Synthesis and characterization of a new series of blue fluorescent 2, 6-linked 9, 10-diphenylanthrylenephenylene copolymers and their application for polymer light-emitting diodes

Article abstract of DOI:10.1021/ma100195m

A series of new 9, 10-diphenylanthracene-based, 2, 6-linked blue-light-emitting copolymers bearing hole- or electron-transporter as well as bulky substituent were successfully synthesized. Photophysical, thermal, electrochemical, and electroluminescence (EL) properties of these copolymers were studied and characterized. Bright and efficient blue fluorescence in the solid state was achieved by incorporating bulky substituent into the copolymer backbone. Both hole- and electron-transport-substituted copolymers apparently enhanced the electroluminescent performance of their polymeric light-emitting diodes (PLEDs). A diphenylvinyl-bearing copolymer (pDPV) PLED exhibited sky-blue EL (λ^ET_max=473 nm, CIE_x,y=0.16,0.28) with peak luminous efficiency of 2.21 cd/A; a N-carbazole bearing copolymer (pCBZ) PLED displayed a blue EL (λ^EL_max469 nm, CIE_x,y=0.15, 0.22) with peak luminous efficiency of 2.15 cd/A OXD-7 (1,3-bis(2-(4-tertbutylphenyl)-1,3,4-oxadiazol-5-yl)benzene) as an electron-transporting dopant was found to improve the performance of PLED significantly. A better balanced hole/electron charge carrier was ascribed to electrontransporting, 1, 3, 4-oxadiazole-bearing copolymer (pOXD) PLED. It showed a very mild efficiency rolls off: only 0.13 cd/A luminous efficiency drops from current densities of 10-100 mA/cm², corresponding to EL brightness of 169-1558 cd/m².

Full text of DOI:10.1021/ma100195m

Macromolecules 2010, 43, 3613–3623 3613
DOI: 10.1021/ma100195m
Synthesis and Characterization of a New Series of Blue Fluorescent
2,6-Linked 9,10-Diphenylanthrylenephenylene Copolymers and Their
Application for Polymer Light-Emitting Diodes
Hung-Yang Chen,†,‡,§ Chin-Ti Chen,*,† and Chao-Tsen Chen*,§
†Institute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, ‡Nano Science and Technology Program,
TIGP, Academia Sinica, Taipei, Taiwan 11529, and §Department of Chemistry, National Taiwan University,
Taipei, Taiwan 10617
Received January 26, 2010; Revised Manuscript Received March 18, 2010
ABSTRACT: A series of new 9,10-diphenylanthracene-based, 2,6-linked blue-light-emitting copolymers
bearing hole- or electron-transporter as well as bulky substituent were successfully synthesized. Photo-
physical, thermal, electrochemical, and electroluminescence (EL) properties of these copolymers were studied
and characterized. Bright and efficient blue fluorescence in the solid state was achieved by incorporating
bulky substituent into the copolymer backbone. Both hole- and electron-transport-substituted copolymers
apparently enhanced the electroluminescent performance of their polymeric light-emitting diodes (PLEDs).
A diphenylvinyl-bearing copolymer (pDPV) PLED exhibited sky-blue EL (λEmLax=473 nm, CIEx,y=0.16, 0.28)
with peak luminous efficiency of 2.21 cd/A; a N-carbazole bearing copolymer (pCBZ) PLED displayed a blue
EL (λEmLax=469 nm, CIEx,y=0.15, 0.22) with peak luminous efficiency of 2.15 cd/A. OXD-7 (1,3-bis(2-(4-tert-
butylphenyl)-1,3,4-oxadiazol-5-yl)benzene) as an electron-transporting dopant was found to improve the
performance of PLED significantly. A better balanced hole/electron charge carrier was ascribed to electron-
transporting, 1,3,4-oxadiazole-bearing copolymer (pOXD) PLED. It showed a very mild efficiency rolls off:
only 0.13 cd/A luminous efficiency drops from current densities of 10-100 mA/cm2, corresponding to EL
brightness of 169-1558 cd/m2.
Introduction
several methods, such as the side-chain groups of the polymer.7
There are many more cases having an extension of the polymer
Blue-light-emitting polymers have been extensively studied
due to practical applications of full-color flat-panel displays or
renovated solid-state lightings (SSLs).1 They can serve as the
light-emitting layer or the host material for luminous dopants
in generating other long-wavelength colors in polymer light-
emitting diodes (PLEDs). Blue color fluorophores commonly
consist of chemical structures like phenyl, carbazole, fluorene,
or heterocyclic rings, such as thiophene, pyridine, and furan.1
Over the past two decades, a variety of the wide-band-gap
polymers with blue-light emission have been developed, they are
poly(p-phenylene), poly(dibenzosilole),2 polyfluorenes, poly-
pyridines, and polycarbazoles, etc. For blue fluorophores, ant-
hracene is the first reported organic material observed for
electroluminescence (EL) in 1963 by Pope and co-workers.3
Since then, many anthracene derivatives have been developed
and applied for organic light-emitting diodes (OLEDs) because
of their pure blue fluorescence and high solution fluorescence
quantum yields (Φfs). Among them, 9,10-substituted anthra-
cene derivative has been one of practical chemical structures for
blue-emitters4 or host materials for downhill energy transfer to
green- or red-emitters.5 In the molecular design of these materi-
als for OLEDs, electron/hole transporting4a,d,e as well as bulky
π-conjugated moieties4b,c are often incorporated into the
anthracene derivatives. Many of them exhibit excellent efficien-
cies. However, for PLEDs, few anthracene-based polymeric
materials have shown satisfactory EL efficiency so far.6-9 The
anthracene unit can be incorporated into the polymer chain through
chain through 9,10-linked anthrylene,6 but very few along 2,6-
positions of anthracene derivatives.8 In spite of high solution
fluorescence quantum yields (Φf), when fabricated into PLEDs,
none of these anthracene-derived blue light-emitting polymers
exhibited luminous efficiency more than 1 cd/A, and the electro-
luminance of these polymers rarely exceeded 1000 cd/m2. More-
over, either 9,10- or 2,6-linked anthrylene polymers often show
red-shifting EL having downgrade blue color purity. Therefore,
the development of efficient and bright anthracene-derived blue
light-emitting polymers for PLEDs applications still remains a
challenge.
Inthis work, we report a series of new 9,10-diphenylanthracene
(DPA)-derived blue fluorescent copolymers. Since the extension
of the copolymer chain is through the 2,6-positions of DPA, these
copolymers enable the chemical grafting with hole-transporting,
electron-transporting, or bulky π-conjugation moiety on DPA
unit. In order to preserve the intrinsic wide-band-gap energy of
DPA for blue EL, we adopt a dioctyl-substituted phenylene unit
as the comonomer for the copolymerization with 2,6-linked
anthrylene derivatives. Such phenylene unit plays a role of the
π-conjugation twister between each 2,6-linked anthrylene unit
in the polymer backbone. The synthesis, characterization, and EL
properties of these novel blue light-emitting copolymers are
reported herein. Influences of the substituted group, including
hole injection/transport, electron injection/transport, and bulky
moieties, on the EL performance of these blue light-emitting
copolymers are also discussed. To the best of our knowledge, the
EL performance of 2,6-linked anthrylenephenylene copolymers
reported here is one of the best among all anthracene-derived
PLEDs reported so far. Our results also provide insightful
*Corresponding authors. E-mail: cchen@chem.sinica.edu.tw, chenct@
ntu.edu.tw.
r 2010 American Chemical Society
Published on Web 03/30/2010
pubs.acs.org/Macromolecules

3614 Macromolecules, Vol. 43, No. 8, 2010
Chen et al.
information about the judicious design of blue-light-emitting
polymers based on DPA fluorophores for PLED applications.
