Synthesis and characterization of polyfluorenes containing bisphenazine units

Article abstract of DOI:10.1002/pola.23966

Novel conjugated copolymers based on 9,9-dioctylfluorene and bisphenazine (BP) were synthesized by Suzuki polymerization. Energy transfer from the conjugated main chain to the BP moieties was observed. Full energy transfer was achieved when the molar cont

Full text of DOI:10.1002/pola.23966

Synthesis and Characterization of Polyfluorenes Containing
Bisphenazine Units
MING WANG,1,2 HUI TONG,1 YANXIANG CHENG,1 ZHIYUAN XIE,1 LIXIANG WANG,1 XIABIN JING,1 FOSONG WANG1
1State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
2Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
Received 8 December 2009; accepted 4 February 2010
DOI: 10.1002/pola.23966
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Novel conjugated copolymers based on 9,9-dioctyl-
fluorene and bisphenazine (BP) were synthesized by Suzuki
polymerization. Energy transfer from the conjugated main chain
to the BP moieties was observed. Full energy transfer was
achieved when the molar content of the bisphenazine was 20%
(20BPPF) in toluene solution. The similar phenomena were
observed even for 1% bisphenazine content copolymer (1BPPF)
in film. The lowest occupied molecular orbital (LUMO) energy
levels (ꢀ3.06 eV) of the copolymers were lower than that of the
polyfluorene homopolymer (PFO; ꢀ2.65 eV), indicating that the
introduction of the BP unit was benefit to electron injection.
Single-layer electroluminescent devices (ITO/PEDOT:PSS/poly-
mer/LiF/Al) were fabricated to investigate their electrolumines-
cence (EL) performances. The maximum brightness and
current efficiency of all BPPF copolymers surpassed the PFO
homopolymer. The best single-layer device was based on
5BPPF, with a maximum brightness of 1532 cd/m2 and current
efficiency of 1.09 cd/A. Much higher efficiency could be
achieved for multilayer EL devices of 5BPPF (ITO/PEDOT:PSS/
PVK/polymer/TPBI/LiF/Al), which showed a maximum current
C
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efficiency of 10.01 cd/A. 2010 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 48: 1990–1999, 2010
KEYWORDS: bisphenazine; energy transfer; electrochemistry;
heteroatom-containing polymers; light-emitting diodes (LED);
polyfluorene
INTRODUCTION Polymer light-emitting diodes (PLEDs) have
received much attention of researchers in the past decades
because of their potential application in the large-area, flat-
panel display field.1–18 Among many promising conjugated
polymers, polyfluorene (PF), and its derivatives have been
widely studied for PLEDs because of their processibility, high
photoluminescence efficiency, and good hole-transporting
properties.19–25 On the basis of the idea of energy transfer
from a host to a dopant, blue-emissive PF has also been
used as a host to realize green, yellow, and even red emis-
sion when a corresponding dopant is incorporated into the
main chain or side chain of PF.26–29 However, high lowest
occupied molecular orbital (LUMO) energy level and poor
electron-transporting properties of PFs result in a high elec-
tron injection barrier and imbalance of charge carrier trans-
port for the PLED application,30 which will greatly limit their
device efficiencies.
composed of dioctylfluorene and dibenzo[a,c]phenazine
units. The dibenzo[a,c]phenazine moieties greatly improved
PLED performance of the copolymers. All the copolymers
achieve much more efficient blue electroluminescence com-
pared to PF homopolymer (PFO), with the maximum bright-
ness of 1260–4600 cd/m2 and luminous efficiency of 0.47–
2.69 cd/A.30
Our group has reported a series of novel acene-type conju-
gated molecules containing n-type pyrazine units in a previ-
ous paper.38 With the increasing the number of pyrazine
rings and the conjugation length of the molecules, the LUMO
energy levels significantly decrease from ꢀ3.24 to ꢀ3.78 eV,
according to the electrochemical data. At the same time, their
absorption and emission peaks are red shifted step by step.
In dilute solution, their emission color can be easily tuned
from blue to even red. Their high electron affinity, high envi-
ronmental stability, and ease of structural modification make
these pyrazine-containing conjugated molecules excellent
candidates as a new class of n-type semiconductors.
