Formation of poly(9,9-dioctylfluorene) β-phase by incorporating aromatic moiety in side chain

Article abstract of DOI:10.1016/j.orgel.2016.08.007

Two novel polyfluorene-based blue-light emitting copolymers consisting of aromatic carbazole or dibenzothiophene-S,S-dioxide (SO) moiety in side chain are designed and synthesized based on a palladium-catalyzed Suzuki polymerization. The UV–vis absorption, photoluminescence characteristics and wide-angle X-ray diffraction measurements of the resulting copolymers demonstrate that the incorporation of such aromatic units can induce the formation of the β-phase. The resulting copolymers consisting of aromatic units exhibit improved electroluminescence properties relative to the pristine PFO film. A maximum luminous efficiency of 2.25?cd?A^?1 with Commission Internationale de l'Eclairage coordinates of (0.17, 0.09) is obtained from the light-emitting device based on the copolymer consisting of SO unit (PFO-SO). Device based on copolymer PFO-SO also exhibits stable electroluminescent spectra and relatively low roll-off of luminous efficiency at high current densities. These observations indicate that the incorporation of aromatic moiety in side chain is a promising strategy to obtain stable blue-light-emitting polyfluorenes with enhanced efficiency.

Full text of DOI:10.1016/j.orgel.2016.08.007

Organic Electronics 38 (2016) 130e138
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Formation of poly(9,9-dioctylfluorene) b-phase by incorporating
aromatic moiety in side chain
Sen Zhao, Junfei Liang, Ting Guo, Yu Wang, Xuan Chen, Denghao Fu, Junbin Xiong,
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 of 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:
Two novel polyfluorene-based blue-light emitting copolymers consisting of aromatic carbazole or
dibenzothiophene-S,S-dioxide (SO) moiety in side chain are designed and synthesized based on a
palladium-catalyzed Suzuki polymerization. The UVevis absorption, photoluminescence characteristics
and wide-angle X-ray diffraction measurements of the resulting copolymers demonstrate that the
Received 12 May 2016
Received in revised form
27 July 2016
Accepted 8 August 2016
incorporation of such aromatic units can induce the formation of the b-phase. The resulting copolymers
consisting of aromatic units exhibit improved electroluminescence properties relative to the pristine PFO
film. A maximum luminous efficiency of 2.25 cd Aꢀ1 with Commission Internationale de l’Eclairage
coordinates of (0.17, 0.09) is obtained from the light-emitting device based on the copolymer consisting
of SO unit (PFO-SO). Device based on copolymer PFO-SO also exhibits stable electroluminescent spectra
and relatively low roll-off of luminous efficiency at high current densities. These observations indicate
that the incorporation of aromatic moiety in side chain is a promising strategy to obtain stable blue-light-
emitting polyfluorenes with enhanced efficiency.
Keywords:
b-Phase
Dibenzothiophene-S,S-dioxide
Poly(9,9-di-n-octylfluorene)
High efficiency
Blue light emission
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
representative cases is the emergence of green-band during the
long-term operation that can be correlated to the formation of the
Polymer light-emitting diodes (PLEDs) have attracted tremen-
dous attention due to their great potential in full-color flat-panel
displays and solid-state lighting based on large area flexible sub-
strates [1e3]. Among the reported conjugated polymers that have
been utilized as the light-emitting layer for PLEDs, blue light-
emitting polymers remain a great challenge owing to their rela-
tively low luminous efficiency and spectra stability compared with
the green and red light-emitting counterparts [4e7]. As one of the
most extensively investigated blue light-emitting polymers,
poly(9,9-di-n-octylfluorene) (PFO) have been regarded as a prom-
ising blue-emitting candidate which has high efficiency owing to
the high photoluminescent quantum yield (PLQY), good charge
transport property, appreciable thermal stability, as well as versa-
tile molecular structures modifying strategies [8e11]. However,
polyfluorenes also face a range of challenges. One of the most
unanticipated excimers or keto-defect in the carbon bridge of
backbone.