N-(3-Bromophenyl)-N-phenylnaphthalen-1-amine, BrNPA. A
mixture of palladium acetate (0.09 g, 4 mol %), 1,1-bis-
(diphenylphosphino)ferrocene (0.444 g, 8 mol %), sodium
tert-butoxide (4.81 g, 50.0 mmol), m-dibromobenzene (3.54 g,
15 mmol), and N-phenyl-1-naphthylamine (2.19 g, 10.0 mmol)
was dissolved in toluene (15 mL) and heated at 110 °C for 48 h
under nitrogen. After cooling to room temperature, the solution
was filtered through Celite. The organic layer was removed
under reduced pressure, and the residue was purified by silica gel
column chromatography using hexanes as an eluent to give the
Experimental Section
General. 1H and 13C NMR spectra were recorded on a Bruker
AV-400 MHz or AV-500 MHz Fourier transform spectrometer
at room temperature. Elemental analyses (on a Perkin-Elmer
2400 CHN elemental analyzer), electron ionization (EI), or fast
atom bombardment (FAB) mass spectroscopy (MS) were per-
formed by the Elemental Analyses and Mass Spectroscopic
Laboratory, respectively, in-house service of the Institute of
Chemistry, Academic Sinica. The number- and weight-average
molecular weight of polymers were determined by gel permea-
tion chromatography (GPC) on a Waters GPC-1515 with a 2414
refractive index detector, using THF as the eluent and poly-
styrene as the standard. UV-vis absorption spectra were
recorded on a Hewlett-Packard 8453 diode array spectrophoto-
meter. Photoluminescence (PL) spectra were recorded on a
Hitachi fluorescence spectrophotometer F-4500. Glass transi-
tion temperatures (Tg’s) and thermal decomposition tempera-
tures (Td’s) of the copolymers were measured via differential
scanning calorimetry (DSC) and thermogravimetric analysis
(TGA) using Perkin-Elmer DSC-6 and TGA-7 analyzer systems,
respectively. The temperatures at the intercept of the curves on
the thermogram (endothermic, exothermic, or weight loss) and
the leading baseline were taken as the estimates for the onset Tg
and Td. Both solution and solid-state fluorescence quantum
yields of the copolymers were determined using the integrating
sphere method described by de Mello et al.10
1
product (1.36 g, 36%). H NMR (CDCl3, 400 MHz, δ/ppm):
7.87 (d, 2H, J=8.40 Hz), 7.77 (d, 1H, J=8.20 Hz), 7.48-7.43 (m,
2H), 7.38-7.34 (m, 1H), 7.30 (dd, 1H, J=7.32 Hz, J=1.12 Hz),
7.22-7.18 (m, 2H), 7.11-7.10 (m, 1H), 7.05 (dd, 2H, J=6.60 Hz,
J=1.16 Hz), 7.00-6.97 (m, 3H), 6.84-6.81 (m, 1H).
3-Bromophenyltriphenylsilane, BrTPS. To a magnetically
stirred solution of m-dibromobenzene (2.60 g, 11.0 mmol) in
anhydrous THF (100 mL) was cooled to -78 °C, 7.20 mL of
n-BuLi (1.6 M in hexane, 11.5 mmol) was added slowly under
nitrogen. After stirred at -78 °C for 3 h, triphenylsilyl chloride
(2.95 g, 10.0 mmol) was added into the solution in one portion.
The reaction mixture was gradually allowed to warm up to room
temperature during 16 h of stirring. The reaction mixture was
poured into water, and then this aqueous solution was extracted
with diethyl ether. The extracts were washed with water and
dried over magnesium sulfate. The crude product was purified
by silica gel column chromatography using hexanes as an eluent
to obtain a white powder (2.75 g, 60%). 1H NMR (CDCl3,
500 MHz, δ/ppm): 7.64 (t, 1H, J=1.10 Hz), 7.55-7.51 (m, 7H),
7.46-7.41 (m, 4H), 7.38-7.35 (m, 6H), 7.22 (d, 1H, J=9.61 Hz).
13C NMR (CDCl3, 400 MHz, δ/ppm): 138.6, 137.8, 136.3, 134.8,
133.3, 132.7, 129.8, 129.6, 128.0, 127.8, 123.0. HRMS (m/z):
[Mþ] calcd for C24H19BrSi 414.0439; found 414.0438.
Materials. All chemicals were purchased from Aldrich and
Acros Chemical Co. and were used without any further puri-
fication. All solvents such as toluene, DMSO, and THF were
distilled after drying with appropriate drying agents. The
˚
dried solvents were stored over 4 A molecular sieves before
5-(3-Bromophenyl)-1H-tetrazole, BrN4. 3-Bromobenzonitrile
(10.92 g, 60 mmol) was added to a dried DMF (300 mL) solution
containing sodium azide (19.50 g, 300 mmol) and ammonium
chloride (16.05 g, 300 mmol). The mixture was slowly heated to
refluxing temperature for 16 h under a nitrogen atmosphere.
Excess salts remained as a suspension even at high temperature.
After cooling the reaction mixture, it was then acidified with a
2 N HCl aqueous solution (attention: hydrazoic acid is formed!)
until acidic conditions were reached, after which a white powder
slowly appeared. The product was isolated by filtration and
washed thoroughly with water to eliminate excess salts. After
drying in a vacuum oven, the product was obtained as a white
usage. Compounds 1,4-dibromo-2,5-dioctylbenzene,11 1-bro-
mo-3-iodobenzene,12 3,6-di-tert-butyl-9H-carbazole,13 2,6-di-
bromoanthraquinone,14 and 1,1-diphenyl-2-(4-bromophenyl)-
ethane, BrDPV,15 were synthesized according to the literature
methods.
2,20-(2,5-Dioctyl-1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane), DOPTMBO. A flask charged with 1,4-dibro-
mo-2,5-dioctylbenzene (4.58 g, 10.0 mmol), bis(pinacolato)-
diboron (5.59 g, 22.0 mmol), PdCl2(dppf) (0.44 g, 6 mol %),
and potassium acetate (5.89 g, 60.0 mmol) in 60 mL of DMSO
was stirred at 80 °C for 6 h under a nitrogen atmosphere. After
cooling, the mixture was diluted with 50 mL of benzene and then
washed with H2O (3 ꢀ 50 mL). After drying over magnesium
sulfate, the organic phase was evaporated under reduced pressure,
and the residual solid was recrystallized from methanol to afford
1
solid with quantitative yields. H NMR (CDCl3, 400 MHz, δ/
ppm): 8.25 (s, 1H), 8.03 (d, 1H, J=7.96 Hz), 7.64 (d, 1H, J=
7.52 Hz), 7.40 (t, 1H, J=7.94 Hz). HRMS (m/z): [Mþ] calcd for
C7H5BrN4 224.9776; found 224.9779.
1
a white crystal (2.03 g, 37%). H NMR (CDCl3, 400 MHz, δ/
2-(3-Bromophenyl)-5-(3-(trifluoromethyl)phenyl)-1,3,4-oxadiazole,
BrOXD. 5-(3-Bromophenyl)-1H-tetrazole (11.41 g, 50.95 mmol)
was dissolved in dried anisole (200 mL) containing 3-(trifluoro-
methyl)benzoyl chloride (10.63 g, 50.95 mmol). 2,4,6-Collidine
(7.4 mL) was added dropwise to the mixture with stirring. After
the addition of collidine, the solution was heated to reflux for 4 h
under a nitrogen atmosphere. The solvent was removed under
reduced pressure, and then the product was purified by silica gel
column chromatography using hexanes containing 10% ethyl
acetate as the eluent to obtain a white powder (13.12 g, 70%). 1H
NMR (CDCl3, 500 MHz, δ/ppm): 8.38 (s, 1H), 8.34 (d, 1H, J=
7.96 Hz), 8.29 (t, 1H, J=1.78 Hz), 8.09 (d, 1H, J=7.86 Hz), 7.82
(d, 1H, J=7.90 Hz), 7.69 (t, 2H, J=7.91 Hz), 7.26 (t, 1H, J=
7.93 Hz). 13C NMR (CDCl3, 500 MHz, δ/ppm): 163.6, 134.9,
131.8 (q, JCF =132 Hz), 130.7, 130.1, 129.8, 129.7, 128.3 (ꢀ2),
125.5, 125.3, 124.5, 123.8, 123.7, 123.2, 122.4. HRMS (m/z): [Mþ]
calcd for C15H8BrF3N2O368.9850;found368.9847. Anal. Found
(Calcd) for C15H8BrF3N2O: C, 48.68 (48.81); H, 2.42 (2.18); N,
7.69 (7.59).
ppm): 7.50 (s, 2H), 2.78 (t, 4H, J=8.00 Hz), 1.51-1.45 (m, 4H),
1.41-1.25 (m, 44H), 0.86 (t, 6H, J=6.80 Hz). 13C NMR (CDCl3,
400 MHz, δ/ppm): 146.1, 136.4, 130.4, 83.2, 35.5, 33.8, 31.9,
29.9, 29.5, 29.3, 24.8, 22.6, 14.1. HRMS (m/z): [Mþ] calcd for
C34H60B2O4 554.4678; found 554.4677. Anal. Found (Calcd) for
C34H60B2O4: C, 73.82 (73.65); H, 10.66 (10.91).