It is well known that introduction of n-type units into conju-
gated polymers could improve their electron injection and
transport properties.31–33 Recently, many PF derivatives con-
taining various n-type building blocks were reported in the
literatures,27,34–37 such as benzothiadiazole, pyridine, naph-
thoselenadiazole, quinoxaline, oxadiazole, etc. For example,
Samon A. Jenekhe’s group have synthesized a series of PFs
In this article, we reported the synthesis and characteriza-
tion of a series of new PFs containing bisphenazine (BP), a
pyrazine-containing conjugated molecule, as the n-type unit
(Chart 1). As shown in Scheme 1, the copolymers were syn-
thesized by the Suzuki coupling reaction, containing 1, 5, 10,
Additional Supporting Information may be found in the online version of this article. Correspondence to: L. Wang (E-mail: lixiang@ciac.jl.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1990–1999 (2010) 2010 Wiley Periodicals, Inc.
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ARTICLE
CHART 1 The structures of the
BP, PFO, BPPF copolymers.
and 20 mol % bisphenazine, denoted 1BPPF, 5BPPF, 10BPPF,
and 20BPPF, respectively. The BP unit was chosen because
two pyrazine rings in its molecular structure could promise
it to be a strong electron acceptor.39 Its introduction into
the main chains of BPPF copolymers may improve the
electron injection properties and enhance the recombination
efficiency of electrons and holes, which would ultimately
obtain highly efficient electroluminescence while using a sta-
ble LiF/Al cathode instead of a very active Ca/Al cathode in
PLEDs. On the other hand, BP unit can also be used as a
dopant to tune the emission color of the copolymers. Photo-
physical properties of the copolymers were studied both in
dilute solutions and in thin films. PLEDs based on these
copolymers as emissive layers were fabricated and
characterized.
lution under an argon atmosphere at a scan rate of 100 mV/
s and using ferrocene as standards. A glass carbon disk (2-
mm diameter) was used as the working electrode with a Pt
wire as the counter electrode and an Ag/AgCl electrode as
the reference electrode.
EL Device Fabrication and Characterization
The ITO glass plates were degreased in an ultrasonic solvent
bath and then dried in a heating chamber at a temperature
of 120 ꢁC. The PLEDs with the configuration of the organic
light-emitting diodes (OLED) were fabricated with the PFO
and copolymers as the emissive material, poly(N-vinyl-carba-
zole) (PVK) as the hole-transport/electron-blocking layer
and 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI) as
the electron transport/hole-blocking layer Type
I ITO/
PEDOT:PSS/polymer/LiF/Al, Type II ITO/PEDOT:PSS/PVK/
polymer/LiF/Al, Type III ITO/PEDOT:PSS/polymer/TPBI/
LiF/Al and Type IV ITO/PEDOT:PSS/PVK/polymer/TPBI/
LiF/Al. The PSS-doped PEDOT was spin-coated onto the
treated ITO at 3000 rpm for 1 min and heated for 1 h at
120 ꢁC to obtain an approximate thickness of 40 nm. PVK
was spin-coated from its 0.5 mg/mL solution in toluene to
give a 20-nm-thick film on PEDOT. This film was dried in a
EXPERIMENTAL
Measurements
1H NMR spectra data were recorded on a Bruker Avance
300-MHz NMR spectrometer in CDCl3 with TMS standard.
