Improved electroluminescence efficiency of polyfluorenes by simultaneously incorporating dibenzothiophene-S,S-dioxide unit in main chain and oxadiazole moiety in side chain

Article abstract of DOI:10.1016/j.polymer.2014.02.032

A series of efficient and spectrally stable blue light-emitting polyfluorene derivatives containing 3,7-dibenzothiophene-S,S-dioxide (SO) unit in main chain and oxadiazole (OXD) moiety in the side chain were synthesized via Suzuki copolymerization. It was realized that the glass transition temperatures of the resulted copolymers PFSO-OXD increased gradually with the content of OXD, while the UV-vis absorption, photoluminescence spectra, as well as electrochemical properties were not significantly influenced by the molar ratio of OXD unit. Apparent solvatochromism of copolymers PFSO-OXD can be realized by varying polarity of solvents from toluene to dichloromethane. Light-emitting devices based on PFSO-OXD exhibited superior performances to those of PFSO and PF-OXD20 due to the more balanced charge carrier mobility of the devices. The electroluminescence spectra of all copolymers are independent with the current densities and thermal annealing. The best device performance was achieved based on PFSO-OXD20 with a maximal luminous efficiency of 4.9 cd A^-1 with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.12). The results indicated that the strategy of concurrently incorporating SO and OXD unit into the main chain and side chain of polyfluorenes, respectively has great potential to achieve efficient blue light-emitting polymers.

Full text of DOI:10.1016/j.polymer.2014.02.032

Polymer 55 (2014) 1698e1706
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Improved electroluminescence efficiency of polyfluorenes by
simultaneously incorporating dibenzothiophene-S,S-dioxide unit in
main chain and oxadiazole moiety in side chain
*
Yong Yang, Lei Yu, Yong Xue, Qinghua Zou, Bin Zhang, Lei Ying , Wei Yang, Junbiao Peng,
Yong Cao
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of
Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of efficient and spectrally stable blue light-emitting polyfluorene derivatives containing 3,7-
dibenzothiophene-S,S-dioxide (SO) unit in main chain and oxadiazole (OXD) moiety in the side chain
were synthesized via Suzuki copolymerization. It was realized that the glass transition temperatures of
the resulted copolymers PFSO-OXD increased gradually with the content of OXD, while the UV-vis
absorption, photoluminescence spectra, as well as electrochemical properties were not significantly
influenced by the molar ratio of OXD unit. Apparent solvatochromism of copolymers PFSO-OXD can be
realized by varying polarity of solvents from toluene to dichloromethane. Light-emitting devices based
on PFSO-OXD exhibited superior performances to those of PFSO and PF-OXD20 due to the more
balanced charge carrier mobility of the devices. The electroluminescence spectra of all copolymers are
independent with the current densities and thermal annealing. The best device performance was ach-
ieved based on PFSO-OXD20 with a maximal luminous efficiency of 4.9 cd Aꢀ1 with the Commission
Internationale de L’Eclairage (CIE) coordinates of (0.16, 0.12). The results indicated that the strategy of
concurrently incorporating SO and OXD unit into the main chain and side chain of polyfluorenes,
respectively has great potential to achieve efficient blue light-emitting polymers.
Received 26 October 2013
Received in revised form
19 January 2014
Accepted 8 February 2014
Available online 18 February 2014
Keywords:
Polyfluorenes
3,7-Dibenzothiophene-S,S-dioxide
Oxadiazole
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
polymers with stable electroluminescence. Representative ap-
proaches include incorporating various electron-deficient moieties
Polymer light-emitting diodes (PLEDs) have attracted consid-
erable attention in recent years due to their great potential for the
fabrication of large-area and flexible displays on the basis of cost-
effective solution-processing techniques [1e4]. In order to ach-
ieve high performance full-color displays, light-emitting polymers
with three primary color of red, green and blue emission are
principally needed [5e7]. While the green and red-emitting poly-
mers have been attained by various systems, efficient and spectrally
stable blue light-emitting polymers with the Commission Inter-
nationale de L’Eclairage (CIE) coordinates of x þ y < 0.3 remain
great challenge because of the trade-off between luminous effi-
ciency and color purity [8,9]. To address this point, extensive efforts
have been devoted to the development of blue light-emitting
[10e12], using star-burst or hyperbranched framework to get rid of
annihilation of excitons [8,13,14], relying on the next-generation
thermal assisted delayed fluorescent emission [15] or hybridized
local and charge transfer state [16] and so forth.
Among the currently available blue light-emitting polymer
systems, recently emerged conjugated copolymers containing
electron-deficient dibenzothiophene-S,S-dioxide (SO) moiety are of
particular interests on account of their high efficiency and excellent
spectral stability [17e23]. SO unit is quite similar to the fluorene
moiety in shape, wherein the C-9 position of fluorene is replaced by
electron-deficient S,S-dioxide in SO moiety [24]. The incorporation
of SO unit into the backbone of conjugated polymers can lead to
decreased lowest unoccupied molecular orbital (ELUMO) energy
levels, which is favorable for electron injection [25]. On the basis of
this strategy, a series of conjugated copolymers based on fluorene
or carbazole as electron-rich moiety, and SO unit which was 2,8-
* Corresponding author. Tel.: þ86 20 87114346 17; fax: þ86 20 87110606.
