Highly efficient and stable blue-light-emitting copolyfluorene consisting of carbazole, oxadiazole, and charge-trapping anthracene groups

Article abstract of DOI:10.1002/pola.24439

This article presents the synthesis and electroluminescent (EL) properties of a stable blue-light-emitting copolyfluorene (P1) consisting of carbazole, oxadiazole and charge-trapping anthracene groups by Suzuki coupling reaction. The hole-transporting car

Full text of DOI:10.1002/pola.24439

Highly Efficient and Stable Blue-Light-Emitting Copolyfluorene Consisting
of Carbazole, Oxadiazole, and Charge-Trapping Anthracene Groups
RUEI-TANG CHEN,1 WEN-FEN SU,2 YUN CHEN2
1Nanopower and Thin Film Technology Center, ITRI South, Industrial Technology Research Institute, Tainan, Taiwan
2Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan
Received 25 August 2010; accepted 9 October 2010
DOI: 10.1002/pola.24439
Published online 3 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: This article presents the synthesis and electrolumi-
nescent (EL) properties of a stable blue-light-emitting copoly-
fluorene (P1) consisting of carbazole, oxadiazole and charge-
trapping anthracene groups by Suzuki coupling reaction. The
hole-transporting carbazole and electron-transporting oxadia-
zole improve charges injection and transporting properties,
whereas the anthracene is the ultimate emitting chromophore.
The thermal, photophysical, electrochemical, and EL properties
of P1 were investigated by thermogravimetric analysis, differ-
ential scanning calorimeter, optical spectroscopy, cyclic vol-
tammetry, and EL devices fabrication and characterization. P1
demonstrated high-thermal stability with thermal decomposi-
tion and glass tranistion temperatures above 400 and 145ꢀC,
respectively. In film state, P1 showed blue emission at 451 nm
attributed to anthracene chromophore. Photophysical and elec-
trochemical investigations demonstrate that effective energy
transfer from fluorene to anthracene segments and charges
trapping on anthracene segments leads to efficient and stable
blue emission originating from anthracence. Polymer light-
emitting diodes using P1 as the emitting layer (ITO/
PEDOT:PSS/P1/Ca/Al) exhibited excellent current efficiency (5.1
cd/A) with the CIE coordinate being (0.16, 0.11). The results
indicate that copolyfluorene is a promising candidate for the
C
V
blue-emitting layer in the fabrication of efficient PLEDs. 2010
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49:
184–191, 2011
KEYWORDS: copolyfluorene; energy transfer; light-emitting
diodes (LED); luminescence; synthesis; trapping
INTRODUCTION Developing stable and highly efficient blue-
light-emitting polymers for full color displays is a great chal-
lenge because of the intrinsically wide band-gap that causes
imbalanced charge injection.1 Polyfluorene (PF) is a promis-
ing blue-light-emitting material that is widely used in PLEDs
because of its excellent thermal and chemical stability and
high photoluminescence (PL) quantum yield.2–4 However, the
highest occupied molecular orbital (HOMO) level of PF
(EHOMO ¼ ꢁ5.8 eV) creates a high hole-injection barrier and
discrepancies in carriers mobility, which lead to unbalanced
charges injection.1,5 Moreover, a major drawback of PF for
blue-emitting PLEDs is color instability attributed to aggre-
gate and/or excimer formation during thermal annealing or
device operation.6–9 To overcome this problem, several
approaches have been adopted to develop new stable blue
emission PF, such as incorporation of bulky groups into the
main or side chains, introduction of spiro-structure or bulky
groups at the C-9 position of the fluorene, and modification
of device structure.10–29 For example, Huang et al. obtained
an efficient color-stable blue PLED with maximum efficiency
of 2.5 cd/A and Commission Internationale de L’Eclairage
(CIE) coordinate of (0.156, 0.080), by incorporating blue-
light-emitting 4-N,N-diphenylaminostilbene and electron-
transporting oxadiazole into the poly[2,7-(9,9-dioctylfluor-
ene)-alt-1,3-(5-carbazolphenylene)] backbone.18 Wu et al.