Elemental analysis was performed by Bio-Rad elemental
analysis system. Thermogravimetric analysis (TGA) measure-
ments were performed on a Perkin-Elmer series 7 analysis
systems under N2 at a heating rate 10 ꢁC/min. Molecular
mass spectra of intermediates were measured by means of
LDI-1700 MALDI-TOF mass spectroscopy. Molecular weight
and polydispersity of the polymer were determined by gel
permeation chromatography (GPC) on a Waters 410 instru-
ment with polystyrene as standards and THF as eluent. UV–
vis absorption (UV) spectra were recorded with a Perki-
nElmer Lambda35 UV/Vis Spectrometer. Photoluminescence
(PL) spectra were obtained on a PerkinElmer LS50B Lumi-
nescence spectrometer. The electroluminescence (EL) spec-
tra, Commission Internationale de L’Eclairage (CIE) coordi-
nates, current–voltage, and brightness–voltage characteristics
of devices were measured with a Spectrascan PR650 spectro-
photometer at the forward direction and a computer-con-
trolled Keithley 2400 under ambient condition. Cyclic vol-
tammetry (CV) was performed on a Chi660b electrochemical
ananlyzer with a three-electrode cell in 0.1 M tetrabutylam-
monium perchlorate (Bu4NClO4) in dry dichloromethane so-
ꢁ
vacuum at 80 C for 1 h. The copolymer layer (about 70 nm)
was then spin-coated onto the PVK/PEDOT/ITO-coated glass
substrate in fresh toluene solution (10 mg/mL). Because of
the partial solubility of PVK in toluene, the interface between
PVK and the copolymers was not flat; the total thickness of
the PVK/copolymer bilayers was measured instead to get
the copolymer layer thickness. Films of TPBI thickness
20 nm were obtained by evaporation from resistively heated
in a vacuum evaporator. Finally, a thin layer of LiF (1 nm)
followed by a layer of aluminum (100 nm) was deposited
in a vacuum thermal evaporator through a shadow mask at
a pressure of 3 ꢂ 10ꢀ4 to 4 ꢂ 10ꢀ4 Pa. All the device fabri-
cation and characterization were carried out at ambient
conditions. The active area of the devices was 12 mm2.
The EL spectra, CIE coordinates, current–voltage and bright-
ness–voltage characteristics of the devices were measured
with
a Spectrascan PR650 spectrophotometer at the
forward direction and a computer-controlled Keithley 2400
instrument.
SYNTHESIS AND CHARACTERIZATION OF POLYFLUORENES, WANG ET AL.
1991

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic routes of com-
pound 5, B2F and polymers.
Materials
Hz, 4H), 4.00 (t, J ¼ 7.2 Hz, 4H), 2.01 (t, J ¼ 6.9 Hz, 4H),
1.85–1.70 (m, 22H), 1.48–1.30 (m, 28H), 0.90 (t, J ¼ 6.4 Hz,
6H). Anal. Calcd. for C62H74N2O4: C, 81.72; H, 8.19; N, 3.07.
Found: C, 81.41; H, 8.14; N, 3.09. m/z [MALDI-TOF]: 912 [M
þ H]þ.
All reagents were purchased from Aldrich, Acros, or
TCI companies. 2,7-di-t-butyl-pyrene-4,5,9,10-tetraones (1),40
3,6-dibromobenzene-1,2-diamine (4),41 9,9-dioctylfluorene-2-
diboronic acid ester (6),42 9,9-dioctylfluorene-2,7-diboronic
acid ester (7),43 and compound 238,44 were synthesized
according to the reported methods.
Compound 5
Synthesis
Compound 3
A solution of compound 3 (1.1 g, 1.2 mmol) and 4 (0.43 g,
1.6 mmol) in 140 mL acetic acid was degassed and heated at
reflux for 20 h. The mixture was cooled to room tempera-
ture, and the precipitated product was filtered. The crude
product was dried under vacuum and purified by column
chromatography on silica gel with chloroform: petroleum
ether (1:2) as the eluent to afford the key compound 5 as a
yellow solid (1.2 g, 85%).
Into a stirred solution of 1 (0.78 g, 2.1 mmol) in 100 mL
acetic acid compound 2 (1.0 g, 1.7 mmol) in 30 mL acetic
acid was added dropwise for 30 min at 80 ꢁC. The mixture
was stirred for 4 h, and the precipitate was filtered. This
crude product was dried under vacuum and purified by col-
umn chromatography on silica gel with chloroform : petro-
leum ether (2:1) as the eluent to afford compound 3 as a
red solid (1.14 g, 74%).