E-mail address: msleiying@scut.edu.cn (L. Ying).
http://dx.doi.org/10.1016/j.polymer.2014.02.032
0032-3861/Ó 2014 Elsevier Ltd. All rights reserved.

Y. Yang et al. / Polymer 55 (2014) 1698e1706
1699
linked or 3,7-linked positions as electron-deficient moiety have
been reported [17e23,25]. Highly efficient copolymers are there-
fore accessible based on this strategy with the luminous efficiency
as high as 6.0 cd Aꢀ1 [17]. Even though the incorporation of more
SO unit into the backbone of copolymers may lead to increased
device performance, further improving the molar ratio of SO unit
will lead to broadened emitting spectra due to the gradually
dominated intramolecular charge transfer emission located at ca.
490 nm [26,27], which is unfavorable for the achievement of pure-
blue light-emitting polymers.
It is well-known that the oxadiazole (OXD) unit has high
electron-affinity of ca. 2.16 eV [28,29], and its derivatives of 2-(4-
biphenylyl)5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and
related copolymers has been widely used as electron-injection/
transport materials in organic light emitting diodes [30e33]. Bi-
polar conjugated copolymers bearing OXD pendant chain as
electron-deficient unit demonstrated improved electron-injection
along with balanced charge transportation [34]. In this contribu-
tion, we introduced an alternative approach of incorporating an
additional electron-deficient OXD moiety as the side chain to
construct efficient blue light-emitting fluorene-co-SO-OXD based
copolymers. We surmise that the resulted copolymers that simul-
taneously containing electron-deficient moieties of SO and OXD
moiety in the main chain and side chain, respectively may have
specific advantages including enhanced electron transport prop-
erties that is favorable for the balance of charge carriers in the
active layer, along with the improved solubility of copolymers.
concentrated and washed with petroleum ether for three times to
afford crude product as white power. Then the resulting product
was dissolved in SOCl2 (15 mL) and was heated for 16 h under 70 ꢁC.
After cooling to room temperature, the reaction mixture was
dropped into water, filtered and recrystallized from ethanol to give
the target compound (9.00 g) as white needles. Yield: 76%. 1H NMR
(300 MHz, CDCl3)
d
(ppm): 8.14 (m, 2H), 8.06 (d, J ¼ 8.52 Hz, 2H),
7.56(d, J ¼ 8.52 Hz, 2H), 7.23 (t, J ¼ 8.73 Hz, 2H), 1.35(s, 9H). 13C NMR
(75 MHz, CDCl3)
d (ppm): 166.42, 164.71, 163.56, 163.06, 155.46,
129.23, 129.11, 126.76, 126.09, 120.96, 120.40, 120.36, 116.55, 116.26,
35.11, 31.12.
2.2. 2,7-Dibromo-9,9-bis(4-(4-(5-(4-tert-butylphenyl)-2-
oxadiazolyl)phenyloxy)phenyl)fluorene (3)
A mixture of compound 1 (5.08 g, 10 mmol), 2 (7.4 g, 25 mmol)
and sodiumhydride (0.96 g, 40 mmol) was dissolved in N,N-dime-
thylformide (DMF, 300 mL) and stirred at 150 ꢁC under argon for
20 h. The reaction mixture was poured in water. The organic layer
was then extracted by dichloromethane, dried over magnesium
sulfate (MgSO4) and concentrated. The crude product was purified
by chromatography on silica gel using petroleum ether/dichloro-
methane/ethyl acetate (8/1/1, v/v/v) as the eluent followed by
recrystallization from ethanol to give the target compound (7.4 g)
as white needles. Yield: 70%. 1H NMR (300 MHz, CDCl3)
d (ppm):
8.10 (d, J ¼ 8.85 Hz, 4H), 8.05 (d, J ¼ 8.55 Hz, 4H), 7.63 (d, J ¼ 8.55 Hz,
2H), 7.54 (m, 8H), 7.20 (d, J ¼ 8.82 Hz, 4H), 7.15 (d, J ¼ 8.85 Hz, 4H),
7.03 (d, J ¼ 8.79 Hz, 4H), 1.37 (s, 18H). 13C NMR (75 MHz, CDCl3)
2. Experimental
d (ppm): 164.61, 164.07, 160.14, 155.42, 155.36, 152.97, 140.20,
138.09, 131.36, 129.68, 129.38, 128.94, 126.84, 126.17, 122.16, 121.93,
121.23, 119.70, 119.03, 118.93, 64.74, 35.21, 31.25.
All commercially available reagents were distilled according to
standard procedure prior to use. 2,7-Dibromo-9,9-bis-(4-
hydroxyphenyl)fluorene (1) [35], 2,7-dibromo-9,9-dioctylfluorene
2.3. General procedures of Suzuki copolymerization, taking PFSO as
an example
(4),
2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
dioctylfluorene (5) and 3,7-dibenzothiophene-S,S-dioxide (6)
were synthesized according to the previously reported procedures
[17,36].
