Hyperbranched oxadiazole-containing polyfluorenes: Toward stable blue light PLEDs

Article abstract of DOI:10.1021/ma050833f

The synthesis of hyperbranched oxadiazole-containg polyfluores using 'A₂ + A₂^′ + B₃' approach based on Suzuki polycondensation was investigated. The molecular structures of the sample polymer were characterized with high-resolution NMR spectroscopy and elemental analysis. The solubility was found to be poor in organic solvents for hyperbranched polymers with higher contents of 2-(4-bromophenyl)-5-(3,5-dibromophenyl)-1,3,4-oxadiazole monomer. The sample polyfluores showed stable blue light emission even after being annealed in the elevated temperatures in the N₂ and air.

Full text of DOI:10.1021/ma050833f

Macromolecules 2005, 38, 6755-6758
6755
good quality amorphous films.14 In this communication,
to avoid the excimer formation and to improve the
charge transport-balance properties of PFs, we intro-
duce a heteroaryl moiety, oxadiazole, as a branching
unit into the PF to make the polymers with hyper-
branched structure. A novel “A2 + A′2 + B3” approach
based on Suzuki polycodensation was developed for the
hyperbranched polymer synthesis. The conventional
methods to synthesize the hyperbranched polymers are
the AB2 and A2 + B3 approaches.15 The resulting hyper-
branched oxadiazole-containing polyfluorenes show stable
blue light emission even in the air at the elevated
temperatures. Moreover, the PLED devices have been
fabricated on the basis of the resulting copolymers, and
the EL characteristics have been investigated.
Hyperbranched Oxadiazole-Containing
Polyfluorenes: Toward Stable Blue Light
PLEDs
Yu Xin,† Gui-An Wen,† Wen-Jing Zeng,‡ Lei Zhao,†
Xing-Rong Zhu,† Qu-Li Fan,† Jia-Chun Feng,†
Lian-Hui Wang,† Wei Wei,† Bo Peng,†
Yong Cao,‡ and Wei Huang*,†
Institute of Advanced Materials (IAM), Fudan University,
Shanghai 200433, P. R. China, and Polymer Optoelectronic
Materials and Devices, South China University of
Technology, Guangzhou 510640, P. R. China
Received April 20, 2005
Revised Manuscript Received June 23, 2005
Introduction. The discovery of electroluminescent
poly(p-phenylene) (PPV) in 19901 has attracted consid-
erable interest for the application in the display tech-
nology2 for their merits, such as low-cost solution
processing, good thermal stability, flexibility, and so on.
Polymer light-emitting diodes (PLEDs) will require high
quantum efficiency so as to satisfy its future com-
mercialization. For improving the quantum efficiency,
it is critical to achieve both efficient charge injection and
balanced mobility of both charge carriers inside the
electroluminescent (EL) polymers. It can be obtained by
multilayer PLEDs fabricated with the additional hole-
transporting and electron-transporting layer besides the
electroluminescent polymer layer. Alternatively, conju-
gated polymers with transport-balance properties of
electrons and holes should be developed so as to obtain
the single-layer PLEDs. In previous work, our group has
developed a series of p-n diblock conjugated copoly-
mers3 with improved EL properties by introducing a
heteroaryl moiety, oxadiazole, to the backbone of con-
jugated polymers, and these polymers showed unique
and interesting optical and electrochemical properties.4
Oligo- and polyfluorene (PFs) derivatives are promis-
ing blue light-emitting materials, for they exhibit excel-
lent chemical and thermal stabilities and produce light
with high quantum yields.5 Problems encountered with
these materials are their tendency to aggregate and
formation of excimers in the solid state leads to blue-
green emission and fluorescence quenching and the
decrease in electroluminescence efficiency.6 To minimize
this tendency for aggregation and avoid this detrimental
excimer emission, several attempts have been made to
use longer and branched side chains or bulky substit-
uents,7 copolymerization techniques,8 dendrimer attach-
ment,9 the end-capping of PFs with bulky groups,10 the
cross-linking of PFs,11 an oligomer approach,8,12 etc.
Since the highly branched and globular features can
reduce or eliminate strong intermolecular interactions
and aggregation, electroactive and light-emitting den-
drimers and hyperbranched polymers are of current
interest for developing efficient electroluminescent de-
vices for displays and lighting and other photonic
devices.13 Moreover, it is likely that PLEDs made of
three-dimensional polymers should have improved sta-
bility and efficiency and also make the materials form
Results and Discussion. a. Synthesis and Struc-
tural Characterization. A novel “A2 + A′2 + B3”
Suzuki polycondensation was developed to synthesize
the hyperbranched polymers (see Scheme 1).16 Since it
has been well established that it tends to produce
insoluble cross-linked polymers in the step-growth po-
lymerization, the reaction conditions, such as temper-
ature, time, concentration, and solvent, which will
influence the polymerization process should be consid-
ered. The Suzuki coupling polycondensation reaction is
much better to be carried out in a dilute solution and
relatively low temperature to reduce the possible cross-
linking between branches; thus, the poor solubility of
the polymer should be avoided. Three monomers, 2,7-
dibromo-9,9-dioctylfluorene (2), 9,9-dioctylfluorene-2,7-
bis(trimethylene boronate) (4), and 2-(4-bromophenyl)-
5-(3,5-dibromophenyl)-1,3,4-oxadiazole (12), were syn-
thesized according to literature procedures.17 The poly-
mers were obtained in toluene with Pd(PPh3)4 (0.5 mol
%) by taking 2, 4, and 12 in different molar ratios.
Because random hyperbranched polymer synthesized by
the one-pot Suzuki approach has a large number of
bromo- and boronic ester end groups which are detri-
mental for light-emitting applications, the monofunc-
tional monomers 2-bromo-9,9-dioctylfluorene (6) and
9,9-dioctylfluorene-2-boronic acid (7) were therefore
used to cap the end groups. The reaction mixture
containing monomers 2, 4, and 12 was stirred and
refluxed for 5 days at the polymerization stage. Com-
pounds 6 and 7 were then added to the reaction mixture
together with additional Pd(PPh3)4 (0.5 mol %). The
reaction was continued for further 2 days. Slightly
yellow amorphous materials were obtained through
standard workup. The molar ratio was found to be a
key factor on the solubility of hyperbranched products.
The solubility was poor in common organic solvents for
hyperbranched polymers with higher contents of 12. The
polymers in the molar ratios (monomer 2/4/12) of 5:6.5:1
(POF5) and 10:11.5:1 (POF10) are soluble in common
organic solvents, e.g., toluene, THF, chloroform, and
methylene chloride. However, under molar ratios of
0:3:2 (POF) and 3:3.5:1 (POF3), the polymers cannot be
soluble or soluble partially in common organic solvents.
The molecular structures of the polymers were char-
acterized with high-resolution NMR spectroscopy and
elemental analysis (see Supporting Information). In the
proton NMR spectra, the characteristic signals of the
branching units and fluorene units were observed at
around δ ) 8.3 and 7.7 ppm, respectively. The ratios
† Fudan University.
‡ South China University of Technology.
* To whom correspondence should be addressed: Tel +86 21
5566 4188; Fax +86 21 6565 5123; e-mail wei-huang@fudan.edu.cn.
10.1021/ma050833f CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/13/2005