1H NMR (300 MHz, CDCl3): d (ppm) 9.73 (d, J ¼ 5.1 Hz, 4H),
8.28 (s, 2H), 7.92 (s, 2H), 7.18 (d, J ¼ 3.9 Hz, 4H), 6.82 (d, J
¼ 3.9 Hz, 4H), 3.97 (t, J ¼ 6 Hz, 4H), 1.83–1.70 (m, 22H),
1.49–1.30 (m, 28H), 0.91 (t, J ¼ 6.4 Hz, 6H). Anal Calcd for
1H NMR (300 MHz, CDCl3): d (ppm) 9.69 (s, 2H), 8.64 (s,
2H), 8.40 (s, 2H), 7.28 (d, J ¼ 6.4 Hz, 4H), 6.89 (d, J ¼ 7.2
1992
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
C68H76Br2N4O2: C, 71.57; H, 6.71; N, 4.91. Found: C, 71.19;
20BPPF: Yield: 70%, 1H NMR (300 MHz, CDCl3) d (ppm):
9.85 (m, 1H), 8.51 (m, 0.5H), 8.22 (s, 0.5H), 7.85–7.68 (m,
6H), 7.31 (m, 1H), 6.84 (m, 1H), 4.00 (s, 1H), 2.20 (br, 3H),
1.90–0.98 (m, 37H), 0.95–0.60 (m, 11.5H). Calculated: C,
87.12; H, 9.32; N, 2.21. Found: C, 86.90; H, 9.52; N, 1.88.
H, 6.51; N, 4.12. m/z [MALDI-TOF]: 1141 [M þ H]þ.
Compound B2F
Model compound (B2F) was prepared by the Suzuki reac-
tion. To a flask containing a degassed solution of 5 (0.23 g,
0.2 mmol), 9,9-dioctylfluorene-2-diboronic (0.21 g, 0.44
mmol) acid ester in 20 mL toluene added 10 mL of an aque-
ous solution of 2 M K2CO3. The Pd(PPh3)4 (5 mg, 0.004
mmol) in 10 mL toluene was added into the mixture. The
mixture was stirred 2 days at 90 ꢁC and be cooled to the
room temperature. The aqueous phase was extracted with
CH2Cl2. The CH2Cl2 extracts were combined with the organic
phase and washed with H2O. The solution was dried with an-
hydrous Na2SO4. After most of the solvent was removed, the
crude product was dried under vacuum and purified by col-
umn chromatography on silica gel with chloroform: petro-
leum ether (1:1) as the eluent to afford the product as a yel-
low solid (0.32 g, 92%).
1
The H NMR spectra of other copolymers were similar to the
spectra of 20BPPF copolymer, except that the relative inten-
sities of signals are different due to the different composition
of the copolymers.
RESULTS AND DISCUSSION
Synthesis and Characterization of the Copolymers
The synthetic routes of the monomers and the correspond-
ing copolymers were outlined in Scheme 1. The compound 3
was synthesized according to the reported methods, from
2,7-di-t-butyl-pyrene-4,5,9,10-tetraones (1) and the com-
pound 2 in 74% yield. The key monomer 5 was obtained by
the reaction of 3 and 3,6-dibromobenzene-1,2-diamine (4) in
a high yield (85%). By the Suzuki coupling reactions of 5, 6,
and 7 in an argon atmosphere with Pd(PPh3)4 as the cata-
lysts, we obtained the model compound B2F and polymers.
The molar ratio of BP in the copolymers was controlled by
adjusting the molar ratio of 5, 6, and 7. The molar ratio
between the dibromides and the diboronic acid ester was
1:1. All the copolymers could be soluble in common organic
solvents, such as toluene, chloroform, dichloromethane, and
tetrahydrofuran (THF).
1H NMR (300 MHz, CDCl3): d (ppm) 9.79 (s, 2H), 9.68 (s,
2H), 8.54 (s, 2H), 8.44 (d, J ¼ 8.1 Hz, 2H), 8.16 (s, 2H), 8.00
(d, J ¼ 8.1 Hz, 2H), 7.89 (d, J ¼ 7.8 Hz, 2H), 7.76 (s, 2H),
7.49–7.27 (m, 10H), 6.87 (d, J ¼ 8.1 Hz, 4H), 3.99 (t, J ¼ 6.3
Hz, 4H), 2.08–0.60 (m, 124H). Anal Calcd for C126H158N4O2:
C, 85.96; H, 9.05; N, 3.18. Found: C, 85.89; H, 9.08; N, 3.12.
m/z [MALDI-TOF]: 1760 [M þ H]þ.