Compound 4 (642.6 mg, 1.0 mmol), 5 (438.7 mg, 0.8 mmol), and
3,7-dibromo-dibenzothiophene-S,S-dioxide
(6)
(74.8
mg,
Nuclear magnetic resonance (NMR) spectra were recorded on a
Bruker DRX 300 spectrometer (operating at 300 MHz for 1H NMR,
and 75 MHz for 13C NMR) in deuterated chloroform solution with
tetramethylsilane as a reference. Gel permeation chromatography
(GPC) measurements were performed in tetrahydrofuran (THF)
with a Waters 2410 refractive index detector. Cyclic voltammetry
(CV) measurement was carried out at a scan rate of 50 mV/s at room
temperature under argon using a CHI660A electrochemical work-
station with tetra(n-butyl)ammonium hexafluorophosphate (n-
Bu4NPF6, 0.1 M) in acetonitrile as the electrolyte. Thermogravi-
metric analyses (TGA) were performed on a Netzsch TG 209 at a
heating rate of 20 ꢁC minꢀ1. The differential scanning calorimetry
(DSC) was measured on a Netzsch DSC 204 under nitrogen flow at a
heating rate of 10 ꢁC minꢀ1. UV-vis absorption spectra were
recorded with a HP 8453 spectrophotometer. Photoluminescence
(PL) spectra were recorded with a spectrofluorometer (Spex
Fluorolog-3). PL quantum yields were measured using an IS080
LabSphere integrating sphere with excitation by a 325 nm HeCd
laser (Melles Griot).
0.2 mmol), tetrakis(triphenylphosphine)palladium, (Pd(PPh3)4,
3 mg), tetraethylammonium hydroxide aqueous solution (Et4NOH,
20 wt/v%, 2 mL) and toluene (15 mL) was stirred under argon and
heated to 100 ꢁC. The solution was kept at 100 ꢁC with vigorous
stirring under argon for 24 h. Then phenylboronic acid (20.0 mg)
was added and the reaction was allowed to stirring for 6 h. And
then bromobenzene (0.2 mL) was added and the reaction was
allowed to stirring for another 6 h. After cooling to room temper-
ature, the solution was precipitated into methanol (200 mL) and
filtered. The solids were re-dissolved in dichloromethane and
washed with water for three times. After removal of the volatile
under reduced pressure, the concentrated solution was precipita-
tion in methanol to give target copolymer. The resultant polymers
were washed by using Soxhlet extraction by using acetone and
dried under vacuum to give the 663 mg of yellowish fibers with
yields of 90%. 1H NMR (300 MHz, CDCl3)
7.86e7.61 (br, Ar H), 1.15 (m, CH2), 0.84e0.79 (m, CH3). Anal. Calcd.
(%) for [(C29H40 90(C12H6O2S)10]n: C 88.40; H 9.88; S 0.86. Found: C
d (ppm): 8.20 (br, Ar H),
)
87.85; H 10.05; S 1.05.
2.1. 2-(4-Fluorobenzoyl)-5-[4-tert-butylphenyl]-[1,3,4]oxadiazole
2.4. PF-OXD20
(2)
Monomer 3 (424.4 mg, 0.4 mmol), 4 (642.6 mg, 1.0 mmol), 5
A mixture of 4-t-butylbenzhydrazide (7.68 g, 40 mmol), trie-
thylamine (4.04 g, 40 mmol) and chloroform (200 mL) was stirred
at about 0 ꢁC in a 250 mL three-neck flask. Then 4-fluorobenzoyl
chloride (6.32 g, 40 mmol) dissolved in chloroform (20 mL) was
added to the mixture dropwise. After 2 h, the reaction mixture was
(329.0 mg, 0.6 mmol), yield: 76%. 1H NMR (300 MHz, CDCl3)
d
(ppm): 8.11e8.02 (br, Ar H), 7.78e7.43 (br, Ar H), 7.18e7.04 (br, Ar
H), 1.36e1.13 (m, CH2), 0.84e0.74 (m, CH3). Anal. Calcd. (%) for
[(C29H40 80(C61H48O4N4)20]n: C 86.63; H 8.48; N 2.28. Found: C
86.08; H 9.08; N 2.26.
)

1700
Y. Yang et al. / Polymer 55 (2014) 1698e1706
Scheme 1. Synthesis of monomer and copolymers.
2.5. PFSO-OXD10
8.20e8.02 (br, Ar H), 7.77e7.43 (br, Ar H), 7.18e7.04 (br, Ar H), 1.43e
1.14 (m, CH2), 0.83e0.75 (m, CH3).
Monomer 3 (212.2 mg, 0.2 mmol), 4 (642.6 mg, 1.0 mmol), 5
(329.0 mg, 0.6 mmol), 6 (74.8 mg, 0.2 mmol), yield: 57%. 1H NMR
2.9. Device fabrication and characterization
(300 MHz, CDCl3)
H), 7.19e7.04 (br, Ar H), 1.43e1.14 (m, CH2), 0.88e0.80 (m, CH3).
