2H), 6.52 (d, 4H), 6.47 (d, 2H), 1.49 (s, 9H). C-NMR
(
100 MHz, DMSO-d , ppm): d 164.59, 164.29, 155.24, 151.08, 146.91, 129.78,
6
1
Calcd. for C H N O: C, 81.15; H, 6.60; N, 8.87; O, 3.38. Found: C, 80.98; H,
28.18, 126.84, 126.18, 125.84, 124.55, 121.55, 121.40, 116.49, 35.25, 31.33. Anal.
3
0
27
3
6
.58; N, 8.49; O, 3.16.
Synthesis of Bis-(4-bromo-phenyl)-f4-[5-(4-tert-Butyl-
benzyl)-[1,3,4]oxadiazol-2-yl]-phenylg-amine (4)
A solution of 2.25 g (12.62 mmol) of NBS in 10 mL of DMF was added dropwise to a
solution of 2.49 g (5.26 mmol) of compound 3 in 30 mL of DMF under nitrogen
ꢀ
atmosphere at 5 C for a period of 30 min. After being stirred for 6 hours at room
temperature, a portion of 100 mL of water was added into a reaction mixture then
extracted with 100 mL of EA three times. The combined organic layer was washed
with aqueous sodium bisulfite (10 wt.%) and then dried over anhydrous MgSO4.
The solvent was removed by evaporation under reduced pressure. The crude product
was purified by column chromatography on silica gel using EA=DCM (1:4). The
þ
ꢀ
yield of light yellow solid was 0.86 g (18.1%). MS [M ], m=z 633. mp: 252–254 C.
1
H-NMR (400 MHz, CDCl , ppm): d 7.31 (m, 8H), 7.01 (d, 2H), 6.61 (m, 2H),
.47 (d, 4H), 1.48 (s, 9H). C-NMR (100 MHz, CDCl , ppm): d 164.62, 163.31,
3
1
3
6
1
1
3
55.32, 151.15, 146.93, 129.82, 128.23, 126.88, 126.22, 125.89, 124.59, 121.53,
21.41, 116.43, 35.29, 31.35. Anal. Calcd. for C H Br N O: C, 60.31; H, 4.41;
3
0
25
2
3
Br, 25.89; N, 6.81. Found: C, 60.12; H, 4.19; N, 6.36.
Polymerization of N-(4-(5-(4-tert-Butylphenyl)-1,3,4-oxadiazol-2-yl)phenyl)-
n-(4-(9,9-dihexyl-2-methyl-9H-fluoren-7-yl)phenyl)-4-methylbenzenamine
(
TPAOXD-PF)
A portion of 0.39 g (0.5 mmol) of compound 4, 0.2511 g (0.5 mmol) of 2-(2-(1,3,
-dioxaborinan-2-yl)-9,9-dihexyl-9H-fluoren-7-yl)-1,3,2dioxaborinane, tetrakis-
triphenylphosphine palladium) (Pd(PPh ) ) (0.01 mmol), several drops of surfactant
2
(
3
4

Synthesis and Characterization of Di-hexyl-fluorene and Triphenylamine 121
(aliquat 336) were dissolved in a mixture of 3 mL of degassed toluene and 2 mL of
degassed K CO (2 M aq). The mixture was stirred at 80 C for 2 days under argon
ꢀ
2
3
atmosphere. At the end of polymerization, 0.1 mL of 1-bromo-4-tertbutylbenzene
was added as a monofunctional end-capping reagent. After being stirred the mixture
for 12 hours, 60 mg of phenyl boronic acid was added and stirred for 12 hours. After
cooling to room temperature, the reaction mixture was poured into 250 mL of meth-
anol. The precipitate was filtered and dissolved in 40 mL of chloroform, extracted
with 100 mL of deionized water three times. The organic layer dried over anhydrous
MgSO and the solvent was removed by evaporation under reduced pressure. The
4
polymer was dissolved in small amount of DCM and poured into a stirred methanol.
The polymer was collected by filtration and dried under vacuum for 12 hours.
1
H-NMR (400 MHz, CDCl , ppm): d 8.09–8.01 (br, Ar-H), 7.89–7.83 (br, Ar-H),
3
7
Ar-H), 6.61–6.60 (br, Ar-H), 6.41–6.39 (br, Ar-H), 2.01–1.99 (br, -CH CH -),
.71–7.68 (br, Ar-H), 7.48–7.46 (br, Ar-H), 7.35–7.34 (br, Ar-H), 6.92–6.91 (br,
2
2
1
.41–1.40 (br, -C(CH ) ), 0.98–0.97 (br, -CH ).
2 4 3
Polymerization of N-(4-Butylphenyl)-n-(4-(9,9-dihexyl-2-methyl-9H-fluoren-
-yl)phenyl)-4-methylbenzenamine (TPAPF)
7
TPAPF was synthesized by the Suzuki coupling reaction between bis-(4-bromo-
phenyl)-(4-butyl-phenyl)-amine and 2-(2-(1,3,2-dioxa-borinan-2-yl)-9,9-dihexyl-
9
H-fluoren-7-yl)-1,3,2-dioxaborinane. Similar conditions were used as in the
1
polymerization of TPAOXDPF. H-NMR (400 MHz, CDCl , ppm): d 8.02–7.97
3
(br, Ar-H), 7.77–7.62 (br, Ar-2H), 7.60–7.37 (br, Ar-2H), 7.28–6.84 (br, Ar-8H),
6
1
.63–6.45 (br, Ar-6H), 2.68–2.64 (br, -CH ), 1.98–1.68 (br, -CH - -CH -),
2 2 2
.39–1.34 (br, (CH ) ), 1.03–0.97 (br, -CH ).
2
9
3
Results and Discussion
The synthetic routes to the monomers and polymers are shown in Scheme 1. All
1
13
the compounds were characterized by H-NMR, C-NMR and MASS. The
polymerization was conducted by the well known Suzuki coupling reaction.
The chemical structure of TPAOXDPF and TPAPF were confirmed by the
1
H-NMR. The number average molecular weights of TPAPF and TPAOXDPF
measured by GPC were 11,300 and 8,200 with polydispersity index (PDI) 1.98
and 1.83, respectively. As shown in Figure 1, the degradation temperature (Td,
the point of 5% weight loss of polymer) of TPAPF and TPAOXDPF appeared
ꢀ
at 346 and 393 C, respectively, in the TGA thermogram. TPAOXDPF exhibited
higher Td than that of TPAPF because of OXD side chain on the polymer
backbone.
As shown in Figure 2, TPAPF solution in chloroform and film show the absorp-
tion maximum (UVmax) at 386 and 388 nm, respectively. The PL maximum (PLmax
)
wavelength of TPAPF solution and film are appeared at 432 and 435 nm, respect-
ively. TPAOXDPF solution in chloroform and film show UVmax at 374 and
3
76 nm, respectively (Fig. 3). The PLmax of TPAOXDPF solution and film are
appeared at 451 and 478 nm (Fig. 3), respectively. The band gap energies figured
out from the UV-Vis spectrum of TPAOXDPF and TPAPF films are 2.87 and