1H NMR spectra of the copolymers were in good agreement
with their structures. The number-average molecular weights
(Mn) of these polymers were determined to be 7.3 ꢂ 104 to
12.7 ꢂ 104 with a polydispersity index of 2.5–3.3 by GPC
using polystyrene as the standard in THF (Table 1). The
thermal properties of the polymers were evaluated by TGA.
The TGA measurement revealed that the onset decomposi-
tion temperatures of the BPPF copolymers and PFO homo-
polymer under nitrogen were similar at 436 ꢁC, which indi-
cated their good thermal stability (see in Fig. S1 in
Supporting Information).
General Polymerization Procedure
The polymer was prepared from palladium-catalyzed Suzuki
coupling reaction. All handling of catalysts and polymeriza-
tion was done in an argon atmosphere. For example, for the
synthesis of 5BPPF, to a stirred solution of 9,9-dioctylfluor-
ene-2,7-diboronic (0.2792 g, 0.5 mmol) acid ester, 9,9-dioc-
tyl-2,7dibromofluorene (0.2194 g, 0.4 mmol), and compound
5 (0.1141 g, 0.1 mmol) in 10 mL toluene and 5 mL 2 M
K2CO3 solution in water were added the catalysts, Pd(PPh3)4
(5.8 mg, 5 mol %) in 5 mL toluene and Aliquat 336 (0.1 g,
ꢁ
0.25 mmol). The mixture was stirred for 2 days at 80 C and
be cooled to the room temperature. The mixture was poured
into methanol. The precipitate was collected by filtration,
dried and then dissolved in chloroform. The solution was
washed with water and dried with anhydrous Na2SO4. After
most of the solvent was removed, the residue was poured
into stirred methanol to obtain polymer fiber. This polymer
fiber was further purified by extracting with acetone for 24
h. The reprecipitation procedure in chloroform/methanol
was then repeated three times. The final product was dried
in vacuum. Random copolymers (BPPF) and the correspond-
ing poly(9,9-dioctylfluorene) (PFO) were synthesized by
Suzuki coupling polymerization. The molar ratio of BP in the
copolymers was controlled by adjusting the molar ratio
between 9,9-dioctylfluorene and 5. Yield: 70–83%.
Photophysical Properties
Normalized optical absorption spectra of B2F, PFO, and four
BPPF copolymers in toluene solutions (10ꢀ5 M) were shown
in Figure 1. The main absorption band, which was associated
with the p–p* transition of the conjugated backbone,45 pro-
gressively blue-shifted from 389 nm in PFO homopolymer to
370 nm in 20BPPF, indicating the reduction of the effective
conjugation length as reported in other papers.27,46 The full
width at half-maximum (FWHM) of the absorption spectra
increased with increasing BP content, suggesting greater in-
homogeneous broadening due to the local disorder induced
by the presence of BP units.27 For all copolymers, a new
absorption band at 436 nm was found, and it increased in
intensity with increasing BP amount from 1 to 20%. At the
same time, a similar absorption peak at 436 nm was also
observed for the model compound B2F in toluene solution
(10ꢀ5 M). Thus, it is reasonable to assign the new absorption
band at 436 nm to the B2F moieties in copolymers.
PFO: Yield: 82%, 1H NMR (300 MHz, CDCl3) d (ppm): 7.85
(br, 2H), 7.68 (br, 4H), 2.15 (br, 4H), 1.15 (br, 20H), 0.81 (br,
10H). Calculated: C, 89.63; H, 10.37. Found: C, 89.85; H,
10.14.
SYNTHESIS AND CHARACTERIZATION OF POLYFLUORENES, WANG ET AL.
1993

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Molecular Weight and Thermal Properties of
Polymers
a
a
b
Polymer
Mn
Mw/Mn
Td (ꢁC)
PFO
8.9 ꢂ 104
7.3 ꢂ 104
8.3 ꢂ 104
8.2 ꢂ 104
12.7 ꢂ 104
2.5
2.6
2.8
3.1
3.3
438
434
436
437
436
1BPPF
5BPPF
10BPPF
20BPPF
a
Molecular weights were determined by GPC using polystyrene as a
standard.
b
Onset decomposition temperature measured by TGA under nitrogen.