PFO can form various morphological features through different
processing conditions [12e14]. Such morphological features can
affect device performance and emission color. A well-studied case is
the PFO
polymer chain, which can result in an extended conjugation length
[15e17]. In addition to inducing PFO -phase through physically
controlling PFO film forming conditions [18e20], the PFO -phase
b-phase, a phase with a planar zigzag conformation of
b
b
can also be induced by chemically end-capping PFO chain end-
groups by the electron-deficient oxadiazole and triazole moieties,
which gives the enhanced color purity of blue emission associated
with the improved device efficiency [21]. The PFO b-phase can also
be formed by introducing ambipolar unit into the backbone of PFO
[22]. By studying the morphology evolution of penta-fluorene
oligomers thin film through temperature-dependent photo-
luminescence spectra, it is noted that the b-phase of oligofluorenes
can be formed by cooling an amorphous thin-film to 80 K followed
by slowly returning back to room-temperature [23]. The investiga-
tion of optoelectronic properties of oligofluorenes indicates that b-
* Corresponding author.
** Corresponding author.
E-mail addresses: msleiying@scut.edu.cn (L. Ying), psjbpeng@scut.edu.cn
(J. Peng).
http://dx.doi.org/10.1016/j.orgel.2016.08.007
1566-1199/© 2016 Elsevier B.V. All rights reserved.

S. Zhao et al. / Organic Electronics 38 (2016) 130e138
131
phase can be formed in oligofluorenes consisting of more than 15
repeat units [24]. Recently, Xie and Huang introduced a remarkable
progress in realizing polyfluorene with -phase based on a smart
molecular design strategy of utilizing supramolecular steric hin-
drance, which can be achieved by virtue of the introduction of hy-
drophobic interactions of the in-plane alkyl chains cross the steric
hindrance. Of particular interest is that the thin film of the resulting
copolymer PODPF exhibited excellent spectral stability without
green band emission after thermal annealing at 200 ꢁC under ni-
trogen and even underair [25]. Our previous studyalso reveals that a
2.2. Thermal properties
b
Differential scanning calorimetry (DSC) and thermal gravimetric
analysis (TGA) measurements are used to evaluate the thermal prop-
erties of the resulting polymers, with relevant characteristics shown in
Fig. 1. No obvious glass transition temperature can be observed during
the heating process. However, it is interesting to note that copolymers
PFO-Cz and PFO-SO exhibit the melting temperature (Tm)of154ꢁCand
156 ꢁC, respectively during the heating process. During the cooling
procedure, one notes the crystallization temperature (Tc) of 79 ꢁC and
83 ꢁC for copolymers PFO-Cz and PFO-SO, respectively [8,22].
Moreover, the decomposition temperatures (Td, corresponding to the
temperature at 10% weight loss) of these copolymers are higher
than 380 ꢁC, indicating good thermal stability of these copolymers.
Detailed thermal properties data are summarized in Table 1.
certain amount of b-phase can be formed bysolvent vapor annealing
of a blue light-emitting poly-(dibenzothiophene-S,S-dioxide-co-
dioctyl-2,7-fluorene) (PFSO10), leading to the obviously enhanced
luminous efficiency from 2.77 cd Aꢀ1 for the pristine device to
4.8 cd Aꢀ1 for device after solvent vapor treatment [19]. The
b-phase
of poly(9,9-dioctylfluorene) (PFO) can also be induced through
addition of a high boiling-point solvent of 1-chloronaphthalene,
where the improved external quantum efficiency of 1.85% associ-
ated with color purity of (0.16,0.10) can be achieved [26].
In this article, we report a novel strategy to induce the formation
of b-phase in PFO film, which is carried out by chemically incor-
porating an electron-rich carbazole (Cz) unit as the donor moiety or
electron-deficient SO unit as the acceptor moiety in side chain. It is
noted that the electroluminescence (EL) performance can be
remarkably improved upon the incorporation of aromatic Cz or SO
unit. The champion device performance with the maximum lumi-
nous efficiency of 2.25 cd Aꢀ1 is achieved with the Commission
Internationale de l’Eclairage (CIE) coordinates of (0.17, 0.09) for
device based on polymer consists of SO unit in the side chain.