9-(3-Bromophenyl)-3,6-di-tert-butylcarbazole, BrCBZ. A mixture
of 1-bromo-3-iodobenzene (5.47 g, 19.39 mmol), 3,6-di-tert-butyl-
9H-carbazole (5.31 g, 19.00 mmol), copper powder (3.66 g, 58.17
mmol), and potassium carbonate (8.02 g, 58.17 mmol) in DMF
(44 mL) was stirred at refluxing temperature for 24 h under a
nitrogen atmosphere. After cooling to room temperature, the reaction
mixture was diluted with ethyl acetate and filtered through Celite.
The filtrate was poured into water and then extracted with ethyl
acetate. The combined organic phase was washed with brine and
dried over magnesium sulfate. The subjection of the crude mixture to
silica gel chromatography (hexanes) afforded colorless oil (5.56 g,
1
68%). H NMR (CDCl3, 500 MHz, δ/ppm): 8.10 (d, 2H, J =
1.90 Hz), 7.71 (t, 1H, J=1.89 Hz), 7.54-7.48 (m, 2H), 7.46-7.41
(m, 3H), 7.33 (d, 2H, J=8.75 Hz), 1.44 (s, 18H).
2,6-Dibromo-9,10-diphenylanthracene, Br2DPA. To a solu-
tion of bromobenzene (9.36 g, 60.0 mmol) in anhydrous THF

Article
Macromolecules, Vol. 43, No. 8, 2010 3615
(300 mL) at -78 °C, 22.8 mL of n-BuLi (2.5 M in hexane,
57.0 mmol) was slowly added. The mixture was stirred at -78 °C
for 2 h, and then 2,6-dibromoanthraquinone (5.46 g, 15.0 mmol)
was added into the mixture immediately. The resulting mixture
was allowed to warm up to room temperature slowly and then
stirred for 24 h. The reaction mixture was poured into water and
then extracted with ethyl acetate. The combined organic extracts
were washed with brine and dried over magnesium sulfate. After
the organic solvent was removed under reduced pressure, the
residue was purified by column chromatography on silica gel
(ethyl acetate/hexanes, 1:4) to obtain the diol intermediate. A
mixture of the diol intermediate, potassium iodide (4.98 g,
30.0 mmol), sodium hypophosphite hydrate (10.60 g, 100.0 mmol
was stirred in acetic acid (100 mL) at refluxing temperature for
16h. After coolingtoroom temperature, the reaction mixturewas
poured into a mixture of ice and water. The precipitate was
filtered and washed with water. The residue was purified by
column chromatography on silica gel (dichloromethane/hexanes,
1:5) and then recrystallized from THF to afford a yellow powder
(3.50 g, 48%). 1H NMR (CDCl3, 500 MHz, δ/ppm): 7.80 (d, 2H,
J = 1.89 Hz), 7.62-7.55 (m, 6H), 7.52 (d, 2H, J = 9.47 Hz),
7.42-7.40 (m, 4H), 7.36 (dd, 2H, J=9.50 Hz, J=1.97 Hz). 13C
NMR (CDCl3, 500 MHz, δ/ppm): 137.7, 136.8, 131.1, 130.8,
129.1, 128.9, 128.7, 128.1, 120.2. HRMS (m/z): [Mþ] calcd for
C26H16Br2 485.9619; found 485.9612. Anal. Found (Calcd) for
C26H16Br2: C, 63.94 (63.96); H, 3.35 (3.30).
7.41 (d, 2H, J=9.5 Hz). 13C NMR (CDCl3, 500 MHz, δ/ppm):
164.7, 163.6, 138.9, 135.6, 134.6,134.5, 131.9 (q, JCF=132 Hz),
130.8, 130.1, 129.9, 129.8 (ꢀ2), 129.3, 129.2, 128.7, 128.5, 128.4,
127.0, 124.6, 124.4, 124.3, 123.8, 122.4, 120.9. HRMS (m/z): [Mþ]
calcd for C44H22Br2F6N4O2 910.0015; found 910.0014. Anal.
Found (Calcd) for C44H22Br2F6N4O2: C, 57.94 (57.92); H, 2.51
(2.43); N, 6.06 (6.14).
(3,30-(2,6-Dibromoanthracene-9,10-diyl)bis(3,1-phenylene))-
bis(triphenylsilane), Br2TPS. Br2TPS was synthesized from 3-
bromophenyltriphenylsilane, BrTPS (2.75 g, 6.6 mmol), and 2,6-
dibromoanthraquinone (0.80 g, 2.2 mmol) using a synthetic
method similar to Br2DPA. The desired product was obtained
by silica gel flash column chromatography (dichloromethane/
hexanes, 10:1) and then recrystallized from THF to afford a pale
yellow powder (0.36 g, 40%). 1H NMR (CDCl3, 400 MHz,
δ/ppm): 7.85 (s, 2H), 7.71 (dd, 2H, J=6.8 Hz, J=0.4 Hz), 7.66
(d, 2H, J=9.2 Hz), 7.63-7.58 (m, 14H), 7.55 (d, 2H, J=9.6 Hz),
7.42-7.34 (m, 22H). 13C NMR (CDCl3, 400 MHz, δ/ppm):
138.8, 137.0, 136.8, 136.4, 136.3, 136.2, 136.0, 135.1, 135.0, 133.9,
133.8, 132.4, 130.9, 129.7, 129.1, 128.8, 128.7, 128.3, 128.2, 128.1,
128.0, 127.9, 120.2. HRMS (m/z): [Mþ] calcd for C62H44Br2Si2
1002.1348; found 1002.1351. Anal. Found (Calcd) for C62H44Br2Si2:
C, 73.85 (74.10); H, 4.62 (4.41).
N,N0-(3,30-(2,6-Dibromoanthracene-9,10-diyl)bis(3,1-phenylene))-
bis(N-phenylnaphthalen-1-amine), Br2NPA. Br2NPA was syn-
thesized from N-(3-bromophenyl)-N-phenylnaphthalen-1-amine,
BrNPA (1.12 g, 3.01 mmol), and 2,6-dibromoanthraquinone
(0.364 g, 1.0 mmol) using a similar method for DiBrDPA. The
desired product was obtained by silica gel flash column chromato-
graphy (dichloromethane/hexanes, 1:10) as a bright-yellow
2,6-Dibromo-9,10-bis(4-(2,2-diphenylvinyl)phenyl)anthracene,
Br2DPV. Br2DPV was synthesized from 1,1-diphenyl-2-(4-bromo-
phenyl)ethene (2.09 g, 6.27 mmol) and 2,6-dibromoanthra-
quinone (0.98 g, 2.67 mmol) by using a synthetic method similar
to Br2DPA. The desired product was obtained by silica gel flash
column chromatography (dichloromethane/hexanes, 1:5) and
then recrystallized from THF to afford a yellow powder (1.04 g,
1
powder (0.49 g, 53%). H NMR (CDCl3, 500 MHz, δ/ppm):
7.99 (t, 2H, J=9.0 Hz), 7.80-7.84 (m, 4H), 7.68-7.72 (m, 4H),
7.35-7.48 (m, 12H), 7.13-7.28 (m, 12H), 6.84-6.96 (m, 6H).