prepared blue-emitting copolyfluorene with CIE coordinate
of (0.14, 0.15) and maximum efficiency of 2.07 cd/A, which
possessed electron-rich triphenylamine moieties in the
polymer backbone and electron-deficient oxadiazole pendent
groups.30 Liu et al. reported stable blue emission [CIE coor-
dinates: (0.16, 0.19)] of copolyfluorene containing electron-
withdrawing dibenzothiophene-S,S-dioxides with current
efficiency of 6.0 cd/A.22 Huang et al. reported blue-light-
emitting copolyfluorenes containing pendant carbazole and
triphenylamine groups as hole-transporting moieties to pro-
mote hole injection by creating graded HOMO levels.19 The
current efficiency and CIE coordinate of the blue-emitting
device were 4.57 cd/A and (0.16, 0.10), respectively. The
current efficiency (4.57 cd/A) was claimed as the highest
one for pure blue PLED at that point in time.19 Although the
device efficiency could be enhanced to 13.1 cd/A, the CIE
coordinate was shifted to (0.19, 0.20) due to the presence of
green tail emission. These results indicate that both balanced
charges injection/transport and suppressed aggregate/
Additional Supporting Information may be found in the online version of this article. Correspondence to: Y. Chen (E-mail: yunchen@mail.ncku.edu.
tw)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 184–191 (2011) 2010 Wiley Periodicals, Inc.
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SCHEME 1 Synthesis of P1, P0, and model compounds (M1–M3).
excimer formation are required to obtain highly efficient and
stable pure blue-emitting PF derivatives.
bazole and oxadiazole. Moreover, the anthracene derivatives
possess excellent PL and electroluminescent (EL) character-
istics,32–36 as well as pure blue emission with a peak at
425–450 nm.37 Therefore, to improve the performance fur-
ther we incorporate emitting anthracene, hole-transporting
carbazole, and electron-transporting oxadiazole moieties into
PF main chain simultaneously. The anthracene acts as pure
blue emitter, whereas the carbazole and oxadiazole are used
to balance charges injection and transport. The purpose of
Previously, we successfully developed a series of stable blue-
light-emitting copolyfluorenes; the best current efficiency
(2.4 cd/A) was obtained for copolyfluorenes containing elec-
tron-transporting oxadiazole.31 The enhancement in device
performance was attributed to improved electron injection
and transport. The HOMO and lowest unoccupied molecular
orbital (LUMO) levels of anthracene lie between those of car-
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Photophysical Properties of Model Compounds (M1–M3) and Copolyfluorene (P1)
Solutiona
Film
UV–vis
PL kmax
UV–vis
PL kmax
b
kmax (nm)
(nm)
/
PL
kmax (nm)
(nm)
P1
M1
M2
M3
a
392
423, 446
389, 402
388, 404
444
0.88
0.11
0.92
0.66
391
434, 451
407
316
318
336
335
411
382, 400
384, 402
454
In chloroform (1 ꢂ 10ꢁ5 M).
Quantum yield: the value was estimated using quinine sulfate (10ꢁ5 M in 1 N H2SO4(aq), /PL ¼ 0.55) as a
b
standard.
our molecular design is to create a graded energy of HOMO
and LUMO levels to facilitate hole and electron injection and
charge transport to the emitting anthracene chromophore.
Moreover, the band-gap of anthracene segments is smaller
than that of fluorene segments, facilitating the energy trans-
fer from fluorene to anthracene segments. This molecular
design successfully implements a pure blue-light-emitting
material; the current efficiency and maximum luminance of
its single layer device reach 5.1 cd/A and 2670 cd/m2,
respectively, with the CIE coordinate being (0.16, 0.11). To
our best knowledge, the efficiency is the highest in pure blue
PLED.