2.3. Photophysical properties
Fig. 2 shows the normalized UVevis absorption and photo-
luminescent (PL) spectra of polymers in toluene solution with
concentration of about 1 ꢂ 10ꢀ5 g mLꢀ1 and as thin films. In toluene
solution, the absorption profiles of both PFO-SO and PFO-Cz show
the maximum absorption at about 390 nm, which can be attributed
to the p-p* transition [27]. The PL spectra of PFO-SO and PFO-Cz are
quite comparable with that of PFO, which exhibit three character-
istic peaks located at 415, 440 and 470 nm.
The UVevis absorption of copolymers PFO-Cz and PFO-SO as
thin films exhibit slightly broadened profiles relative to that of PFO.
It is interesting to note that a new peak located at 436 nm emerged
in both absorption profiles of PFO-Cz and PFO-SO, indicating the
2. Results and discussion
formation of PFO
the absorption spectra, the content of
about 1% and 7% for PFO-Cz and PFO-SO, respectively.
The observation of the formed -phase can also be reflected in the
b
-phase in such thin films. By deconvolution of
b-phase is determined to be
2.1. Synthesis and characterization
b
The detailed synthesis of monomers and polymers is shown in
Scheme 1. Compound 2-(8-bromooctyl)dibenzo[b,d]thiophene (3)
is synthesized based on the alkylation reaction of 2-bromo-dibenzo
[b,d]thiophene (2) and 1,8-dibromooctane at ꢀ78 ꢁC. Compound 2-
(8-bromooctyl)dibenzo[b,d]thiophene-5,5-dioxide (4) is prepared
in a good yield of 85% based on an oxidizing reaction using
hydrogen peroxide as the oxidizer. The alkylation of carbazole by
1,8-dibromooctane gives the intermediate compound 9-(8-
bromooctyl)-9H-carbazole (5) in a good yield. The monomer 9-
(8-(2,7-dibromo-9-octyl-9H-fluoren-9-yl)octyl)-9H-carbazole
(M1) is synthesized based on the reaction of 4 and 5 in a good yield
of 85% in the presence of sodium hydroxide. Similarly, the mono-
corresponding PL spectra. As shown in Fig. 2b, PL profiles for these
copolymers are apparently different. The PL spectrum of PFO spin-
casted from toluene solution exhibits characteristic amorphous
phase, with emission peaks located at 426 (0-0 band), 450 (0e1
band), and 475 nm (0e2 band). In contrast, the PL spectra of the PFO-
Cz and PFO-SO show emission peaked at 440 nm (0-0 band), 462 nm
(0e1 band) and 497 nm (0e2 band), which can be attributed to the
PFO
ray diffraction (XRD) patterns of such thin films (see Fig. S3 in the SI).
One can clearly see a characteristic peak with 2
value of about 7.0ꢁ
for both PFO-SO and PFO-Cz films, which can be correlated to the
characteristic peak of PFO -phase [8,22,28]. In contrast, the
b-phase. The existence of PFO b-phase can also be revealed by X-
q
b
mer
2-(8-(2,7-dibromo-9-octyl-9H-fluoren-9-yl)octyl)dibenzo-
morphology of the pristine PFO film that processed under the same
condition is featureless. The combination of XRD patterns and those
observations of UVevis and PL spectra confirms the formation of PFO
[b,d]thiophene-5,5-dioxide (M2) can be synthesized based on the
reaction of 2,7-dibromo-9-octyl-9H-fluorene (6) and 4 in a yield of
87%. The molecular structures of the target monomers are
confirmed by 1H NMR and 13C NMR spectra (shown in Fig. S1 and
Fig. S2 in the Supporting Information).
b-phase in such thin films. While the emission of PFO-Cz exhibits
obvious characteristic emission corresponding to PFO -phase, one
b
can still observe the existence of a slight shoulder peak located at
422 nm (0-0 band for the amorphous phase), implying the incom-
The target polymers are synthesized based on the Suzuki poly-
merization, which is carried out by copolymerizing the monomer
2,7-bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
dioctylfluorene (M3), 2,7-dibromo-9,9-dioctylfluorene (M4), with
1 mol% of dibromo monomer M1 or M2, with relevant copolymers
denoted as PFO-Cz and PFO-SO, respectively. Both copolymers can
be easily dissolved in common organic solvents, such as tetrahy-
drofuran (THF), chloroform (CHCl3), chlorobenzene and dichloro-
benzene. The number average molecular weight (Mn) estimated by
gel permission chromatography (GPC) is 30.5 and 73.0 kDa, with
polydispersity index (PDI) of 1.55 and 1.62 for PFO-Cz and PFO-SO,
respectively. Detailed molecular weight data are summarized in
Table 1.