The normalized PL emission spectra of PFO and the four
copolymers in 10ꢀ5 M toluene solutions were shown in Fig-
ure 2(a). The PL spectrum of PFO showed the blue emission
with peaks at 414 nm and 438 nm. Besides the two similar
blue emission peaks at 414 and 440 nm, a new green-yellow
emission peak at 523 nm was observed in the PL emission
spectrum of 1BPPF. As the BP content increased in the
copolymers, the blue emission steadily decreased and a new
green-yellow emission increased in intensity. In 20BPPF, only
the green-yellow emission at 523 nm was observed. As the
emission peak of B2F in toluene was at 511 nm, which was
very close to the green-yellow emission of copolymers, sug-
gesting this new green-yellow emission was resulted from
FIGURE 1 Normalized optical absorption spectra of 10ꢀ5 M sol-
utions of model compound (B2F), PFO and BPPF copolymers
in toluene solution.
maximum peak was red-shifted with increasing the solvent
polarity. Such positive solvatochromism indicates an intramo-
lecular charge transfer (ICT) excited state with a large dipole
moment.47 To further clarify its optical properties, the geom-
etry of B2F was optimized by density functional theory cal-
culations at the B3LYP/6-31G* level.48 The HOMO and LUMO
molecular orbitals (see in Fig. S2 in Supporting Information)
indicated that the LUMO and HOMO were localized on the
BP unit and the polymer main chain, respectively, which also
supported the existence of charge transfer.
the B2F moieties. Considering
a large spectra overlap
between the blue emission of PFO (414 and 438 nm) and
the absorption band of B2F (436 nm), energy transfer from
the conjugated main chain to the B2F moieties would hap-
pen. Thus, energy transfer would be more and more efficient
as the BP content increased. Finally, the original blue emis-
sion disappeared for 20BPPF, indicating a complete energy
transfer. The PL characteristics of B2F in dilute solution
(10ꢀ5 M) were investigated in different solutions with
increasing solvent polarity of toluene, THF, acetone, and DMF
[Fig. 2(b)]. The emission peaks of these solutions were at
511, 529, 543, and 555 nm, respectively. The PL emission
The normalized optical absorption spectra of thin films of
PFO and the four copolymers were shown in Figure 3(a).
Their absorption spectra were similar to those in toluene
solutions. The main absorption band was also progressively
blue-shifted from 380 nm in PFO homopolymer to 365 nm
in 20BPPF. Similarly, a new long wavelength band at 439 nm
was observed for BPPF copolymers, and its intensity
increased with increasing BP content. The absorption onsets
(471 nm) of the four copolymers were very close, which is
much red-shifted compared with that of PFO (413 nm). The
FIGURE 2 (a) Normalized PL emission (380 nm excitation) spectra of 10ꢀ5 M solutions of B2F, PFO, and BPPF copolymers in tolu-
ene and (b) normalized PL emission (410 nm excitation) spectra of 10ꢀ5 M solutions of B2F in toluene, THF, acetone, and DMF.
1994
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 3 (a) Normalized optical absorption and (b) normalized PL emission (380 nm excitation) spectra of thin films of PFO and
BPPF copolymers in toluene.
optical band gaps, calculated from these absorption onsets,
were 3.00 eV for PFO and 2.63 eV for copolymers, respec-
tively. Figure 3(b) showed the PL emission spectra of the co-
polymer in thin films. Again, only blue emission was
observed for PFO as in its solution. However, the PL spectra
of copolymers in thin films were different from those in sol-
utions. Even for 1BPPF, no blue emission peak could be
found in thin film and only a green-yellow emission around
522 nm was observed, suggesting much more efficient
energy transfer for copolymers in thin film. With increasing
BP content, the PL spectra were steadily red-shifted with
emission maximum ranging from 522 nm (1BPPF) to 560
nm (20BPPF), possibly due to the larger ICT effect of the
excited state in going from 1BPPF to 20BPPF, as other PFs
containing n-type units.30
way as PFO. All the four copolymers had similar HOMO
(around ꢀ5.69 eV) and LUMO (around ꢀ3.06 eV) energy lev-
els. Their LUMO energy levels were much lower than PFO.