Photophysical Properties
Table 1 lists the photophysical data of P1 and model com-
pounds (M1–M3). P1 and P0 exhibit a major absorption
peak at 390 nm in film state, which is attributed to the p–p*
transition of conjugated PF segments [Fig. 1(a)]. The absorp-
tion and PL spectra in CHCl3 are almost identical to those of
PF due to very low contents of the comonomer units (carba-
zole, oxadiazole, and anthracene) in the main chain (Fig. S1
in Supporting Information). However, in solid state PL spec-
tra, the intensity of the longer wavelength peak (ꢃ446 nm)
of P1 is enhanced and surpasses that in solution state. This
RESULT AND DISCUSSION
Synthesis and Characterization
Scheme 1 illustrates the synthetic routes of the blue-light-
emitting polymer (P1) and three model compounds (M1–M3)
corresponding to chromophores. The dibromo monomers
of 3,6-dibromo-9-(3,5-dimethyl-4-(octyloxy)phenyl-9H-carba-
zole (2), 2-(4-bromom-2-methylphenyl)-5-(4-bromo-3-meth-
ylphenyl)-1,3,4-oxadiazole (3), and 9-(3,4-bis(decyloxy)-
phenyl)-2,7-dibromo-9-(2,5-dimethylphenyl)-9H-fluorene (5)
were prepared according to our previously reported proce-
dures.31,38–40 The P1 and model compounds were prepared
by the palladium-catalyzed Suzuki coupling reaction.41 A co-
polymer P0 without anthracene moiety was also synthesized
by analogous procedures for comparative study. Their struc-
tures were satisfactorily characterized by using 1H and 13C
NMR spectroscopy, mass spectrometry, and elemental analy-
sis. P1 and P0 are soluble in common organic solvents such
as toluene, THF, and chloroform. The weight-average molecu-
lar weight (Mw) and the polydispersity index of P1 were 1.8
ꢂ 105 and 2.0, whereas those of P0 were 1.3 ꢂ 105 and 1.8,
respectively. The thermal decomposition temperature (Td)
and glass transition temperature (Tg) of P1 is 406 ꢀC (ꢃ5%
ꢀ
loss) and 145 C, as examined by thermogravimetric analysis
(TGA) and differential scanning calorimeter (DSC). The Tg of
P1 is much higher than 97 ꢀC for poly(9,9-dioctylfluorene)
(PFO).42 The high Tg is conducive to reducing crystallization
and to retaining long-term morphological stability when
applied as an active layer in EL devices.43
FIGURE 1 The absorption and photoluminescence spectra in
film state: (a) P1 and P0 and (b) model compounds M1–M3.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
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TABLE 2 Electrochemical Properties of Model Compounds
(M1–M3) and Copolyfluorenes (P0, P1)
after the thermal annealing (Supporting Information Fig. S2)
also suggests the formation of fluorenone moiety. Current
photophysical results reveal that P1 is a potential blue-emit-
ting material applicable in PLEDs.
Eox vs.
Ag/AgCl
(V)a
Eox vs.
FOC (V)b
EHOMO
(eV)c
ELUMO
(eV)d
Eogpt
(eV)e
Electrochemical Properties
P1
P0
M1
M2
M3
a
1.18
1.18
1.07
1.63
1.14
0.71
0.71
0.60
1.16
0.67
ꢁ5.51
ꢁ5.51
ꢁ5.40
ꢁ5.96
ꢁ5.47
ꢁ2.59
ꢁ2.59
ꢁ2.08
ꢁ2.65
ꢁ2.57
2.92
2.92
3.32
3.31
2.90
The electrochemical behaviors of P1, P0, and model com-
pounds (M1–M3) were investigated by cyclic voltammetrys
(CVs), using ITO as the working electrode (deposited with
the compounds to be measured). The CVs of P1, P0, and
M1–M3 in anodic scans are shown in Supporting Informa-
tion Figures S3 and S4, with the characteristic electrochemi-
cal data summarized in Table 2. The anodic scans of M1–M3
are apparently irreversible and their HOMO levels are esti-
mated to be ꢁ5.40, ꢁ5.47, and ꢁ5.96 eV from onset oxida-
tion potentials. Electrochemical irreversibility is caused by
slow electron exchange of the redox species with the work-
ing electrode.45 Therefore, the irreversible anodic scans of
M1–M3, deposited on ITO glass, are due to its slow electron
exchange with ITO glass (working electrode). The LUMO lev-
els are ꢁ2.08, ꢁ2.57, and ꢁ2.65 eV, respectively, as esti-
mated from the HOMO levels and optical band-gaps (Eogpt).