6756 Communications to the Editor
Macromolecules, Vol. 38, No. 16, 2005
Scheme 1
between the branching unit and fluorene of these two
polymers can be estimated from NMR spectra, that is
9:1 for POF5 and 17:1 for POF10, respectively. The
number-average molecular weight and the weight-
average molecular weight were determined by gel
permeation chromatography (GPC) against standard
polystyrene. The former for the polymers were 18 000
and 15 200, while the latter for the polymers were
47 000 and 37 000. The thermal properties of the
hyperbranched polymers were investigated using ther-
mogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). No distinct glass transitions were
observed for the polymers in the DSC curve (10 °C/min).
TGA reveals good thermal stabilities of these polymers
with the onset decomposition temperature (Td) of 424.8
and 425.6 °C under nitrogen, respectively. They are
much higher than those for the linear polyfluorene.18
b. Optical Properties. The photophysical charac-
teristics of the polymers were investigated in solution
and in the thin film. The UV-vis absorption and
photoluminescence (PL) spectra for POF10 in toluene
and in the film are shown in Figure 1. Solid films on
quartz plates used for fluorescence were prepared by
spin-coating with 10-3 mol/L CHCl3 solution at 1500
rpm. The polymer shows the maximum absorptions in
376 nm, attributed to π-π* transition of the polymer.
The PL spectrum in solution peaked at 418 nm. When
in film, there is a slight red shift by about 7 nm, and
the PL spectral pattern is similar to the one for polymer
in solution. The spectroscopic results of POF5 are almost
the same. Figure 2 shows the normalized PL emission
spectra of POF10 in the film annealed at room temper-
ature, 100, 150, and 200 °C for 1 and 3 h under a
nitrogen atmosphere. After the polymers annealing at
different temperatures for 1 h, there is nearly no change
in the PL spectra. With the annealing time prolonged,
the PL spectra show a little change at the range of above
460 nm at 150 and 200 °C. However, there was no
additional band emerged, which would change the pure
blue emission to an undesirable blue-green color.19 The
same phenomena were observed for POF5 under the
same condition. Interestingly, similar results can be
seen after annealing in the air, shown in Figure 3. The
normalized PL emission spectra of POF10 show very
little characteristic broad band for the aggregation in
the film annealed at room temperature, 100, 150 and
200 °C for 0.5 and 1 h in the air. To our best of
knowledge, no similar stable polyfluorene derivatives
annealed in the elevated temperatures in the air have
been reported. Apparently, with the introduction of
oxadiazole ring as the branching unit, the red-shift
phenomena of polyfluorenes are reduced markedly, and
the aggregation and excimer-forming of polymer chains
can be avoided in the hyperbranched fluorene-oxadia-
zole copolymers.
The PL quantum yields (ΦF) of POF5 and POF10 in
toluene were measured with 9,10-diphenylanthracene
as a reference standard (cyclohexane solution, ΦF
)
0.9).20 The quantum efficient yield reaches 0.46 and
0.54, respectively, which is similar to the hyperbranched
polyfluorene in the previous report.21 The absolute
quantum efficiencies (ηqe) of solid films were measured
Figure 1. Absorption and emission spectra of the POF10 in
solution (toluene) and in thin film.