13C NMR (CDCl3, 400 MHz, δ/ppm): 148.9, 148.1, 148.0, 143.4,
143.3, 138.4, 136.5, 135.3, 131.0, 130.5, 129.4, 129.2, 128.9,
128.7, 128.6, 128.5, 128.4, 127.1, 126.6, 126.5, 126.3, 126.2,
126.1, 124.2, 124.1, 124.0, 123.9, 122.6, 122.5, 122.3, 120.6,
120.5, 120.0. HRMS (m/z): [Mþ] calcd for C58H38Br2N2
920.1402; found 920.1412. Anal. Found (Calcd) for C58H38Br2N2:
C, 74.09 (75.49); H, 4.17 (4.15); N, 3.19 (3.04).
1
46%). H NMR (CDCl3, 500 MHz, δ/ppm): 7.76 (d, 2H, J=
1.9 Hz), 7.50 (d, 2H, J=9.3 Hz), 7.39-7.41 (m, 8H), 7.29-7.36
(m, 16H), 7.23 (d, 2H, J=4.3 Hz), 7.15 (d, 4H, J=8.14 Hz), 7.12
(s, 2H). 13C NMR (CDCl3, 500 MHz, δ/ppm): 143.6, 143.3,
140.3, 137.3, 136.6, 135.9, 130.8, 130.7, 130.4, 129.7, 129.1, 128.8,
128.7, 128.3, 127.7, 120.1. HRMS (m/z): [Mþ] calcd for C54H36Br2
842.1184; found 842.1174. Anal. Found (Calcd) for C54H36Br2:
C, 76.77 (76.78); H, 4.31 (4.30).
Copolymer pDPA. Aqueous potassium carbonate (2.0 M, 5.0 mL)
and three drops of phase transfer reagent Aliquate 336 were added
to a mixture of monomers, Br2DPA (0.49 g, 1.0 mmol) and 2,20-(2,5-
dioctyl-1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)
(0.55 g, 1.0 mmol), in freshly distilled toluene (20.0 mL). The mixture
was degassed by the freeze-pump-thaw method, and then Pd-
(PPh3)4 (12.0 mg, 1.0 mol %) was added under a nitrogen atmo-
sphere. The mixture was vigorously stirred at 85-90 °C for more
than 48 h under a nitrogen atmosphere. Polymer end-groups were
then capped by heating the mixture at 85-90 °C for 12 h with
phenylboronic acid (0.24 g) and then for another 12 h with 1-bromo-
4-tert-butylbenzene (0.43 g). After cooling to room temperature, the
mixture was poured into a solution (400 mL) of methanol and
deionized water (10:1) with vigorous stirring. The resulting precipi-
tate was isolated by filtration and redisolved in chloroform. It was
then further purified by a short column packed with neutral
aluminum oxide. The polymer solution was precipitated in acetone/
methanol (volume ratio=1:1), and the resulting copolymer pDPA
was obtained as a pale yellow powder with isolated yields of 87%
(0.55 g). 1H NMR (CDCl3, 400 MHz, δ/ppm): 7.71-7.65 (m, 4H),
7.57-5.51 (m, 10H), 7.36-7.31 (m, 2H), 7.12 (s, 2H), 2.50 (s, 4H),
1.30 (s, 4H), 1.25-1.20 (m, 20H), 0.79 (t, 6H, J=7.0 Hz). 13C NMR
(CDCl3, 400 MHz, δ/ppm): 140.7, 139.0, 138.0, 137.8, 137.0, 131.3,
131.1, 129.6, 129.2, 128.5, 127.7, 127.5, 126.7, 126.6, 32.7, 31.9, 31.4,
29.6, 29.3, 29.2, 22.6, 14.1. Anal. Found (Calcd): C, 89.35 (91.37); H,
8.26 (8.63).
9,90-(3,30-(2,6-Dibromoanthracene-9,10-diyl)bis(3,1-phenylene))-
bis(3,6-di-tert-butyl-9H-carbazole), Br2CBZ. Br2CBZ was syn-
thesized from 9-(3-bromophenyl)-3,6-di-tert-butylcarbazole,
BrCBZ (5.28 g, 12.2 mmol), and 2,6-dibromoanthraquinone
(1.46 g, 4.0 mmol) using a synthetic method similar to Br2DPA.
The desired product was obtained by silica gel flash column
chromatography (dichloromethane/hexanes, 1:5) and then re-
crystallized from THF to afford a white powder (2.50 g, 60%).
1H NMR (CDCl3, 500 MHz, δ/ppm): 8.12 (d, 4H, J=13.1 Hz),
8.02 (s, 2H), 7.86-7.80 (m, 4H), 7.71 (d, 2H, J=9.3 Hz), 7.65
(d, 2H, J=15.5 Hz), 7.56-7.47 (m, 12H), 1.45 (s, 18H), 1.43
(s, 18H). 13C NMR (CDCl3, 500 MHz, δ/ppm): 143.2, 139.3, 138.9,
138.8, 138.7, 135.9, 130.8, 130.2, 129.6, 128.9, 128.7, 128.6,
126.0, 123.8, 123.6, 120.7, 116.3, 109.2, 31.9. HRMS (m/z):
[Mþ] calcd for C66H62Br2N2 1040.3280; found 1040.3293. Anal.
Found (Calcd) for C66H62Br2N2: C, 75.75 (76.00); H, 6.07
(5.99); N, 2.72 (2.69).
5,50-(3,30-(2,6-Dibromoanthracene-9,10-diyl)bis(3,1-phenylene))-
bis(2-(3-(trifluoromethyl)phenyl)-1,3,4-oxadiazole), Br2OXD. Br2OXD
was synthesized from 2-(3-bromophenyl)-5-(3-(trifluoromethyl)-
phenyl)-1,3,4-oxadiazole, BrOXD (1.104 g, 3.00 mmol), and 2,6-
dibromoanthraquinone (0.364 g, 1.0 mmol) using a synthetic
method similar to Br2DPA. The desired product was obtained by
silica gel flash column chromatography (dichloromethane/ethyl
acetate, 1:5) and then recrystallized from THF to afford a white
powder (1.26 g, 57%). 1H NMR (CDCl3, 500 MHz, δ/ppm): 8.42
(t, 2H, J=6.5 Hz), 8.34 (d, 2H, J=6.5 Hz), 8.31 (d, 2H, J=
7.5 Hz), 8.22 (d, 2H, J = 14.0 Hz), 7.84 (q, 2H, J = 7.0 Hz),
7.74-7.77 (m, 4H), 7.63-7.69 (m, 4H), 7.51 (d, 2H, J=9.5 Hz),
Copolymer pDPV. pDPV was obtained as a yellow powder
with isolated yields of 69% (0.68 g). It was prepared from the
reaction of Br2DPV (0.82. g, 1.0 mmol) and 2,20-(2,5-dioctyl-1,4-
phenylene)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.55 g,

3616 Macromolecules, Vol. 43, No. 8, 2010
Scheme 1. Synthesis and Chemical Structures of 2,6-Linked Anthrylenephenylene Copolymers
Chen et al.
1.0 mmol) according to the procedure described for the synthesis
of pDPA. 1H NMR (CDCl3, 400 MHz, δ/ppm): 7.61-7.81
(m, 4H), 7.23-7.50 (m, 26H), 6.96-7.15 (m, 8H), 2.42-2.63
(m, 4H), 1.32-1.47 (m, 4H), 1.09-1.30 (m, 20H), 0.71-0.81
(m, 6H). 13C NMR (CDCl3, 400 MHz, δ/ppm): 143.4, 143.1,
140.4, 140.1, 137.8, 137.4, 136.7, 131.1, 130.4, 129.5, 129.1,
128.7, 128.2, 127.9, 127.6, 127.5, 126.6, 32.8, 31.8, 31.4, 29.6,
29.3, 29.2, 22.6, 14.1. Anal. Found (Calcd): C, 90.87 (92.45); H,
7.47 (7.55).