plete energy transfer from the amorphous phase to the
b-phase of
PFO. The fluorescence quantum yields ( PL) of copolymers PFO-Cz
Ф
and PFO-SO are evaluated to be 45.7% and 43.3%, respectively,
which are much higher than that of 32.7% for PFO film. The optical
band gap (Eogpt) as estimated from the onset absorption is 3.00, 2.92
and 2.94 eV for copolymers PFO, PFO-Cz and PFO-SO, respectively.
Detailed photophysical parameters are summarized in Table 2.
2.4. Electrochemical properties
Electrochemical properties of copolymers are examined by cy-
clic voltammetry (CV). Fig. 3 shows the p-doping traces of PFO, PFO-

132
S. Zhao et al. / Organic Electronics 38 (2016) 130e138
Scheme 1. Synthetic route for monomers and copolymers.
Table 1
highest occupied molecular orbital (HOMO) levels of the co-
Molecular weight and thermal properties of polymers.
polymers are calculated according to the equation of
EHOMO ¼ ee(Eoxþ 4.80e0.38) (eV), where Eox is the onset of the
oxidation potentials relative to Ag/Agþ. Thus, the EHOMO of co-
polymers is calculated to be ꢀ5.83, ꢀ5.79 and ꢀ5.81 eV, respec-
tively. Unfortunately, we failed to record any reliable n-doping
process. Thus, to get the lowest unoccupied molecular orbital en-
ergy levels (ELUMO) of these polymers, the ELUMO is estimated based
on the difference between EHOMO and the optical band gap (Eogpt),
which is determined to be ꢀ2.83, ꢀ2.87 and ꢀ2.87 eV for co-
polymers PFO, PFO-Cz and PFO-SO, respectively. In considering that
the small molar ratio of Cz and SO unit is attached to the PFO
backbone through a non-conjugated fashion, which will not disturb
a
Polymer
M
n (kDa)
PDI
Tc/ꢁC
Tm/ꢁC
Td /ꢁC
PFO
PFO-Cz
PFO-SO
30.5
65.0
73.0
1.51
1.55
1.62
93
79
83
156
154
156
385
396
410
a
Corresponding to 10 wt% weight loss.
Cz and PFO-SO. The oxidation potentials of polymers are calibrated
by ferrocene/ferrocenium redox couple (Fc/Fcþ), which is measured
to be 0.38 V. It is assumed that the redox potential of Fc/Fcþ has an
absolute energy level of 4.80 V to vacuum [26]. Therefore, the

S. Zhao et al. / Organic Electronics 38 (2016) 130e138
133
(b)
110
100
90
(a)
PFO-Cz
PFO-SO
PFO-Cz
PFO-SO
PFTA
80
70
60
50
40
30
0
50
100
150
200
250
300
100
200
300
400
500
600
700
800
Temperature ( C)
Temparature ( C)
Fig. 1. DSC and TGA curves of copolymers.
(b)
(a)
PFO
1
1
PFO
PFO-Cz
PFO-SO
1
0.8
0.6
0.4
0.2
0
1
PFO-Cz
PFO-SO
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
350
400
450
500
550
600
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 2. UVevis absorption and PL spectra of copolymers in toluene solution with concentration of 1 ꢂ 10ꢀ5 g mLꢀ1 (a) and as thin films (b).
Table 2
Photophysical data and energy levels of the polymers.
Fc/Fc
PFO
HOMO/LUMOa Egopt b ФPL
abs
PL
c
Polymer lmax
lmax
UVevis
PL
In solution (nm)
As thin films
(eV)
(eV)
(%)
(nm)
PFO
PFO-Cz
PFO-SO
388
417
388
425 ꢀ5.83/e2.83
3.00
2.92
2.94
32.7
45.7
43.3
388, 436 414
389, 437 417
383, 434 437 ꢀ5.79/e2.87
PFO-Cz
383, 435 437 ꢀ5.81/e2.87
a
Estimated from the HOMO levels and optical band gaps.