By similar procedures, the HOMO and LUMO levels of P1
and P0 are estimated to be ꢁ5.51 eV and ꢁ2.59 eV. Both
HOMO and LUMO levels of M3 lie between those of M1
(HOMO ¼ ꢁ5.40 eV) and M2 (LUMO ¼ ꢁ2.65 eV) (Fig. 2),
indicating that in P1 the anthracene moiety acts as a trap-
ping site for both holes and electrons.18 Moreover, step
energy ladders are formed to facilitate hole and electron
injection and transport. For instance, the hole is injected first
into carbazole units and then transported to anthracene
chromophore, whereas the electron is injected to oxadiazole
groups followed by transporting to anthracene chromophore.
Subsequently, the hole and electron are trapped on anthra-
cene chromophore to recombine as an exciton. Therefore,
these improvements in charges injection/transport and trap-
ping are expected to enhance device efficiency. Moreover,
model compound M3 exhibits stable and reversible oxidation
Onset oxidation potentials obtained from cyclic voltammograms.
EFOC ¼ 0.47 V vs. Ag/AgCl.
b
c
EHOMO ¼ ꢁ(Eox, FOC þ 4.8 V) eV.
d
e
ELUMO ¼ Eg þ EHOMO
.
Band-gaps obtained from onset absorption (konset) using Eg ¼ 1240/
konset
.
emission is mainly contributed by the anthracene chromo-
phore whose intensity is enhanced by the energy transfer
from fluorene segments.32,37 The PL spectrum of P1 film
confirms the presence of anthracene chromophore in poly-
mer chain. The lower band-gap (2.81 eV) of anthracene com-
pared with the fluorene segments (2.90 eV) facilitates the
energy transfer. The possibility of energy transfer can be
evaluated by comparing the absorption and PL spectra of the
model compounds (M1, M2, M3) representing the three
emitting chromophores that comprise the P1 main chain. M3
shows absorption peaks at 365, 382, and 400 nm [Fig. 1(b)],
which are attributed to the characteristic p–p* transitions of
anthracene chromophore,36 and its PL emission appears at
454 nm. The PL emission spectra of M1 and M2 peak at 407
and 411 nm, respectively, which overlap extensively with the
absorption band of M3 (the model of anthracene) above 370
nm. This overlap confirms that the emission enhancement
(ꢃ446 nm) in solid state is attributed to efficient energy
transfer. The absorption and PL spectra of P1 in film state
are basically similar to those in solution state, although the
solid PL spectrum shows slight a red-shift relative to the so-
lution one. This can be ascribed to its noncoplanar main
chain and bulky side chain, which prevent intermolecular
interaction such as aggregation.32 Color stability of P1 film
was investigated by annealing at 200 ꢀC for 30 and 60 min
in air. The PL spectrum of P1 changes only slightly
(decreased intensity at 451 nm and enhanced intensity at
434 nm) after the thermal annealing (Supporting Informa-
tion Fig. S2), indicating that the blue emission (430–450 nm)
is quite stable under the thermal treatment. This color stabil-
ity is mainly contributed from the bulky pendant aromatic
groups and noncoplanar main-chain structure. The relative
intensity change of the emission at 451 nm and 434 nm af-
ter P1 film was subjected to the thermal annealing is prob-
ably due to the interruption of photo-excited energy transfer
from PF segments to anthracene chromophores. Slight fluore-
none chromophore, formed after thermal annealing at 200
ꢀC in air, traps the photo-excited exciton to interrupt the
energy transfer.44 The PL spectral tailing at 500–600 nm
FIGURE 2 Energy band diagrams of P1 and model compounds
M1–M3.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Electroluminescence Properties of PLED
Figure 4 shows the current density-bias-luminance (J-B-L)
characteristics of the EL device with a configuration of ITO/
PEDOT:PSS/P1 or P0/Ca (50 nm)/Al (100 nm), and the cor-
responding characteristic data are summarized in Table 3. It
is known that PF has a higher energy barrier for hole injec-
tion and relatively poor electron injection and transport abil-
ity.1 Therefore, the incorporation of carbazole and oxadiazole
groups is expected to effectively improve transport and
injection of holes and electrons. This is confirmed by the
high maximum current efficiency (5.1 cd/A) attained in the
EL device using P1 as the emitting layer, because the graded
energy levels in P1 facilitate injection and transport of holes
and electrons. Moreover, the charges are mainly trapped in
anthracene moiety due to the large energy barriers, that is,
the energy difference between the LUMO of M1 and M3
(0.49 eV) and between the HOMO of M2 and M3 (0.49 eV)
(Fig. 2). Consequently, the charges are efficiently recombined
at anthracene segment. The emission is blue with EL spectral
peak at about 450 nm (with two shoulders at 428 and 477
nm). The emission band is similar to its PL spectrum (Fig.