Copolymer pCBZ. pCBZ was obtained as a slight yellow
powder with isolated yields of 68% (0.81 g). It was prepared
from the reaction of Br2CBZ (1.04 g, 1.0 mmol) and 2,20-(2,5-
dioctyl-1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)
(0.55 g, 1.0 mmol) according to the procedure described for the
synthesis of pDPA. 1H NMR (CDCl3, 500 MHz, δ/ppm): 8.13-
8.07 (m, 4H), 7.92-7.85 (m, 4H), 7.82-7.70 (m, 6H), 7.59-7.42
(m, 8H), 7.41-7.33 (m, 4H), 7.21 (s, 2H), 2.54 (s, 4H), 1.45-1.33
(m, 40H), 1.08-0.84 (m, 20H), 0.66-0.61 (m, 6H). 13C NMR
(CDCl3, 500 MHz, δ/ppm): 143.0, 140.7, 138.9, 138.8, 138.6,
138.0, 136.4, 131.2, 129.9, 129.7, 129.5, 129.2, 128.9, 128.2, 127.4,
126.5, 125.4, 123.5, 116.2, 109.4, 34.7, 32.8, 32.4, 32.0, 31.7, 31.4,
29.5, 29.2, 29.1, 29.0, 22.5, 14.0. Anal. Found (Calcd): C, 88.49
(89.14); H, 8.37 (8.50); N, 2.74 (2.36).
Copolymer pOXD. pOXD was obtained as a pale yellow
powder with isolated yields of 84% (0.89 g). It was prepared
from the reaction of Br2OXD (0.91 g, 1.0 mmol) and 2,20-(2,5-
dioctyl-1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)
(0.55 g, 1.0 mmol) according to the procedure described for the
synthesis of pDPA. 1H NMR (CDCl3, 400 MHz, δ/ppm): 8.24-
8.38 (m, 8H), 7.58-7.83 (m, 12H), 7.32-7.41 (m, 2H), 7.08 (s, 2H),
2.36-2.56 (m, 4H), 1.15-1.38 (m, 4H), 0.85-1.13 (m, 20H),
0.62-0.71 (m, 6H). Anal. Found (Calcd): C, 74.83 (75.12); H,
5.57 (5.73); N, 5.35 (5.31).
Copolymer pTPS. pTPS was obtained as an off-white powder
with isolated yields of 52% (0.45 g). It was prepared from the
reaction of Br2TPS (0.75 g, 0.75 mmol) and 2,20-(2,5-dioctyl-1,4-
phenylene)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.42 g,
0.75 mmol) according to the procedure described for the synthe-
sis of pDPA. 1H NMR (CDCl3, 400 MHz, δ/ppm): 7.74-7.86
(m, 4H), 7.47-7.68 (m, 24H), 7.34-7.43 (m, 6H), 7.10-7.33 (m,
12H), 2.35-2.60 (m, 4H), 1.19-1.31 (m, 4H), 0.88-1.10 (m,
20H), 0.59-0.72 (m, 6H). 13C NMR (CDCl3, 500 MHz, δ/ppm):
147.5, 144.2, 140.9, 139.1, 138.8, 138.5, 138.4, 137.9, 137.6,
137.3, 137.1, 137.0, 136.4, 136.3, 135.6, 134.7, 134.1, 134.0,
133.9, 132.8, 131.2, 131.0, 129.7, 129.5, 129.4, 129.3, 129.2, 128.0,
127.9, 126.9, 126.7, 126.6, 126.4, 126.3, 35.6, 33.6, 32.7, 31.9,
31.8, 31.5, 31.3, 29.9, 29.7, 29.5, 29.4, 29.2, 29.1, 24.9, 22.7, 22.5,
14.1, 13.9. Anal. Found (Calcd): C, 84.84 (87.90); H, 7.06 (7.20).
Copolymer pNPA. pNPA was obtained as a slight yellow
powder with isolated yields of 77% (0.34 g). It was prepared
from the reaction of Br2NPA (0.38 g, 0.41 mmol) and 2,20-(2,5-
dioctyl-1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(0.23 g, 0.41 mmol) according to the procedure described for the
synthesis of pDPA. 1H NMR (CDCl3, 400 MHz, δ/ppm): 7.91-
8.08 (m, 2H), 7.59-7.89 (m, 6H), 7.26-7.52 (m, 12H), 6.95-7.23
(m, 18H), 6.78-6.91 (m, 2H), 2.31-2.71 (m, 4H), 1.31-1.49
(m, 4H), 0.92-1.28 (m, 20H), 0.68-0.86 (m, 6H). 13C NMR
(CDCl3, 400 MHz, δ/ppm): 148.8, 148.2, 143.6, 141.0, 139.9,
137.9, 137.1, 135.3, 131.1, 129.1, 128.4, 127.1, 126.3, 124.6, 124.3,
122.6, 121.9, 120.6, 32.9, 31.8, 31.5, 29.7, 29.3, 29.1, 24.9, 22.6,
14.1. Anal. Found (Calcd): C, 86.49 (90.18); H, 7.00 (7.19); N,
3.07 (2.63).
Fabrication and Characterization of PLED. The ITO glass
plate was exposed on oxygen plasma at a power of 50 W for
5 min after ultrasonicating sequentially in detergent, deionized
water, acetone, and isopropyl alcohol. A hole injection layer,
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:
PSS) (Baytron P VPCH 8000 from H. C. Starck), was spin-
coated on the ITO glass to achieve a thickness around 50 nm. The
film was baked at 200 °C for 5 min in a nitrogen glovebox after
drying at 140 °C for 10 min in the air. Inside a nitrogen glovebox,
a hole-transporting material, poly(N-vinylcarbazole) (PVK),
dissolved in chlorobenzene (10 mg mL-1) was spin-coated on
top of PEDOT:PSS layer to achieve a thickness about 25 nm. In
order to reduce intermixing upon spin-coating the light-emitting
copolymer, PVK layer was bake-dried at 200 °C for 1 h. On top
of ITO/PEDOT:PSS/PVK, a thin layer (∼100 nm) of light-
emitting copolymer was spin-coated from its chlorobenzene solu-
tion (10 mg mL-1), and the copolymer layer was bake-dried at
140 °C for 1 h inside the nitrogen glovebox. The hole-blocking/
electron-transporting layer, 2,20,200-(1,3,5-phenylene)tris(1-phenyl-
1H-benzimidazole) (TPBI), was deposited by vacuum thermal
deposition (8 ꢀ 10-6 Torr) to achieve a thickness of 25 nm. Finally,
a thin layer of electron-injection layer, LiF (about 1.0 nm), and a
layer of aluminum (120 nm) were directly vacuum-deposited
thereon as the cathode of the devices. The active areas of the
devices were 4 mm2, and a glass substrate was used to encapsu-
late the device in a nitrogen-filled glovebox (with O2 and H2O

Article
Macromolecules, Vol. 43, No. 8, 2010 3617
Scheme 2. Preparation of DOPTMBO, BrCBZ, BrOXD, BrTPS, and BrNPA, and Six 2,6-Dibromosubstituted DPA Derived Monomersa
a Reagents and conditions: (a) PdCl2(dppf), KOAc, DMSO, 80 °C; (b) Cu, K2CO3, DMF, reflux; (c) NaN3, NH4Cl, DMF, reflux;
(d) 3-(trifluoromethyl)benzoyl chloride, 2,4,6-collidine, anisole, reflux; (e) n-BuLi, THF, -78 °C; (f) Pd(OAc)2, dppf, NaOtBu, toluene, 110 °C; (g) i:
n-BuLi, THF, -78 °C, 1 h; ii: 2,6-dibromoanthraquinone, -78 °C ∼ rt; (h) KI, NaH2PO2 xH2O, HOAc, reflux, 16 h.