Anal. Calcd. (%) for [(C29H40 80(C12H6O2S)10(C61H48O4N4)10]n: C
d (ppm), 8.20e8.02 (br, Ar H), 7.84e7.43 (br, Ar
Polymers were dissolved in p-xylene. Patterned ITO coated glass
substrates were cleaned with acetone, detergent, deionized water
and 2-propanol followed by an ultrasonic bath. After treating with
oxygen plasma, 40 nm of poly-(3,4-ethylenedioxythiophene)
doped with poly(styrenesulfonic acid) (PEDOT:PSS, Batron-P 4083,
Bayer AG) was spin-coated onto the ITO substrates followed by
drying in a vacuum at 80 ꢁC for 12 h. A thin film of polymers was
coated onto the anode by spin-casting inside a dry box. The film
thickness of the active layers was around 75e80 nm, measured
with an Alfa Step 500 surface profiler (Tencor). Finally, 1.5 nm of CsF
followed by 120 nm of aluminum (thickness monitored with a
STM-100/MF-Sycon quartz crystal) were subsequently evaporated
on the top of an EL polymer layer in a vacuum of 3 ꢂ 10ꢀ4 Pa.
Currentevoltage (IeV) characteristics were recorded in the nitro-
gen dry-box using a Keithley 236 source-measurement unit and a
calibrated silicon photodiode. EL spectra and CIE color coordinates
were measured by a PR 705 photometer (Photo Research).
The hole- and electron-only devices were used as ITO/
PEDOT:PSS(40 nm)/copolymer (169 nm)/MoO3(10 nm)/Al and ITO/
Al(30 nm)/polymer (169 nm)/Ca (3 nm)/Al, respectively. To fabricate
the hole-only device, a layer of MoO3 (10 nm) instead of cesium
fluoride was thermally deposited on top of the emission layer with a
protective aluminum layer, with the remaining steps the same as
those for the bipolar device. For the electron-only device, ITO glass
was deposited with a layer of aluminum (30 nm) to replace the
)
86.76; H 8.87; N 1.33; S 0.76. Found: C 86.30; H 8.87; N 1.34; S 1.05.
2.6. PFSO-OXD20
Monomer 3 (424.4 mg, 0.4 mmol), 4 (642.6 mg, 1.0 mmol), 5
(219.4 mg, 0.4 mmol), 6 (74.8 mg, 0.2 mmol), yield: 46%. 1H NMR
(300 MHz, CDCl3)
H), 7.19e7.04 (br, Ar H), 1.43e1.13 (m, CH2), 0.88e0.80 (m, CH3).
Anal. Calcd (%) for [(C29H40 70(C12H6O2S)10(C61H48O4N4)20]n: C
d (ppm): 8.20e8.02 (br, Ar H), 7.78e7.42 (br, Ar
)
85.49; H 8.08; N 2.37; S 0.68. Found: C 85.05, H 8.45, N 2.26, S
1.07.
2.7. PFSO-OXD30
Monomer 3 (636.5 mg, 0.6 mmol), 4 (642.6 mg, 1.0 mmol), 5
(109.7 mg, 0.2 mmol) and 6 (74.8 mg, 0.2 mmol), yield: 48%. 1H
NMR (300 MHz, CDCl3) d (ppm): 8.20e8.02 (br, Ar H), 7.78e7.42 (br,
Ar H), 7.18e7.04 (br, Ar H), 1.43e1.12 (m, CH2), 0.83e0.75 (m, CH3).
2.8. PFSO-OXD40
Monomer 3 (848 mg, 0.8 mmol), 4 (642.6 mg, 1.0 mmol), 6
(74.8 mg, 0.2 mmol), yield: 51%. 1H NMR (300 MHz, CDCl3)
d (ppm):

Y. Yang et al. / Polymer 55 (2014) 1698e1706
1701
Fig. 1. DSC (a) and TGA (b) curves of copolymers.
PEDOT:PSS film, then a thick copolymer layer (141 nm for PFSO,
209 nm for PF-OXD20 and 169 nm for PFSO-OXD20) was spin-cast
fromp-xylene solution on top of the aluminum layerand annealed at
100 ꢁC for 20 min. Profilometry (Tencor Alfa-Step 500) was used to
determine the thickness of the films. Finally, 1.5 nm of CsF followed
by 100 nm of aluminum (thickness monitored with a STM-100/MF
(TGA) with corresponding characteristics shown in Fig. 1. One can
realize that copolymer PFSO, which do not contain OXD moiety in
the backbone, exhibited typical liquid crystal (LC) like transition at
about ca. 156 ꢁC, which is quite close to that of poly(9,9-
dioctylfluorene) (PF) of ca. 160 ꢁC [38]. In contrast, all the other
copolymers containing OXD in side chains did not show distinct LC
transition, as the bulky OXD side chain may obstruct the formation
of ordered structures. From Fig. 1a one can obviously recognize that
the glass transition temperature (Tg) of copolymers gradually
increased from 113 to 160 ꢁC with the molar ratio of the incorpo-
rated OXD moiety increased from 10 to 40 mol%, indicating that the
incorporated OXD moiety in side chain is favorable for thermal
stability of copolymers. It is also worth pointing out that copolymer
PFSO-OXD20, which simultaneously comprising SO and OXD
moieties in main chain and side chain, respectively, exhibited
higher Tg of 140 ꢁC than that of 103 ꢁC and 125 ꢁC for its counter-
parts of PFSO and PF-OXD20, respectively. The fact demonstrated
that the combination of both SO and OXD moiety are favorable for
the thermal property of copolymers. Further TGA measurements
illustrated that all copolymers exhibit good thermal stability, with
onset decomposition temperatures (Td) in the range of 417e435 ꢁC
under a nitrogen atmosphere, as can be referred in Fig. 1b. The
outstanding thermal stability of copolymers is essentially positive
for the long-term stability of fabricated device. The corresponding
thermal properties data are summarized in Table 1.