Onset of the absorption spectra.
Measured in solid states.
b
c
PFO-SO
0
0.5
1
1.5
2
2.5
Potential (V)
the electron distribution along the polymer backbone, thus these
copolymers exhibit quite comparable EHOMO/ELUMO. The detailed CV
results are summarized in Table 2.
Fig. 3. CV curves of copolymers.
PEDOT:PSS/polymer (80 nm)/CsF (1.5 nm)/Al (80 nm). Fig. 4a shows
the EL spectra of these polymers. The EL spectra are quite analogous
for all copolymers, with the maximum emission peaked at about
440 nm. The CIE coordinates for PFO-SO is (0.17, 0.09), which lies in
deep-blue region. This high color purity in blue emission is due to
the lack of apparent long-wavelength emission in the EL spectrum.
We note that the emission spectra of PL and EL are different, which
can be attributed to different mechanism in these process. In PL, the
2.5. Electroluminescence properties
The electroluminescence of these copolymers are evaluated
based on polymer light-emitting devices with configuration of ITO/
Table 3
EL characteristics of the copolymers.
mechanism is Fӧrster energy transfer. In EL, the PFO
b-phase acts as
Polymer
V
on (turn on)
LEmax (cd Aꢀ1
)
Lmax (cd mꢀ2
)
CIE (x, y)
the electron-trap, where the LUMO of PFO -phase is located
b
PFO
PFO-Cz
PFO-SO
4.6
3.4
4.4
0.6
1.4
2.25
2691
4916
5101
0.18, 0.17
0.18, 0.10
0.17, 0.09
0.12 eV below that of the amorphous phase [28]. Thus, the PL
spectra of the resulting copolymers exhibit the combination of both

134
S. Zhao et al. / Organic Electronics 38 (2016) 130e138
(b)
(a)
6 mA cm-2
1
1
PFO
12 mA cm-2
60 mA cm-2
120 mA cm-2
180 mA cm-2
PFO-Cz
0.8
0.6
0.4
0.2
0
0.8
PFO-SO
0.6
0.4
PFTA
0.2
0
400
450
500
550
600
650
700
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 4. EL spectra of the resultant copolymers (a) and PFO-SO at various current densities (b).
amorphous phase and
emission is observed.
b
-phase, while in EL spectra, only
b
-phase
Moreover, from Fig. 5b one notes that the device based on PFO-
Cz shows the lowest turn on voltage of 3.4 V among these devices,
and both the current density and luminance of device based on
PFO-Cz is much higher than the other devices at a given driving
voltage, which can be understood as that the attached electron-
donating Cz unit can effectively facilitate hole injection from the
anode. The incorporation of SO unit leads to the slightly decreased
current density, which might be correlated to the formation of
Of particular interest is that these EL spectra exhibit excellent
stability under various current densities. For instance, the EL pro-
files remain nearly unchanged with the current density increased
from 6 to 180 mA cmꢀ2 for device based on PFO-SO as the emissive
layer (Fig. 4b).
Fig. 5 shows the luminous efficiency (LE) as a function of current
density (J) (LE e J) characteristics (a), and current density (J) and
luminance (L) as functions as driving voltage (V) (J e V e L) char-
acteristics (b) of devices based on PFO, PFO-Cz and PFO-SO. It is
worth noting that both devices based on PFO-Cz and PFO-SO as the
electron-trap as a result of the high content of PFO
knowledge, this is the first report of improving blue light-emitting
efficiency and color purity of PFO by inducing the formation of
b-phase. To our
b
-
phase through incorporating organic units into side chain as
additives.