3
concentration below 0.1 ppm). Current-voltage-luminance
characteristics of the devices were measured under ambient
conditions using a Keithley 2400 SourceMeter and a PR-650
SpectraScan spectrophotometer (Photo Research), both of
which were computer controlled with a Labview program.
elemental analysis, 1H/13C NMR, and HRMS for structural
identification and purity check. Except for HRMS, a similar
set of structure examination was applied to the six copolymers
reported herein.
As data shown in Table 1, these copolymers have weight-
average molecular weights (Mw) of 5300-46 200 with poly-
dispersity indices (PDI, Mw/Mn) of 1.40-2.39. Except for
pTPS, Mws of these copolymers are higher than 10 000 g/
mol. The significantly smaller Mw of pTPS may probably
due to the steric hindrance caused by the bulky meta-
substituted-triphenylsilane preventing its copolymer forma-
tion. The thermal properties of these copolymers were
evaluated by TGA and DSC measurements under a nitrogen
atmosphere (Table 1). All of these copolymers exhibit good
thermal stability with decomposition temperatures (Tds)
above 400 °C which are determined by 5% weight loss of
the copolymer. These copolymers exhibit relatively high glass-
transition temperatures (Tgs) above 320 °C due to the rigid
polycyclic aromatic hydrocarbon (PAH) backbone. This
ensures the morphological stability of these copolymers
fabricated as thin films. Tgs of 2,6-linked 9,10-diphenyl-
anthracene copolymers have not been reported in the litera-
ture so far.8,16a,17
Results and Discussion
Synthesis and Characterization. As shown in Scheme 1, all
new 2,6-linked diphenylanthracene-based copolymers were
synthesized through the Suzuki cross-coupling reactions in
good yields from substituted diphenylanthracene monomer
and comonomer DOPTMBO, 2,20-(2,5-dioctyl-1,4-phenylene)-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane). DOPTMBO was
synthesized from 1,4-dibromo-2,5-dioctylbenzene and bis-
(pinacolato)diboron catalyzed by PdCl2(dppf) (dppf =
1,10-bis(diphenylphosphino)ferrocene) with moderate yields
(Scheme 2). In addition to the inhibition of emission wave-
length from red-shifting, comonomer DOPTMBO was chosen
to solubilize 2,6-linked anthracene-based copolymers. Sub-
stituted diphenylanthracene monomers, Br2DPA, Br2DPV,
Br2CBZ, Br2OXD, Br2TPS, and Br2NPA, were synthe-
sized from 2,6-dibromoanthraquinone14 following pertinent
literature procedure in 40-60% isolated yields.16 Para-
substituted diphenylvinylbromobenzene precusor, BrDPV,
is a known compound which was synthesized according
to the literature procedures.15 Previously unknown meta-
substituted bromobenzene BrCBZ, BrOXD, BrTPS, and
BrNPA were prepared via various synthetic routes shown
in Scheme 2. Comonomer DOPTMBO and six 2,6-dibromo-
substituted DPA derived monomers were characterized with
Photoluminescent Characterization. Figure 1 shows the
UV-vis absorption and fluorescence spectra of these copoly-
mers in the dilute solution of chloroform. The absorption
peaks around 360-430 nm with a characteristic feature of
vibronic absorption are attributed to the π to π* transition of
anthracene moiety,18 which is the main π-conjugation moiety
responsible for emission (fluorescence). Except for pDPV,

3618 Macromolecules, Vol. 43, No. 8, 2010
Table 1. Molecular Weight, Thermal, and Photophysical Characterizations of Copolymers
Chen et al.
Tg
(oC)
λFmLax, Φf,b fwhmc
λFmLax, Φf,b fwhmc
Mwa (ꢀ104)
3.7
Mna (ꢀ104)
PDIa
Td (oC)
(nm, %, nm)d
(nm, %, nm)e
pDPA
pDPV
pCBZ
pOXD
pTPS
1.71
1.56
0.72
1.93
0.43
0.66
2.16
1.75
1.4
2.39
1.22
1.67
328
327
326
326
324
325
430
418
435
444
467
407
431 (452), 73, 58
457, 85, 60
436 (456), 84, 53
436 (454), 66, 51
436 (455), 86, 50
441 (458), 34, 58
438 (456), 23, 60
463, 20, 59
466, 37, 68
444, 27, 59
436 (456), 62, 55
454, 20, 68
2.73
1.02
4.62
0.53
1.1
pNPA
a Analytical GPC was obtained in tetrahydrofuran (THF) using polystyrene as standard. b Using 9,10-diphenylanthracene as the standard reference
(Φf = 0.96). c Fwhm is the full width at half-maximum of emission bands. d In chloroform. e As thin films spin-coated from chlorobenzene solution.
Figure 1. Normalized absorption (left) and emission (right) spectra of copolymers in chloroform.
Figure 2. Left: fluorescence images of copolymers as spin-coated thin films on quartz substrate. Right: normalized emission spectra of pTPS in
chloroform solution and as thin film.
these copolymers exhibit essentially same fluorescence spectra
in the deep blue area with two to three vibronic emission
bands extended into long wavelength region of the visible
spectrum. In addition to the less discernible vibronic emis-
sion side bands, pDPV shows the most red-shifted wave-
length λ
FL
max
of 457 nm in solution. This may be due to the
π-conjugation nature of the diphenylvinyl substituent on the
para-position of 9,10-diphenylanthracene.
Vision-wise, all copolymers exhibited blue to sky blue
fluorescence in the thin film state (Figure 2, left). Spectros-
copy-wise, compared with those in solution, all copolymers
displayed a slight red-shifted fluorescent emission with less
than 10 nm of red-shifting wavelength (Δλ), except for pCBZ
(Δλ=30 nm) and pNPA (Δλ=14 nm) (Table 1 and Figure 3).
This is a good indication of limited conformation change for
the π-conjugated polymer backbone from solution to thin
film state. Especially for the fluorescence spectra of pTPS in
dilute solution and in the thin film are almost identical with
Δλ only 1 nm (Figure 2, right). This can be rationalized by the
inhibition of the intermolecular π-π interactions among
polymer chains due to the bulky tetraphenylsilane molecular
structure.
Figure 3. Normalized emission spectra of copolymers in thin film state.
On the other hand, a bathochromic shift of the emission
spectra was observed for the monomer Br2NPA in solutions
with increasing solvent polarity (Figure 4). Such solvato-
chromism usually happen to molecules with the polarity high
in the excited state but low in the ground state. However, no
solvatochromic shift was observed for the copolymer pNPA

Article
Macromolecules, Vol. 43, No. 8, 2010 3619
in different solutions with varied polarity. This result implies
that a molecular dipole moment is significant in the excited
state of Br2NPA, and it may be ascribed to the donor
(diarylamine substituents)-acceptor (anthracene moiety)
electron transfer character of Br2NPA. For copolymer
pNPA, such dipole is significantly reduced by the disoriented
dipole in various directions along the whole π-conjugated
polymer backbone. Molecular dipoles have been verified as
an adverse effect on photoluminescence or electrolumine-
scence.19 Similar character of electron transfer (judged by
the shifting of emission wavelength in solution) was not
observed for another arylamine-bearing DPA moiety
Br2CBZ. This is consistent with the exceptionally low solu-
tion fluorescence quantum yield (Φf) happened on pNPA
instead of pCBZ (Table 1).
chloroform solution, all copolymers have reasonably high
66-86%, except relatively low 34% of pNPA. The relatively
low Φf of pNPA may be attributed to the above-mentioned
donor-acceptor electron transfer character between
naphthylphenyl amine and anthracene moieties. In the solid
state, pTPS possesses a small fluorescence quenching with Φf
as high as 62%, the highest among all copolymers. Relatively
slight solid state fluorescence quenching was also observed
forpCBZinthinfilmstate (Φf=37%). Thecommonstructural
feature of pTPS and pCBZ is the sterically hindered substitu-
ent on the anthracene-based 2,6-linked polyanthrylenepheny-
lene backbone. The bulky substituents effectively prevent the
polymer chain from π-π interactions and alleviate the solid-
state fluorescence quenching, which is a common problem of
anthracene-based fluorophores4b,d or polymers.