Sycon quartz crystal) were thermally evaporated through
a
shadow mask at a base pressure of 3.0 ꢂ10ꢀ4 Pa to form the cathode.
3. Results and discussion
3.1. Synthesis of monomer and copolymers
The detailed procedure for the synthesis of monomer and co-
polymers are illustrated in Scheme 1. Monomer 2,7-dibromo-9,9-
bis(4-(4-(5-(4-tert-butylphenyl)-2-oxadiazolyl)phenyloxy)phenyl)
fluorene (3) was prepared based on the aromatic nucleophilic
substitution reaction of compounds 2,7-dibromo-9,9-bis-(4-
hydroxyphenyl)fluorene (1) and 2-(4-fluorobenzoyl)-5-[4-tert-
butylphenyl]-[1,3,4]oxadiazole (2), since the electron-withdrawing
OXD group can effectively stabilize the negative charge developed
through a stabilized transition state (Meisenheimer complex) to
lower the activation energy for the displacement reaction of the
aryl fluorides group [37], which accordingly lead to relatively high
yield of 70%. Suzuki copolymerization was carried out with molar
feed ratio of monomer 3, 2,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9,9-dioctylfluorene
(4),
2,7-dibromo-9,9-
3.3. Photophysical properties
dioctylfluorene (5) and 3,7-dibenzothiophene-S,S-dioxide (6) of
0:50:40:10, 20:50:30:0, 10:50:30:10, 20:50:20:10, 30:50:10:10 and
40:50:0:10, and the resulted copolymers were referred as PFSO,
PF-OXD20, PFSO-OXD10, PFSO-OXD20, PFSO-OXD30 and PFSO-
OXD40, respectively. The copolymers were end-capped with phe-
nylboronic acid and bromobenzene to remove active end-groups of
polymer main chain.
All copolymers can be readily dissolved in common organic
solvents including toluene, chloroform, tetrahydrofuran (THF),
chlorobenzene, etc, illustrating great potential for solution pro-
cessing procedures. The number average molecular weight (Mn)
and polydispersity indices (PDI) of copolymers were evaluated by
gel permeation chromatography (GPC) with THF as eluent and
linear polystyrene as standard, which were in the range of 7.9e
12.2 kDa and 1.7e2.3, respectively.
Fig. 2 showed the UV-vis absorption spectra of all copolymers
in toluene solution with concentration of 1 ꢂ 10ꢀ5 mol Lꢀ1 and in
thin films. The strong peak at ca. 390 nm can be attributed to the
pe
p* transitions of PF backbone, while the additional absorption
peaked at ca. 300 nm arised with the incorporation of OXD unit. It
was also found that the intensity of the peak located at ca. 300 nm
increases with increasing OXD unit content. Moreover, the ab-
sorption profiles of all copolymers that containing SO unit in main
Table 1
Molecular weights and thermal properties of copolymers.
Copolymer
Mn/kDa
PDI
Tg/ꢁC
Td/ꢁC
PFSO
9.3
11.8
10.4
12.2
12.0
7.9
1.7
2.1
1.7
2.3
2.2
2.3
103
125
113
140
158
160
428
431
426
417
435
433
PF-OXD20
PFSO-OXD10
PFSO-OXD20
PFSO-OXD30
PFSO-OXD40
3.2. Thermal properties
Thermal properties of copolymers were evaluated by differen-
tial scanning calorimetry (DSC) and thermogravimetric analyses

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Y. Yang et al. / Polymer 55 (2014) 1698e1706
Fig. 2. UVevis absorption of copolymers in toluene solution (a) and in film (b).
chain exhibited nearly identical absorption profiles in the range of
335e440 nm, along with a slightly broadened absorption profiles
with respect to copolymer PF-OXD20 in both toluene solution and
thin films. This observation might be attributed to the effects of
intramolecular charge transfer (ICT) due to the incorporation of
the electron-deficient SO unit in the backbone of polyfluorene,
which agrees with previously reported SO based copolymer sys-
tems [17,27]. The UV-vis absorption profiles of copolymers in
chloroform and chlorobenzene solution (Figure S1 in the
Supporting Information) are quite comparable to those of in
toluene solution.
By comparing the absorption profiles of copolymers in toluene
solution (Fig. 2a) with a concentration of ca. 1 ꢂ10ꢀ5 mol mLꢀ1 and
in film (Fig. 2b), one can realize that the full width at half maximum
(FWHM) increased from 50 to 57 nm for PF-OXD20, and from 53 to
68 nm for other copolymers containing SO unit. The broadened
absorption profiles revealed stronger aggregation as can be attrib-
uted to the increased molecular interaction due to the incorporated
SO unit. It was also recognized that the intensity of the peak at ca.