1), with the emission intensity at 428 nm (main emission of
fluorene segment) degenerating significantly, indicating effi-
cient energy transfer from fluorene to anthracene segments.
To confirm the charge-trapping role of antharcene moiety, a
reference EL device using P0 as the emitting layer was also
fabricated to compare with P1-based device, in which P0 is
the copolyfluorene without anthracene chromophore. The
maximum current efficiency of the P0-based device was 1.3
cd/A, much inferior to that of P1-based device (5.1 cd/A)
(Table 3). The current efficiency of P1-based device first
increases to about 5.1 cd/A and then decreases quickly to
about 2 cd/A after 20 mA/cm2 (Supporting Information Fig.
S5). The initial quick decrease in current efficiency might be
due to interaction of the calcium cathode with P1 under de-
vice operation.46,47 Current results indicate that anthracene
chromophore plays an important role and effectively enhan-
ces the device performance.
FIGURE 3 Solution cyclic voltammograms of model com-
pounds M3.
and reduction properties in solution (Fig. 3). Reversibility in
reduction and oxidation is essential for materials to be
applied in organic EL devices; because the main reactions in
an EL device that occur at the cathode and anode during de-
vice operation are reduction and oxidation, respectively.
Emission spectral stability of the P1-based EL device was
compared with that of P0- and PFO-based ones. As shown in
Figure 5, under a luminance of 300 cd/m2, the emission
spectrum of P1-based device changes only slightly at first
and remains almost unchanged after 2 min. However, the
emission spectra of P0-based device changes continuously
TABLE 3 Optoelectronic Properties of Electroluminescent
Devicesa
Von
(V)b
Lmax
(cd/m2)c
LEmax
(cd/A)d
Emission
CIE 1931
(x, y)e
(kem, nm)
P1
P0
a
6.8
6.9
2670
1700
5.1
1.3
450
425
(0.16, 0.11)
(0.17, 0.08)
FIGURE 4 Current density-bias-brightness characteristic of
PLED devices with a structu re of (a) ITO/PEDOT:PSS/P1/Ca (50
nm)/A1 (100 nm) and (b) ITO/PEDOT:PSS/P0/Ca (50 nm)/A1
(100 nm). Inset: EL spectra of the devices at about 500 cd/m2
and real color images.
Device configuration: ITO/PEDOT:PSS/P0 or P1/Ca/Al.
b
c
Defined as the turn-on voltage required for a luminance of 10 cd/m2.
Maximum luminance.
d
e
Maximum luminance efficiency.
The CIE coordinates at maximum luminance.
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ARTICLE
chain structure in P1 effectively reduce the intermolecular
interactions. The CIE chromaticity coordinate of the blue
color emitted from P1-based device barely changes from
(0.17, 0.07) to (0.17, 0.10) within 5 min (at 300 cd/m2). The
results demonstrate that the copolyfluorene P1 is a promis-
ing blue-emitting material applicable in the fabrication of
PLEDs.