The results confirmed that the importing of the BP units
really improved the electron injection. The energy levels of
PFO and BPPF copolymers in single-layer devices were
shown in Figure 4. For BPPF copolymers, the energy barrier
for electron injection was lower than PFO, which will facili-
tate the electron injection. However, the electron injection
barrier is still much higher than the hole injection, suggest-
ing the introduction of an electron transport layer is more
favorable in multilayer devices than the introduction of a
hole transport layer.
EL Properties
To investigate the EL properties and performances of the
polymers, the EL devices were fabricated with the configura-
tion of ITO/PEDOT:PSS (40 nm)/polymer (70 nm)/LiF (1
nm)/Al (100 nm).51 Instead of using very active Ca/Al cath-
ode for PF-based PLEDs as reported in many literatures,
much more stable LiF/Al cathode was selected here because
of the better electron injection ability of these copolymers.
Their EL spectra were shown in Figure 5. In comparison
with the blue-emissive PFO homopolymer, the EL emission
maximum of all the copolymers was about 520 to 534 nm,
and the EL spectra of four copolymers exhibited very good
spectral stability: the EL spectra kept unchanged when the
voltage increased from 8 to 16 V.52 The EL spectra of the
copolymers were nearly the same as the PL emission in thin
film, indicating that EL and PL phenomena originated from
Electrochemical Properties
The electrochemical properties of B2F and polymers in
dichloromethane were studied in a three-electrode electro-
chemical cell with Bu4NClO4 (0.1 M) as electrolyte and Ag/
AgCl as reference electrode. The highest occupied molecular
orbital (HOMO) and the LUMO energy level were determined
according to the empirical formula EHOMO ¼ ꢀ[4.8 ꢀ EFOC
þ
Eoonxset] and ELUMO ¼ ꢀ[4.8 ꢀ EFOC þ E
] eV.49 The half-
red
onset
wave potential (EFOC) of ferrocene (the standard) was 0.56
eV. B2F exhibited two reversible redox processes in the nega-
tive potential region, however only irreversible oxidation
process was observed. The onset reduction and oxidation
potentials were ꢀ1.10 and 1.48 eV for B2F, respectively, (see
in Fig. S3 in Supporting Information). Thus, LUMO and
HOMO energy levels of B2F were calculated to be ꢀ3.14 and
ꢀ5.72 eV. Similarly, HOMO level of PFO (ꢀ5.65 eV) was
obtained. However, no reduction process of the PFO was
found, so its LUMO energy level (ꢀ2.65 eV) was calculated
from the corresponding HOMO energy level and the band
gap (3.00 eV). The LUMO energy level of the B2F was much
low than the LUMO energy level of PFO, suggesting the intro-
duction of BP might be benefit to the electron injection.
Again, the reduction process of four copolymers were hardly
observed, perhaps because of the low content of BP units.50
Thus, their LUMO energy levels were obtained in the same
FIGURE 4 Energy levels of PFO and BPPF copolymers.
SYNTHESIS AND CHARACTERIZATION OF POLYFLUORENES, WANG ET AL.
1995

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 The EL spectra of Type I diodes (ITO/PEDOT/polymer/LiF/Al) based on BPPF copolymers under 8, 10, 12, 14, and 16V.
the same excited state.46 As shown in Table 2, the CIE coor-
dinates of the EL emission of the copolymers indicated the
green-yellow emission of the devices.
ciency among the copolymers was also given by 5BPPF, with
a maximum current efficiency of 1.09 cd/A, which is 17
times higher than that of PFO devices. Overall, incorporation
of BP unit in PFO main chain led to more stable EL spectra
and higher current efficiency.