Comparison of these copolymers in solution and in thin
film state reveals that all six copolymers suffer from medium
to strong fluorescence quenching in solid states (Table 1). In
Energy Levels Determination. The energy level of OLED
materials is an important parameter that governs the effi-
ciency of charge injection and cross-layer charge transport of
holes or electrons. A low-energy photoelectron spectrometer
(Riken-Keiki AC-2) was employed to estimate ionization
potentials of these copolymers, which would be referred to
the energy level of the highest occupied molecular orbitals
(HOMOs). The HOMO energy levels were measured from
copolymers in thin film state, which were prepared by spin-
coating copolymer solutions (in chlorobenzene) on quartz
plates. As shown in Figure 5 (left) and Table 2, HOMO
energy levels of these copolymers were found to be around
5.76-5.99 eV below vacuum level. This difference can be
attributed to the electron-donating ability from different
charge-transporting substituents on the 9,10-diphenyl-
anthracene moiety. For example, with the strong electron-
donating diarylamine, pNPA possesses the lowest HOMO
energy level among these copolymers.
Since the magnitude of the electrochemical oxidation
potential reflects the ease of removing electrons from the
HOMO, we can easily differentiate the electron-donating
Figure 4. Normalized emission spectra of monomer Br2NPA in hex-
anes, toluene, ethyl acetate, 1,4-dioxane, chloroform, tetrahydrofuran,
and pNPA in chloroform.
Figure 5. Low-energy photoelectron spectra of copolymers as solid thin films (left) and differential pulsed voltammograms of copolymers coated on
glassy carbon electrodes in acetonitrile containing 0.1 M n-Bu4NClO4 at a scan rate of 100 mV s-1 (right).
Table 2. Electrochemical Properties and Energy Levels of the Copolymers
Ega (eV)
E
(V)
HOMODPV c (eV)
LUMODPV d (eV)
HOMOAC-2 e (eV)
LUMOAC-2 d (eV)
DPV b
oxd,1
pDPA
pDPV
pCBZ
pOXD
pTPS
2.84
2.76
2.83
2.83
2.87
2.77
1.42
1.4
1.36
1.44
1.43
1.19
-5.79
-5.77
-5.73
-5.81
-5.8
-2.95
-3.01
-2.9
-2.98
-2.93
-2.79
-5.89
-5.85
-5.79
-5.99
-5.88
-5.76
-3.05
-3.09
-2.96
-3.16
-3.01
-2.99
pNPA
-5.56
a Eg is the energy band gap estimated from the low-energy edge of the absorption spectra from the polymer thin film. b The first oxidation potential
obtained by differential pulsed voltammetry from the polymer thin film on a glassy carbon rod working electrode. c HOMO = -4.8 eV - e(Eoxd,1
-
Eoxd,ferrocene), where Eoxd,ferrocene is the oxidation potential of ferrocene (0.43 V vs Ag/AgNO3) determined by differential pulsed voltammetry in a 0.1 M
solution of n-Bu4NClO4 in acetonitrile. d LUMO = HOMO þ Eg. e HOMO energy level was determined by low-energy photoelectron spectrometer
(Riken-Keiki AC-2).

3620 Macromolecules, Vol. 43, No. 8, 2010
Chen et al.
Figure 6. Electroluminescence (EL) characteristics of pOXD with the device configuration ITO/PEDOT:PSS/PVK/pOXD/cathodes.
ability of these copolymers via the electrochemical method.
Additional data provided by differential pulsed voltammetry
(DPV) have been used to show the electron-donating ability
of these copolymers. Basically, a similar trend of the HOMO
energy levels, determined by DPV, was comparable with
those measured by AC-2. As shown in Figure 5 (right) and
Table 2, it can be expected for pNPA having a smallest
oxidation potential and the highest HOMO energy level
among copolymers due to its built-in electron-rich moiety
(naphthylphenylamine). Similarly, pCBZ has a second small
oxidation potential and the second high HOMO energy level
next to that of pNPA. This is because the N-carbazole moiety
is less electron-rich than that of naphthylphenylamine. On
the other hand, it is reasonable to find pOXD with the
highest oxidation potential and the lowest HOMO energy
level among copolymers due to its built-in electron-deficient
moiety (oxadiazole heterocyclic ring). By calculating the
optical band gaps (from the on-set absorption energy of thin
film) and HOMO energy levels (obtained from AC-2 or
DPV), the LUMO energy level of each copolymer can be
obtained, and they are summarized in Table 2. For a similar
reason (built-in electron- deficient moiety), the deepest
LUMO energy level was found for pOXD among these
copolymers.
Hole-Blocking/Electron-Transporting and Cathode Con-
figuration of PLEDs. The initial examinations of electro-
luminescent properties for these copolymers were conducted
by fabricating multilayer devices with the configuration of
ITO/PEDOT:PSS/PVK/copolymers/cathodes, where PEDOT:
PSS and PVK were used as hole injection and hole trans-
porting layers, respectively. Four different cathode configura-
tions, TPBI/LiF/Al, CsF/Ca/Al, TPBI/CsF/Al, and CsF/Al,
were tested to investigate the cathode effect on the electrolumi-
nescence of PLEDs. TPBI (2,20,200-(1,3,5-phenylene)-tris-
(1-phenyl-1H-benzimidazole) was used as the hole-blocking/
electron-transporting layer after the polymer light-emitting
layer. A thin layer of LiF or CsF was used as the electron-
injection layer followed by Ca and/or Al as the cathode
material. Figure 6 showed the EL characteristics based on
copolymer pOXD with four different cathode configurations
with or without TPBI.
It is apparent that devices incorporated with TPBI show
higher luminous efficiencies than those without TPBI. It is
also clear that the device taking Ca as the cathode shows a
slightly higher EL efficiency than that without Ca. We
presume a better charge balance between electron and hole
in these devices (ones with TPBI and/or Ca). And this may be
attributed to the blocking of hole carriers and the enhance-
ment of the electron-injection because of TPBI or the low
work function Ca as the cathode material. However, an
emission shoulder was observed around 500 nm in the green
portion of EL spectra, whenever the devices were incorpo-
rated with either TPBI as the hole-blocking/electron-trans-
porting layer or the low work function Ca as the cathode
material. Such enhanced-green EL has been reported before,
and it can be attributed to the optical microcavity effect,20
which improves light out coupling in the green portion of the
EL spectrum. We have observed a similar effect taking place
on other anthrylene copolymers such as pDPV and pCBZ.
Nevertheless, more electrons are able to penetrate into the
active layer (with the aid of electron-transporting TPBI or
low work function Ca), and the exciton profile (light-emitting
zone) within the active layer may be shifted away from the
cathode. This significantly reduced exciton quenching adjacent
to cathode surface, and thus EL efficiency is enhanced.
OXD-7-Doped PLEDs. Similar enhancement of the green
portion of EL spectra has been reported before on a solution-
processed blue electrophosphorescent OLEDs doped with
electron-transporting OXD-7 (1,3-bis(2-(4-tert-butylphenyl)-
1,3,4-oxadiazol-5-yl)benzene).20c,d Such enhancement was
also observed in our copolymer PLEDs when doped with
OXD-7. Here, we adopted the device configuration of

Article
Macromolecules, Vol. 43, No. 8, 2010 3621
Figure 7. Electroluminescence (EL) characteristics of OXD-7-doped PLED devices with the device configuration of ITO/PEDOT:PSS/PVK/pOXD:
OXD-7 (x wt %, x = 0.0, 9.1, 23.1, 28.6, 33.3, 37.5)/CsF(1 nm)/Al(120 nm).