emissive layer show luminous efficiencies of 1.4 and 2.25 cd Aꢀ1
,
respectively, both of which are much higher than that obtained
from the device based on PFO as the emissive layer. Detailed elec-
troluminescent performances of the resulting devices are summa-
rized in Table 3. The improved performance can be attributed to the
increased PL quantum efficiency of such copolymers as a result of
2.6. Charge carrier mobility
To investigate the effect of incorporating aromatic Cz or SO units
on the charge carrier transport properties in the emissive layer, we
fabricate the single charge carrier devices. The device architectures
for hole- and electron-only devices are ITO/PEDOT:PSS (40 nm)/
polymer (80 nm)/MoO3 (10 nm)/Al (100 nm) and ITO/ZnO(40 nm)/
polymer(80 nm)/CsF(1.5 nm)/Al(100 nm), respectively. Fig. 6 shows
the characteristic J e V characteristics of devices. We note that the
electron mobility slightly decrease in PFO-Cz and PFO-SO films,
the formation of PFO
b-phase, or might be correlated to more
balanced charge carrier transport in the emissive layer. We note
that the obtained luminous efficiencies are relatively low, which
can be attributed to the large hole injection barrier between the
PEDOT:PSS (work function of ꢀ5.2 eV) and the resulting copolymer
films (work function ~ ꢀ5.80 eV), leading to the inefficient hole
injection that is responsible for the relative low luminous effi-
ciencies. In this regard, it is rational to suppose that the luminous
efficiency can be enhanced upon employing appropriate device
structures.
which is understandable since the PFO
electron-trap that can effectively decrease the electron flux [29].
This issue can also be confirmed since the PFO-SO film with higher
b-phase is essentially
content of
b-phase exhibited lower electron mobility. It is also
(a)
7000
6000
3
2.5
2
(b)
10
PFO
PFO
PFO-Cz
PFO-SO
PFO-Cz
PFO-SO
10
0.1
5000
4000
3000
2000
1000
0
1.5
1
0.001
10
PFTA
0.5
0
10
0
2
4
6
8
10
12
1
10
100
Voltage (V)
Current density (mA cm )
Fig. 5. LE e J (a) and J e V e L (b) characteristics of copolymers. Device structure: ITO/PEDOT:PSS/polymer/CsF/Al.

S. Zhao et al. / Organic Electronics 38 (2016) 130e138
135
10
10
10
10
10
10
(c)
(a) 10
10
(b)
PFO-SO
PFO
PFO-Cz
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
hole
electron
hole
electron
hole
electron
0
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
Voltage (V)
Voltage (V)
Voltage (V)
Fig. 6. Current densities as a function of driving voltage of devices based on PFO (a), PFO-Cz (b) and PFO-SO (c).
worth pointing out that the decreased electron mobility leads to
more balanced charge carrier transport across the entire applied
voltage range for devices based on PFO-Cz and PFO-SO than that for
PFO. As shown in Fig. 6c, for device based on PFO-SO the electron
and hole densities are very comparable under the same driving
voltage, indicating the significantly enhanced balance of charge
carrier in the emissive layer. This observation is consistent with the
fact that the device based on PFO-SO exhibits the highest luminous
efficiency.
device performances can be primarily attributed to the increased
fluorescence quantum yields, more balanced electron/hole flux as
well as the increased roughness of film surfaces due to the for-
mation of PFO b-phase. It is also worth pointing out that the elec-
troluminescent spectra exhibited excellent stability even at higher
current densities. These observations indicate that incorporation of
a small amount of aromatic units in PFO side chain can be an
effective strategy for improving the performances of blue light
emitting PLEDs.
2.7. Film morphology
4. Experimental section
Tapping mode atomic force microscopy (AFM) measurements
are carried out to get insight into the effects of incorporated aro-
matic unit on the film morphology, with relevant images shown in
Fig. 7. The films are spin-casted from toluene solution on the top of
the prefabricated PEDOT:PSS layer. The PFO film exhibits quite
smooth morphology with root-mean-square (RMS) value of 0.5 nm.
In contrast, the films based on copolymers consisting of Cz and SO
moiety in the side chains exhibit rougher morphology, with RMS
values of 1.0 nm and 1.6 nm for PFO-Cz and PFO-SO, respectively.
The increased roughness indicate that the incorporation of Cz and
SO can induce the reorganization of PFO-Cz and PFO-SO chains.
These observations associated with the obviously increased device
performances highlight that the incorporation Cz or SO unit can be
an effective strategy for attaining the improved blue light-emission
of polyfluorenes.