EXPERIMENTAL
Materials
9,10-Dibromoanthracene and [9,9-di(p-tolyl)-fluoren-2-yl]
boronic acid were purchased from Aldrich and Luminescence
Technology, respectively, and used without further purifica-
tion. Model compounds (M1–M3) and copolyfluorene (P1)
were prepared by the Suzuki coupling reaction using tetra-
kis(triphenylphosphine)palladium [Pd(PPh3)4] as the cata-
lyst, which was procured from Strem. Dibromo monomers
3,6-dibromo-9-(3,5-dimethyl-4-(octyloxy)phenyl-9H-carbazole
(2), 2-(4-bromom-2-methylphenyl)-5-(4-bromo-3-methylphenyl)-
1,3,4-oxadiazole (3), and 9-(3,4-bis(decyloxy)phenyl)-2,7-di-
bromo-9-(2,5-dimethylphenyl)-9H-fluorene (5) were prepared
according to our previously reported procedures.31,38–40
Synthesis of 9-(3,5-Dimethyl-4-(octyloxy)phenyl-3,6-
bis[9,9-di(p-tolyl-fluorene-2-yl)]-9H-carbazole (M1)
[9,9-Di(p-tolyl)-fluoren-2-yl]boronic acid (1 g, 2.5 mmol),
3,6-dibromo-9-(3,5-dimethyl-4-(octyloxy)phenyl-9H-carbazole
(0.56 g, 1 mmol), Pd(PPh3)4 (30 mg, 0.026 mmol), and sev-
eral drops of Aliquat 336 were added to a mixture consisting
of toluene (12 mL) and aqueous solution of K2CO3 (2 M, 6
mL). The mixture was first purged with Argon and stirred
vigorously at 90 ꢀC for 48 h. The cooled solution was
quenched with water and extracted twice with CHCl3. The
combined organic extracts were dried (MgSO4) and concen-
trated by rotary evaporation. Column chromatography on
silica gel (CHCl3/hexane) afforded products as white solids
of M1.
1
Yield: 68%. H NMR (400 MHz, CDCl3): d 8.34–8.33 (d, J ¼ 4
Hz, 2H), 7.86–7.74 (m, 9H), 7.60–7.58 (m, 2H), 7.39–7.36 (m,
6H), 7.19–7.17 (m, 10H), 7.05–7.03 (m, 9H), 3.78–3.76 (d, J
¼ 8 Hz, 2H), 2.40 (s, 6H), 2.28 (s, 12 H), 1.82–1.80 (t, J ¼ 8
Hz, 1H), 1.40–1.38 (m, 8H), 1.03–0.93 (m, 6H). Anal. Calcd.
for C82H73NO (%): C, 90.48; H, 6.76; N, 1.29. Found (%): C,
90.06; H, 6.76; N, 1.29. ESI-MS (m/z): calcd: 1087.57; found:
1088.
FIGURE 5 Variations of EL spectra with operating time for P1-,
P0-, and PFO-based devices [ITO/PEDOT:PSS/P1, P0 or PFO/Ca
(50 nm)/Al (100 nm)]; the EL spectra were recorded at about
300 cd/m2. The values in the parentheses represent the CIE
coordinates.
Synthesis of 2-(3-Methyl-4-[9,9-di(p-tolyl-fluorene-2-
yl)]phenyl)-5-(2-methyl-4-[9,9-di(p-tolyl-fluorene-2-
yl)]phenyl)-1,3,4-ozadiazole (M2)
over times, indicating that the emission stability in P1-based
device is mainly contributed by the anthracene chromophore.
In contrast, the emission intensity of PFO-based device above
450 nm increases steadily with increasing time, which is at-
tributable to aggregate and/or excimer formation during
operation.6 Hence, the emission stability of P1-based device
is much better than that of PFO-based one. The color stabil-
ity in P1 is linked to effective suppression of aggregate and
excimer formation during device operation. The bulky aro-
matic constituents at C-9 position and noncoplanar main-
The synthetic procedures of M2 were analogous to those
used in the preparation of M1, except 2-(4-bromo-2-methyl-
phenyl)-5-(4-bromo-3-methylphenyl)-1,3,4-oxadiazole was used
instead.