300 nm increases with the molar ratio of OXD unit, which was
consistent with that of in toluene solution. The optical band gap
(Eogpt) as estimated from the onset of absorption as thin films are in
the range of 2.83e2.94 eV as summarized in Table 2.
of poly(9,9-dialkylfluorene) in toluene solution [39]. In contrast,
copolymers PFSO-OXD that contained SO and OXD unit in the main
chain and side chain, respectively, exhibited nearly indistinguish-
able PL spectra with peak emission located at ca. 429 nm, which has
a bathochromic shift of ca. 10 nm with respect to that of PF-OXD20
as can be attributed to the ICT effects. Additionally, distinct sol-
vatochromism of PL spectra was realized by using different solvents
of toluene, chlorobenzene, THF, chloroform as well as dichloro-
methane, which have various parameters of polarity (Table S1 in
the Supporting Information). Taking PFSO-OXD20 as an example,
the well-resolved emission can be realized in non-polar solvent of
toluene and became featureless in polar solvent of dichloro-
methane, while the emission maximum bathochromic shifted from
429 nm in toluene to that of ca. 450 nm in dichloromethane, as
demonstrated in Fig. 3b. It is also worth pointing out that copol-
ymer PF-OXD20 exhibited slightly higher PL quantum efficiency in
toluene, THF as well as dichloromethane solution than the other
copolymers, but much lower PL quantum efficiency than the other
copolymers in chloroform (Figure S2 in the Supporting
Information). However, the PL quantum efficiency in all kinds of
solutions for copolymers containing SO and OXD unit in the main
chain and side chain, respectively, were slightly higher than that of
PFSO, indicating the superiority of the incorporated OXD moiety as
the side chain.
The PL emission of copolymers as thin films was demonstrated
in Fig. 4a. It was realized that the PL emission of copolymers con-
taining SO moiety in the backbone showed nearly indistinguishable
profiles with the maximum emission located at ca. 450 nm, which
has a bathochromic shift of ca. 20 nm relative to that of ca. 427 nm
for PF-OXD20, as can be attributed to the lack of ICT effects in PF-
OXD20. We note that there is ca. 10 nm bathochromic shift of the PL
emission as thin films of all copolymers with respect to those of in
toluene solutions, which is understandable due to the aggregation
of the molecular main chain in solid state. Additionally, as illus-
trated in Table 2, even though copolymers PFSO-OXD showed
slightly lower PL quantum efficiency (QEPL) in thin films, their QEPL
are much higher than copolymer PF-OXD20.
Fig. 3a shows the photoluminescence (PL) spectra of copolymers
in toluene solution with concentration of 1 ꢂ 10ꢀ5 mol Lꢀ1. It was
found that copolymer PF-OXD20 exhibited the maximum photo-
luminescence (PL) emission at 419 nm along with a shoulder peak
at 438 nm, which was quite comparable to that of typical emission
Table 2
Optical and electrochemical characterization of the copolymers.
a
b
c
d
g
Polymer
Eox
Ered
EHOMO
ELUMO
Ecgv e
Eogpt f
QEPL
/V
/V
/eV
/eV
/eV
/eV
/%
PFO
PFSO
1.46
1.54
1.45
1.48
1.52
1.50
1.51
ꢀ2.14
ꢀ1.84
ꢀ2.06
ꢀ1.77
ꢀ1.75
ꢀ1.75
ꢀ1.72
ꢀ5.86
ꢀ5.94
ꢀ5.85
ꢀ5.88
ꢀ5.92
ꢀ5.90
ꢀ5.93
ꢀ2.26
ꢀ2.56
ꢀ2.34
ꢀ2.63
ꢀ2.65
ꢀ2.65
ꢀ2.68
3.61
3.30
3.51
3.24
3.27
3.25
2.23
e
e
2.85
2.94
2.87
2.85
2.85
2.83
61
20
39
56
51
51
PF-OXD20
PFSO-OXD10
PFSO-OXD20
PFSO-OXD30
PFSO-OXD40
Investigation of thermal stabilities of PL emission of copolymer
films was carried out by spin-coating copolymer films atop pre-
fabricated ITO/PEDOT:PSS, which were then drying under vacuum
followed by annealing at various temperatures from 80 to 160 ꢁC for
2 h in the inert atmosphere. Representative spectra based on
copolymer PFSO-OXD20 were shown in Fig. 4b. No discernable
change was found with respect to that of the pristine film even
when the annealing was performed at the temperature higher than
its Tg for 2 h, illustrating the excellent thermal stability of the
resulted copolymers.
a
Onset of oxidation potential.
Onset of reduction potential.
Calculated from the oxidation potential.
Calculated from the reduction potential.
Calculated from the EHOMO and ELUMO energy levels.
Calculated from the onset of the absorption as thin films.
Measured as thin films.
b
c
d
e
f
g

Y. Yang et al. / Polymer 55 (2014) 1698e1706
1703
Fig. 3. PL spectra of copolymers in toluene solution (a), and in different solvents of PFSO-OXD20 (b) with a concentration of ca. 1 ꢂ 10ꢀ5 mol mLꢀ1
.
Fig. 4. PL spectra of PFSO-OXD20 thermal annealed at various temperature.
3.4. Electrochemical properties
molecular frontier orbitals of the resulted copolymers since the
OXD moiety is linked to polymer backbone through a non-
conjugated linkage. Detailed parameters of CV measurement are
summarized in Table 2.
The electrochemical properties of the copolymers were exam-
ined by cyclic voltammetry (CV) under inert atmosphere with the
graphite, platinum wire and Ag/AgCl as the working electrode,
counter electrode and reference electrode, respectively. All mea-
surements were carried out in tetrabutyl ammonium hexa-
fluorophosphate (Bu4NPF6) solution (0.1 M in acetonitrile) at a scan
rate of 50 mV sꢀ1 with corresponding curve shown in Fig. 5.