Table 3. Electroluminescence Characteristics of PLEDs Containing 2,6-Linked Anthrylenephenylene Copolymersa
EL brightness max,
100 mA/cm2,
luminous efficiency at
10 and 100 mA/cm2
(cd/A)
1931 CIE
chromaticity
(x, y) at 8.0 V
turn-on voltageb
(V)
max efficiency
(cd/A, lm/W)
λEmLax, fwhm
(nm) at 8 V
10 mA/cm2 (cd/m2)
ΔLEc (cd/A)
pDPA
pDPV
pCBZ
pOXD
pTPS
3.8
3.7
3.2
3.3
4.2
3.2
2430, 1053, 119
2810, 1899, 211
1640, 1079, 210
3920, 1558, 169
264, 201, 49
1.17, 1.05
2.10, 1.89
2.12, 1.07
1.68, 1.55
0.50, 0.20
1.36, 0.60
0.12
0.21
1.05
0.13
0.3
1.26, 0.53
2.21, 1.01
2.15, 1.02
1.77, 0.98
0.70, 0.40
1.36, 0.63
469, 103
473, 72
469, 73
468, 85
463, 99
469, 82
0.19, 0.30
0.16, 0.28
0.15, 0.22
0.17, 0.25
0.18, 0.23
0.16, 0.24
pNPA
898, 611, 133
0.76
a Device configuration: ITO/PEDOT:PSS(40-50 nm)/PVK(20-30 nm)/copolymers (80 nm)/TPBI (25 nm)/LiF(1.0 nm)/Al(120 nm). b With
electroluminance at 1 cd/m. c The luminous efficiency difference between current density of 10 and 100 mA/cm2.
ITO/PEDOT:PSS/PVK/pOXD:OXD-7 (x wt %, x=0, 9.1,
23.1, 28.6, 33.3, 37.5)/CsF/Al to eliminate the electronic
effect from TPBI and Ca. The EL results of these OXD-7
doped PLEDs are displayed in Figure 7.
These PLEDs emitted blue to sky-blue EL with main
emission band peaked around 463-473 nm, corresponding
to CIE coordinates of (0.15, 0.22) to (0.19, 0.30). The EL
spectrum of the nonsubstituted polymer pDPA has an emis-
sion shoulder around 500 nm with fwhm (full width at half-
maximum) of 103 nm, which is significantly broader than the
other copolymers. This may be ascribed to the lack of bulky
substituent of pDPA resulting polymer chain aggregation.
Whereas pDPV PLED exhibited the highest luminous effi-
ciency of 2.21 cd/A among all copolymers, pTPS PLED
showed a low efficiency of 0.70 cd/A with maximum lumi-
nance of only 264 cd/m2. The poor PLED performance of
pTPS can be attributed to its low molecular weight and the
poor film morphology in spite of its high solid-state fluores-
cence quantum yields compared with other copolymers,
although poor charge-transporting properties of pTPS due
to the nonplanar and bulky triphenylsilyl group could be an
alternative reason.
As shown in Figure 7, the EL spectrum of the nondoped
PLED device (0 wt %) exhibits a pure blue-light emission
with CIE coordinates (0.15, 0.16). However, the emission
shoulder around 500 nm enlarged when the doping concen-
tration of OXD-7 in the polymer layer increased. Introducing
OXD-7 into the polymer layer as an electron-transporting
moiety also resulted a pronounced enhancement of PLED
brightness and luminous efficiency. Similar to that shown in
the previous section, the enhanced luminous efficiency may
be attributed to a better charge balance between electrons
and holes through the enhancement of electron transport by
OXD-7 dopant. On the basis of the above tests, it is inferable
that these diphenylanthracene-based blue PLEDs are abun-
dant in hole carriers, even though the tested copolymer
pOXD is relatively electron-deficient (electron transporting)
among the series.
Interestingly, we have found that the PLEDs with the
lowest turn-on voltage (3.2 or 3.3 V) belong to three charge-
transporting copolymers (see turn-on voltage data in Table 3).
They are pOXD for electron-transporting and pCBZ and
pNPA for hole-transporting, and all their PLED efficiencies
are superior to the unsubstituted pDPA. The enhanced
PLED efficiency can be ascribed to the reinforcement of
2,6-Linked Anthrylenephenylene Copolymer PLEDs. Hav-
ing above examinations, we chose the most favorable device
configuration, ITO/PEDOT:PSS/PVK/copolymer/TPBI/LiF/
Al, to investigate EL characteristics of all copolymers, and the
results are summarized in Table 3 and Figure 8.

3622 Macromolecules, Vol. 43, No. 8, 2010
Chen et al.
Figure 8. Electroluminescence (EL) characteristics of copolymers based on the device configuration of ITO/PEDOT:PSS(50 nm)/PVK(20 nm)/
copolymers(80 nm)/TPBI(25 nm)/LiF(1 nm)/Al(120 nm).
the electron- or hole-transporting abilities of 9,10-diphenyl-
anthracene derivatives. Such charge-transporting molecular
design has been applied on 9,10-diphenylanthracene-based
small molecule OLEDs before.4a,d,e In addition to the hole-
transporting characteristics, pCBZ PLED showed the blue
EL (CIEx,y =0.15, 0.22) with the highest blue color purity
among all copolymers. Although the maximum luminous
efficiency of pCBZ PLED (2.15 cd/A) is not the best, next to
2.21 cd/A of sky-blue pDPV PLED, we consider pCBZ is the
best blue-light-emitting copolymer regarding the blue color
purity. This can be ascribed to its relatively high solid-state
fluorescence quantum yield (Φf =37%) and its bulky sub-
stituent, which prevents the copolymer chain from aggrega-
tion and hence red-shifting emission wavelength.
The difference between hole- and electron-transport
shows in EL efficiency roll-off. In a range of the current
density between 10 and 100 mA/cm2, the luminous efficiency
of pCBZ PLED has a significant decay (ΔLE=1.05 cd/A).
Such efficiency roll-off was also observed for another hole-
transporting copolymer pNPA (ΔLE =0.76 cd/A). In con-
trast, electron-transporting pOXD PLED showed minor
efficiency roll-off, only 0.13 cd/A of ΔLE (Table 3 and
Figure 8). This result demonstrates that a better balance of
charge transport can be achieved under high current densi-
ties when an electron-transporting moiety is incorporated
into the 9,10-diphenylanthracene-based copolymers. Such
reasoning is also consistent with the observed result of OXD-
7 doped PLEDs, which exhibit enhanced luminous efficiency
with increasing OXD-7 dopant concentrations.
polymers,6-9 which have the EL brightness less than 1000 cd/
m2 and luminous efficiency rarely over 1 cd/A, our judi-
ciously designed copolymers have outperformed the others
in PLED applications.
Conclusion
In this paper, we have successfully designed and synthesized a
series of novel 2,6-linked 9,10-diphenylanthracene-based blue-
light-emitting copolymers. These copolymers are composed of
anthrylene and phenylene as the polymer backbone unit and
a range of functional side groups appended to the 9- and 10-
positions of anthracene core. Both electron-transporting and
hole-transporting as well as bulky substituent as side groups of
the polymer chain were studied for their thermal, electrochemical,
photophysical, and electroluminescent properties. Fundamen-
tally, we have demonstrated that both hole- and electron-trans-
porting substituents apparently enhance the EL performance (EL
color purity and efficiency) of these 2,6-anthrylenephenylene
blue-light-emitting copolymers. A better hole/electron charge
balance is presumably achieved in the electron-transport sub-
stituted copolymer. Our results successfully demonstrate a
rational design of polyanthrylenephenylene for blue PLEDs
applications.
Acknowledgment. This research was supported by the National
Science Council of Taiwan, Taiwan International Graduate Program
of Academia Sinica, and National Taiwan University.
References and Notes
According to the above examinations, we can conclude
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on 9,10-diphenylanthracene, the red-shifting of EL is largely
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