1
Yield: 58%. H NMR (400 MHz, CDCl3): d 8.10–8.08 (d, J ¼ 8
Hz, 1H), 8.03 (s, 1H), 7.97–7.95 (d, J ¼ 8 Hz, 1H), 7.85–7.78
(m, 4H), 7.67–7.64 (m, 2H), 7.55–7.53 (m, 2H), 7.41–7.34
(m, 7H), 7.30–7.28 (m, 2H), 7.16–7.11 (m, 8H), 7.06–7.02
(m, 8H), 2.83 (s, 3H), 2.34 (s, 3H), 2.29 (s, 12 H). Anal. Calcd.
BLUE-LIGHT-EMITTING COPOLYFLUORENE, CHEN, SU, AND CHEN
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
for C70H54N2O (%): C, 89.52; H, 5.80; N, 2.98. Found (%): C,
89.04; H, 5.79; N, 2.90. ESI-MS (m/z): calcd: 938.42; found:
939.
trode as the auxiliary electrode. The electrodes were
immersed in an acetonitrile solution containing 0.1 M (n-
Bu)4NClO4 as the electrolyte. The energy levels were esti-
mated using the ferrocene (FOC) value of ꢁ4.8 eV with
respect to vacuum level, which is defined as zero.48,49 For
the solution CVs experiments, the oxidation and reduction
potentials were detected in dichloromethane (glassy carbon
electrode, 0.1 M n-Bu4NPF6 as a supporting electrolyte) and
in THF (glassy carbon electrode, 0.1 M n-BuNClO4 as a sup-
porting electrolyte), respectively, with Ag/AgCl (saturated
KCl) as the reference electrode.
Synthesis of 9,10-Bis[9,9-di(p-tolyl-fluorene-2-
yl)]anthracene (M3)
The synthetic procedures of M3 were similar to those used
in the preparation of M1, except 9,10-dibromoanthracene
was used instead.
Yield: 86%. 1H NMR (400 MHz, CDCl3): d 7.98–7.95 (d, J ¼
12 Hz, 2H), 7.87–7.85 (d, J ¼ 8 Hz, 2H), 7.74–7.72 (m, 4H),
7.54 (s, 1H), 7.50–7.39 (m, 8H), 7.34–7.30 (m, 2H), 7.28–
7.26 (m, 2H), 7.21 (s, 1H), 7.18–7.13 (m, 8H), 7.01–6.98 (m,
8H), 2.26 (s, 12H). Anal. Calcd. for C68H50 (%): C, 94.19; H,
5.81. Found (%): C, 94.16; H, 5.84. ESI-MS (m/z): calcd:
866.39; found: 867.
Fabrication of EL Devices
The ITO-coated glasses were washed successively in ultra-
sonic baths of neutraler reiniger/deionized water mixture,
deionized water, acetone, and 2-propanol, followed with
cleaning in a UV-O3 chamber. A thick hole-injection layer of
PEDOT:PSS was spin-coated on top of the cleaned ITO glass
and annealed at 150 ꢀC for 15 min in a dust-free atmos-
phere. The emitting layer was formed by spin coating the so-
lution of P1 or P0 in toluene (20 mg/mL) on top of the
PEDOT:PSS layer, then baking at 110 ꢀC for 15 min. The
polymer solution was filtered through a syringe filter (0.2
lm) before the spin-coating. Finally, thick layers of Ca (50
nm) and Al (100 nm) were subsequently deposited as cath-
ode via vacuum deposition under 1 ꢂ 10ꢁ6 Torr. Luminance-
bias, current density-bias, and EL spectral characteristics of
the devices were recorded using the combination of a Keith-
ley power source (model 2400) and an Ocean Optics
usb2000 fluorescence spectrophotometer. The EL device was
fabricated in ambient conditions, and their characterization
was conducted in a glove-box filled with nitrogen.
Synthesis of Copolyfluorene (P1 and P0)
The copolyfluorenes were synthesized according to the pro-
cedures reported previously,31 and afforded as a light-yellow
fiber.