The oxidation potential of polymers were calibrated to the
ferrocene/ferrocenium redox couple (Fc/Fþc ), which was determined
to be 0.40 V under the same experimental conditions. It is assumed
that the redox potential of Fc/Fþc has an absolute energy level
of ꢀ4.8 eV to vacuum [40,41]. Therefore, the highest occupied
molecular orbital energy levels (EHOMO) and LUMO energy levels
(ELUMO) of polymers were calculated according to the equation of
EHOMO ¼ ꢀe(Eox þ 4.40) (eV) and ELUMO ¼ ꢀe(Ered þ 4.40) (eV),
where the Eox and Ered is the onset of the oxidation and reduction
potential relative to Ag/Agþ, respectively [40,41]. It was found that
all copolymers exhibited analogous EHOMO, which located in the
range of ꢀ5.85 w ꢀ5.94 eV and was quite comparable with that of
poly(9,9-dioctylfluorene) (PFO), indicating that the incorporation
of the electron-deficient SO and OXD moieties did not significantly
disturb the EHOMO. The comparatively deep EHOMO of copolymers
would provide superb stability. On the other hand, copolymers
containing SO moiety in the backbone exhibited analogous ELUMO
ranged from ꢀ2.63 to ꢀ2.68 eV, which were distinctly lower than
those of ꢀ2.26 eV and ꢀ2.34 eV for PFO and PF-OXD20, respec-
tively. The decreased ELUMO can be attributed to the incorporated
electron-deficient SO in the backbone of copolymers, whilst the
OXD in the side chain of copolymer would not influence the
Fig. 5. Cyclic voltammograms of polymer thin films in an acetonitrile electrolyte so-
lution of n-Bu4NPF6 (0.1 M).

1704
Y. Yang et al. / Polymer 55 (2014) 1698e1706
pristine device by varying the applied current densities from 6 to
240 mA cmꢀ2 (Fig. 7a), as well as performing thermal annealing at
different temperature of 80, 100, 120 and 160 ꢁC (Fig. 7b). We also
noted that the unexpected low-energy emission band corre-
sponding to the excimers or fluorenone-defects cannot be dis-
cerned even at comparatively high current density of 240 mA cmꢀ2
as well as when the film were thermally annealed at the temper-
ature higher than the glass transition temperature, indicating the
superb EL stability of devices based on the resulted copolymers.
As can be seen in Fig. 8a, devices based on copolymer PFSO and
PFSO-OXD20 exhibited lowest threshold voltage (Vth) of ca. 3.1 V,
which was slightly lower than that of ca. 3.6 V for copolymer PF-
OXD20 that without SO unit in the backbone. The maximum
luminance (Lmax) of devices based copolymers PFSO-OXD are much
higher than that of PFSO and PF-OXD20, which may be attributed
to the more balanced charge carrier transportation in the emissive
layer. Fig. 8b summarized the characteristics of luminous efficiency
(LE) as a function of current density (J). One can clearly observe that,
devices based on copolymer PFSO and PF-OXD20 exhibited mod-
erate performance, which has the maximum LE (LEmax) of 1.8 and
0.9 cd Aꢀ1, respectively. On the contrary, devices based on co-
polymers PFSO-OXD exhibited superior performances to those of
based on PFSO and PF-OXD20, especially when the current density
higher than 10 mA cmꢀ2. Best device performance was achieved
based on copolymer PFSO-OXD20, which has LEmax of 4.9 cd Aꢀ1
and Lmax of 8450 cd mꢀ2 with CIE coordinates of (0.16, 0.12).
Detailed device performances are summarized in Table 3. It is
worth noting that these device performances were achieved based
on standard device structures, further optimization of device
structures may lead to improved performances. Nevertheless,
when comparing to the reported blue light-emitting copolymers
that containing heterocyclic groups such as oxadiazole [43e45],
quinoxaline [10], silafluorene [46,47] and dibenzothiophene [36],
the resulted polymers exhibited obviously improved device per-
formance in terms of relatively high luminous efficiency due to the
incorporated electron-deficient SO moiety. On the other hand, with
respect to the SO based copolymers [17,20,23,25], the advantage of
the resulted polymers involves the achievement of efficient blue
emission at low SO ratio, which can effectively avoid gradually
broadened emitting spectra with increased content of SO unit. The
fact demonstrated that the current strategy can be an effectual
approach to achieve highly efficient blue light emitting polymer
with good color purity as well as excellent spectra stability.
Fig. 6. EL spectra of copolymers.
3.5. Electroluminescence properties
Light-emitting diodes with the configuration of ITO/PEDOT:PSS/
copolymers/CsF/Al were fabricated to investigate the electrolumi-
nescence (EL) performances of the resulted copolymers. Fig. 6
demonstrates the electroluminescence spectra of devices based
on these copolymers at current density of 10 mA cmꢀ2. It was
realized that the EL profiles of all copolymers are quite similar to
their PL spectra, and all copolymers exhibited blue emission
without unexpected emission of low-energy band at ca. 530 nm
corresponding to the fluorenone defects [42]. The EL emission of all
copolymers containing SO unit in the main chain of copolymers
located at ca. 450 nm, which exhibited a distinctly bathochromic
shift relative to that of 425 nm for PF-OXD20. The fact can be
attributed to the ICT effects between the electron-rich fluorene and
electron-deficient SO moiety [17], while the non-conjugated linked
OXD in side chain has little effect on the EL emission profiles.