Luminance–voltage characteristics of the Type I devices were
displayed in Figure 6(a). At the same voltage, the luminances
of copolymers diodes were higher than PFO devices. For all
devices of copolymers, the turn-on voltages were at 6.0–5.5
V, which were lower than that of PFO (7.0 V), suggesting bet-
ter electron injection and transport of copolymers. The maxi-
mum brightness of devices based on copolymers were all
much larger than that of PFO (74.5 cd/m2). Current effi-
ciency–current density characteristics of the Type I device
was displayed in Figure 6(b). The same current density, the
current efficiencies of the copolymers diodes were far
greater than PFO devices. The PFO homopolymer diode had
a maximum current efficiency of 0.06 cd/A. The best effi-
It is well known that introduction of buffer layers might
increase the device efficiencies. According to the HOMO and
LUMO energy levels of PVK and TPBI were estimated to be
ꢀ5.8 /ꢀ2.3 eV and ꢀ6.2/ꢀ2.7 eV, PVK (20 nm) hole-trans-
port layer and TPBI (20 nm) electron transport layer were
used to construct multilayer diodes of 5BPPF: Type II (ITO/
PEDOT/PVK/5BPPF/LiF/Al), Type III (ITO/PEDOT/5BPPF/
TPBI/LiF/Al), and Type IV (ITO/PEDOT/PVK/5BPPF/TPBI/
LiF/Al). All the data of the devices were summarized in Ta-
ble 3. The EL spectra of these diodes were similar to the cor-
responding Type I and were stable when the voltage
TABLE 2 EL Performance Data of the Devices with the Configuration of ITO/PEDOT:PSS/Polymer/LiF/Al
Maximum
Brightness (cd/m2)
Current
Polymer
kmax (nm)
Von (V)
Efficiency (cd/A)
CIE Coordinateb
a
PFO
424
520
532
532
534
7
74.5
620
0.06
0.44
1.09
0.36
0.29
(0.17, 0.11)
(0.31, 0.59)
(0.35, 0.60)
(0.36, 0.59)
(0.36, 0.59)
1BPPF
5BPPF
10BPPF
20BPPF
6
5.5
5.5
5.5
1,532
778
550
a
b
Turn-on voltage.
Measured at the voltage of 8 V.
1996
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ARTICLE
FIGURE 6 Luminance–voltage (a) and current efficiency–current density (b) for the PFO and BPPF copolymers.
TABLE 3 EL Performance Data of the Devices with Different Configuration Based on 5BPPF
Device
Maximum
Brightness (cd/m2)
Current
Configuration
kmax(nm)
Vona(V)
Efficiency (cd/A)
CIE Coordinatesb
Type I
532
532
528
528
5.5
5.5
5.0
5.0
1,532
1,480
5,269
5,939
b
1.09
1.38
(0.35, 0.60)
(0.34, 0.59)
(0.33, 0.59)
(0.33, 0.59)
Type II
Type III
Type IV
8.68
10.01
a
Turn-on voltage.
Measured at the voltage of 8 V.
FIGURE 7 The EL spectra of different device structures for 5BPPF under 6, 8, 10, 12, 14, and 16V.
SYNTHESIS AND CHARACTERIZATION OF POLYFLUORENES, WANG ET AL.
1997

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 8 Luminance–voltage (a) and current efficiency–current density (b) for 5BPPF under different device structures.
increases from 6 to 16 V (Fig. 7). The EL emission maximum
of Types III and IV even exactly matched its PL emission in
thin film, which also proved the EL emission arose solely
from the copolymer layer.
and maximum brightness of 5939 cd/m2 with the CIE coor-
dinates of (0.33, 0.59). These results clearly demonstrated
that BPPF copolymers were novel efficiency green-yellow
emission materials for PLEDs.
Addition of the PVK layer in Type II diode, EL performance
was similar to the corresponding Type I diode with a very
little change of the current efficiency change from 1.09 to
1.38 cd/A. In contrast, introduction of TPBI significantly
increased the current efficiency to 8.68 cd/A in Type III
devices. The maximum brightness (5269 cd/m2) was also
much higher than the Types I and II diodes. Both the PVK
(20 nm) and TPBI (20 nm) were added in Type IV diodes,
which further improved the EL performance. The maximum
current efficiency and the maximum brightness even reached
10.01 cd/A and 5939 cd/m2. Overall, by constructing the
multilayer devices, especially introduction of the electron-
transport layer (TPBI), the EL performances were signifi-
cantly improved compared to the corresponding single-layer
device, as shown in Figure 8.
This work was supported by the Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences (CX07QZJC-
24), the National Natural Science Foundation of China (No.
50803062, 60977026, and 20904055), the Science Fund for
Creative Research Groups (No. 20621401), and the 973 Project
(2009CB623601).
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In this article, we synthesized a series of novel PFs contain-
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