For P1: 1H NMR (400 MHz, CDCl3): d 7.74–7.52 (m), 7.08–
6.93 (m), 6.66–6.61 (m), 3.92 (br), 2.37–2.07 (m), 1.79–1.56
(br), 1.33–1.13 (br), 0.89–0.78 (m). 13C NMR (400 MHz,
CDCl3): d 155.93, 152.95, 148.55, 148.05, 142.47, 140.67,
139.06, 138.22, 137.0, 134.78, 134.31, 132.58, 131.08,
130.23, 127.59, 126.45, 125.64, 124.60, 120.38, 114.89,
113.03, 110.36, 105.99, 67.72, 67.38, 66.17, 65.59, 65.41,
47.82, 42.87, 39.26, 37.39, 36.36, 36.16, 29.94, 29.80, 27.97,
24.69, 22.70, 22.61, 21.52, 21.24, 19.74, 19.68, 19.50, 16.53,
14.15. Anal. Found (%): C, 88.64; H, 8.77; N, <0.2.
For P0: 1H NMR (400 MHz, CDCl3): d 7.74–7.52 (m), 7.21–
7.19 (m), 6.98–6.93 (m), 6.65–6.61 (m), 3.91–3.90 (br),
2.17–2.07 (m), 1.79–1.53 (br), 1.31–1.20 (br), 1.13–1.07 (m),
0.89–0.77 (m). Anal. Found (%): C, 86.71; H, 8.46; N, 0.30.
Characterization
CONCLUSIONS
1H NMR and 13C NMR spectra were recorded with a Bruker
AVANCE 400 MHz NMR spectrometer, with the chemical
shifts reported in ppm using tetramethylsilane (TMS) as an
internal standard. Mass and elemental analyses were carried
out on a Shimadzu QP2010 spectrometer and Heraus CHN-
Rapid elemental analyzer, respectively. Molecular weight and
molecular weight distribution of the copolyfluorenes were
determined by a gel permeation chromatograph (GPC) using
CHCl3 as the eluent and monodisperse polystyrene as cali-
bration standards. TGA was performed under nitrogen
atmosphere using a ꢀPerkinElmer TGA-7 thermal analyzer, at
a heating rate of 20 C/min. The thermal transitional proper-
ties of the P1 were recorded using a DSC, Mettler DSC 1,
with a heating rate of 10 ꢀC/min. Absorption spectra were
measured with a Jasco V-550 spectrophotometer, and PL
spectra were obtained using a Hitachi F-4500 fluorescence
spectrophotometer. The CVs were recorded using a voltam-
metric analyzer (model CV-50W from Bioanalytical Systems)
under nitrogen atmosphere. The measuring cell consisted of
a P1-coated ITO as the working electrode, an Ag/AgCl elec-
trode as the reference electrode, and a platinum wire elec-
We have successfully designed a unique copolyfluorene (P1)
containing carbazole, oxadiazole, and anthracene groups for
blue-light-emitting PLEDs. Its solid-state PL spectra showed
exclusive blue emission (peaked at 451 nm) of anthracene
chromophore because of efficient energy transfer from fluo-
rene segments. In addition, appropriate cascade energy steps
for both HOMO and LUMO levels were successfully formed
from carbazole to anthracene and oxadiazole to anthracene,
respectively, facilitating charges injection and transport. This
not only led to balanced charges injection/transport but also
enhanced recombination efficiency. Polymer light-emitting
diodes (ITO/PEDOT:PSS/P1/Ca/Al) exhibit stable blue-light
emission [CIE coordinate: (0.16, 0.11)] and excellent current
efficiency (5.1 cd/A). Incorporating hole-transporting carba-
zole and electron-transporting oxadiazole simultaneously is
effective in promoting charges injection and transport. Fur-
thermore, in combination with suitable emitting anthracene
chromophore gives rise to stable blue-emitting polymer.
The authors thank the National Science Council of Taiwan for
the financial aid through project NSC 98-2221-E006-003-MY3.
190
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ARTICLE
27 Huang, S.-T.; Liaw, D.-J.; Hsieh, L.-G.; Chang, C.-C.; Leung,
M.-K.; Wang, K.-L.; Chen, W.-T.; Lee, K.-R.; Lai, J.-Y.; Chan, L.-
H.; Chen, C.-T. J Polym Sci Part A: Polym Chem 2009, 47,
6231–6245.
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