Macromolecules, Vol. 38, No. 16, 2005
Communications to the Editor 6757
Figure 2. Photoluminescent spectra of POF10 in thin films annealing at 100, 150, and 200 °C: (a) for 1 h; (b) for 3 h (under N2).
Figure 3. Photoluminescent spectra of POF10 in thin films annealing at 100, 150 and 200 °C: (a) for 0.5 h; (b) for 1 h (in air).
Table 1. EL Performances of POF10 and POF5
device structure
λmax [nm]
voltage [V]
current density [mA/cm2]
brightness [cd/cm2]
QE max [%]
ITO/PEDOT/POF10/Ba/Al
ITO/PEDOT/PVK/POF10/Ba/Al
ITO/PEDOT/POF5/Ba/Al
423
446
423
446
10.4
9.6
11.0
8.6
24.9
6.0
179.1
1.2
63
51
150
7
0.44
1.5
0.15
1.0
ITO/PEDOT/PVK/POF5/Ba/Al
by the integrating sphere, which reaches 0.32 and 0.42
for POF5 and POF10.
being annealed in the elevated temperatures in the N2
and air.
Standard PLED devices were fabricated with the
configuration of ITO/PEDOT (50 nm)/polymer (70 nm)/
Ba (4 nm)/Al (150 nm) and ITO/PEDOT (50 nm)/PVK
(40 nm)/polymer/Ba (4 nm)/Al (150 nm). The EL spectra
of the two polymers are almost similar to the PL spectra
of the corresponding polymer films. The EL perfor-
mances of both POF10 and POF5 are concluded in Table
1. For POF10 the maximum brightness is 165 cd/m2 at
12 V, while it reaches 1150 cd/m2 at 14 V after adding
PVK layer. POF5 has similar results. Apparently, when
the hole-transporting layer PVK was deposited on the
PEDOT, the external maximum quantum efficiencies
improved a lot.
Acknowledgment. This work was financially sup-
ported by the National Natural Science Foundation of
China under Grants 60325412, 90406021, 30425020,
and 50428303 as well as the Shanghai Commission of
Science and Technology under Grants 03DZ11016 and
04XD14002 and the Shanghai Commission of Education
under Grants 03SG03 and 04SG06.
Supporting Information Available: Experimental de-
tails. This material is available free of charge via the Internet
at http://pubs.acs.org.
References and Notes
Conclusion. A novel “A2 + A′2 + B3” approach based
on Suzuki polycodensation was developed for the hy-
perbranched polymer synthesis. The polymer films
exhibited very stable blue light emission even after
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