Investigation of the EL spectral stability was performed by
varying the applied current density during devices operation as
well as thermally annealed the specific device at various temper-
atures for 2 h. Representative EL spectra based on copolymer PFSO-
OXD20 as an example were summarized in Fig. 7. It was recognized
that the EL spectra remain almost unchanged with respect to the
Insight into the impact of the incorporated SO and OXD on the
charge carrier transport properties of copolymers came from the
measurement of space charge limited current (SCLC). Single charge
carrier devices were fabricated with configuration of ITO/
Fig. 7. EL spectra of PFSO-OXD20 under different applied current densities (a) and varied thermal annealing temperatures (b).

Y. Yang et al. / Polymer 55 (2014) 1698e1706
1705
Fig. 8. B e V (a) and L e J (b) characteristics of devices with architecture of ITO/PEDOT:PSS/copolymer/CsF/Al.
Table 3
higher than that of electron mobility in a range of the applied
electric field intensity. The imbalanced charge carrier trans-
portation may lead to inferior device performances at high current,
which is consistent with the fast efficiency roll-off as shown in
Fig. 8b. In contrast, well-balanced hole and electron fluxes can be
realized for copolymer PFSO-OXD20 which simultaneously bearing
SO and OXD unit in the main chain and side chain, respectively. The
fact indicated that our strategy of incorporating OXD moiety into
the backbone of fluoreneecoeSO copolymer can effectively
improve the electron transportation and resulted in balanced
charge carrier across the emissive layer, which can in turn lead to
the improved device performances.
Device performances with structure of ITO/PEDOT:PSS/copolymer/CsF/Al.
a
Copolymer
Von
(V)
LEmax
Lmax
CIEb
(x, y)
(cd Aꢀ1
)
(cd mꢀ2
)
PFSO
3.7
3.6
3.5
3.2
3.8
3.3
1.8
0.9
2.6
4.9
3.8
3.4
6548
2209
7842
8450
9617
6450
(0.15, 0.15)
(0.18, 0.15)
(0.15,0.14)
(0.16, 0.12)
(0.15, 0.13)
(0.15, 0.12)
PF-OXD20
PFSO-OXD10
PFSO-OXD20
PFSO-OXD30
PFSO-OXD40
a
Corresponding to luminance of 1 cd mꢀ2
.
b
Measured at current density of 10 mA mꢀ2
.
PEDOT:PSS/copolymer/MoO3/Al and ITO/Al/copolymer/Ca/Al for
hole-only and electron-only devices, respectively. These so-called
hole-only devices are fabricated by replacing common cathode of
CsF/Al with high work function MoO3/Al. The increasing offset
between the Fermi energy of the cathode and the ELUMO of MoO3
(2.3 eV) reduces the number of injected electrons to levels at which
the injected holes significantly dominate. A similar analysis can be
carried out for the electron-only devices by replacing the ITO
(4.7 eV) with a lower work function metal Al (4.2 eV), which can
make sure that the carriers are almost exclusively electrons as a
result of increasing offset between the Fermi level of the anode and
the HOMO energy level of blue light-emitting polymers (about
5.9 eV) [48]. The resulted current density e voltage (J e V) char-
acteristics based on copolymers PFSO, PF-OXD20 and PFSO-OXD20
as active layer were demonstrated in Fig. 9. It was realized that the
copolymer PFSO that containing SO in the main chain, and PF-
OXD20 that containing OXD in the side chain, exhibited imbal-
anced hole and electron fluxes, as the hole mobility is distinctly
4. Conclusion
In summary, a series of efficient and spectrally stable blue light-
emitting polyfluorenes were synthesized by simultaneously
incorporating SO and OXD moiety into the backbone and the side
chain, respectively. The resulted copolymers exhibited superb
electroluminescence stability, as no low-energy emission can be
observed by varying the applied current densities or thermally
annealed devices at different temperatures. It was also realized that
the efficiencies of all copolymers that simultaneously containing SO
and OXD unit are superior to copolymers that only bearing SO or
OXD unit, which can be attributed to the improved electron
transportation that can in turn lead to well-balanced charge carrier
transportation. Best device performance with the maximum lu-
minous efficiency of 4.9 cd Aꢀ1 and CIE coordinates of (0.16, 0.12)
were achieved based copolymer PFSO-OXD20 as emissive layer.
The results indicated that our strategy by concurrently
Fig. 9. J e V characteristics of single carrier devices of PFSO (a), PF-OXD20 (b) and PFSO-OXD20 (c).

1706
Y. Yang et al. / Polymer 55 (2014) 1698e1706
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The authors are grateful for financial support from the National
Natural Science Foundation of China (grant Nos. 51303056,
21074038, 21303256 and 51273069), the Research Fund for the
Doctoral Program of Higher Education of China (20130172110005)
and the Fundamental Research Funds for the Central Universities,
South China University of Technology (No. 2014ZM0005).
Appendix A. Supplementary data
Supplementary data related to this article can be found online at
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