1
22
Y. S. Park et al.
Scheme 1. Synthesis of monomer and polymer.
2.90 eV, respectively. This indicates that the OXD pendant in TPAOXDPF does not
affect the band gap energy significantly.
The HOMO energy level estimated from the oxidation onset potential of
TPAPF is ꢂ5.28 eV, which is slightly lower than that of TPAOXDPF (ꢂ5.13 eV).
ꢀ
Figure 1. TGA thermograms of the copolymers. (Conditions: N flow, heat from 50 C to
2
ꢀ ꢀ
7
00 C at 10 C=min).

Synthesis and Characterization of Di-hexyl-fluorene and Triphenylamine 123
Figure 2. UV-Vis and PL spectra of TPAPF (a) solution and (b) film.
One can also noticed that the OXD pendant does not affect the HOMO level
significantly. The LUMO energy level of TPAPF and TPAOXDPF are ꢂ2.38 and
ꢂ2.26 eV, respectively. The optical and electrochemical properties of copolymers
were summarized in Table 1.
In order to investigate EL properties, single layer light emitting diodes (ITO=
polymer=Al) were fabricated based on copolymers. As shown in Figure 5, typical
rectification characteristics are observed in the devices. The maximum brightness
2
and efficiency of the device based on TPAOXDPF (167 cd=m and 0.018 cd=A)
2
are higher than those of TPAPF (42.1 cd=m and 0.008 cd=A) based device, indicat-
ing that very strong electronegative OXD group in the side chain improved electron
transporting property. The enhanced device performance is result of the more
Figure 3. UV-Vis and PL spectra of TPAOXDPF (a) solution and (b) film.

1
24
Table 1. Summary of photophysical properties of the polymers
kmax, solution (nm) kmax, thin film (nm)
Y. S. Park et al.
Energy levels
a
HOMO LUMO Eg
b
c
UV
PL
UV
PL
(eV)
(eV)
(eV)
TPAPF
TPAOXDPF
386
374
432
451
388
376
435
478
ꢂ5.28
ꢂ5.13
ꢂ2.38 2.90
ꢂ2.26 2.87
a
Estimated from the onset oxidation potential in the cyclic voltammogram.
Estimated from the HOMO and Egap.
b
c
Figured out from the absorption edge from the UV-Vis spectrum.
Figure 4. Cyclic voltammogram of TPAOXDPF and TPAPF. CVs were performed in poly-
mer solution in 0.1 M Bu NPF solution in DCM at a scan rate of 100 mV=s.
4
6
balanced injection rates of both carriers due to the simultaneous introduction of
TPA and 1,3,4-oxadiazole pendant into polymer.
Conclusion
We synthesized new PF derivatives with TPA and OXD. These copolymers exhibited
good solubility in common organic solvents and good thermal stability. The
efficiency of the device based on TPAOXDPF was higher than that of the device
based on TPAPF. It was suggested that the hole-electron recombination processes
were facilitated in the emitting layer by of OXD pendants.
Acknowledgment
This research was supported by the Converging Research Center Program through
the Ministry of Education, Science and Technology (2010K001079).

Synthesis and Characterization of Di-hexyl-fluorene and Triphenylamine 125
Figure 5. Current density – voltage – brightness curves of (a) ITO=TPAPF=Al and (b) ITO=
TPAOXDPF=Al (filled circle: current density – voltage, triangle: brightness – voltage).
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