4.1. Materials
All manipulations involving air-sensitive reagents were per-
formed under an atmosphere of dry argon. Toluene and THF were
purified by standard procedures and distilled under dry argon
before use. All reagents, unless otherwise specified, were obtained
from Aldrich and Acros Chemical Co. and used as received. 2,7-
dibromofluorene, 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-9,9-dioctylfluorene (M3), 2,7-dibromo-9,9-dioctylfluorene
(M4) were synthesized according to reported procedures [30,31]
4.1.1. Synthesis of monomers
4.1.1.1. 2-Bromodibenzo[b,d]thiophene (2).
Dibenzo[b,d]thiophene (11 g, 60 mmol) and iron (0.17 g, 3 mmol)
were added into reaction vessel under the ice bath, then element
bromine (3.1 mL, 60 mmol) was added into the reaction vessel and
stirring the mixture at room temperature for 16 h. The reaction was
quenched by adding sodium dithionite aqueous solution, and
extracted 3 times by using dichloromethane. After removing the
organic phase under reduced pressure, the crude product was pu-
rified by column chromatography with a yield of 80%. 1H NMR
3. Conclusion
In summary, two novel polyfluorene derivatives are designed
and synthesized by incorporating a small amount of aromatic
carbazole and dibenzothiophene-S,S-dioxide unit into the side
chain of PFO. The device performance in terms of luminous effi-
ciency and luminance can be significantly improved after the
incorporation of such aromatic moieties. The enhancement of
(500 MHz, CDCl3)
(m, 1H), 7.70 (d, J ¼ 7.4 Hz, 1H), 7.54 (m, 1H), 7.24 (m, 2H).
d
(ppm) 8.27 (d, J ¼ 1.9 Hz, 1H), 8.10 (m, 1H), 7.85
Fig. 7. AFM topography of PFO (a), PFO-Cz (b) and PFO-SO (c).

136
S. Zhao et al. / Organic Electronics 38 (2016) 130e138
4.1.1.2. 2-(8-Bromooctyl)dibenzo[b,d]thiophene (3).
8.12e8.08 (m, 2H), 7.53 (d, J ¼ 1.4 Hz, 2H), 7.43 (m, 8H), 7.20 (m, 2H),
4.28 (t, J ¼ 7.3 Hz, 2H), 2.76 (t, J ¼ 6.5 Hz, 2H), 2.08e1.96 (m, 2H),
2-Bromodibenzo[b,d]thiophene (3.3 g, 12.5 mmol) was solved in
dry THF under Ar atmosphere, then n-BuLi (3.3 g, 12.5 mmol) was
added into the reaction vessel and stirring the mixture at ꢀ78 ꢁC for
2 h, then 1,8-dibromooctane (11 g, 45 mmol) was added into the
reaction vessel and stirring the mixture at ꢀ78 ꢁC for 1 h. The re-
action was quenched by adding water, and extracted 3 times by
using dichloromethane. After removing the organic phase under
reduced pressure, the crude product was purified by column
chromatography with a yield of 70%. 1H NMR (500 MHz, CDCl3)
1.90e1.76 (m, 2H), 1.40e1.05 (m, 22H), 0.83 (t, J ¼ 7.2 Hz, 3H). 13
C
NMR (125 MHz, CDCl3)
d (ppm) 148.55, 140.56, 138.66, 132.10,
127.39, 125.69, 122.94, 122.19, 121.40, 120.47, 118.81, 108.80, 87.71,
63.70, 43.21, 40.10, 31.91, 30.01, 29.83, 29.47,29.31, 29.24, 29.09,
27.42, 25.90, 23.39, 22.74, 14.22.
4.1.1.7. 2-(8-(2,7-Dibromo-9-octyl-9H-fluoren-9-yl)octyl)dibenzo
[b,d]thiophene-5,5- dioxide (M2).
d
(ppm) 8.18e8.12 (m,1H), 7.96 (d, J ¼ 1.1 Hz,1H), 7.89e7.82 (m,1H),
7.75 (d, J ¼ 7.2 Hz, 1H), 7.48e7.42 (m, 2H), 7.29 (m, 1H), 3.41 (m, 2H),
2.86e2.74 (m, 2H), 1.85 (m, 2H), 1.71 (m, 2H), 1.49e1.28 (m, 8H). 13
NMR (125 MHz, CDCl3) (ppm) 139.97, 139.36, 136.83, 135.83,
2,7-Dibromo-9-octyl-9H-fluorene (5 g, 11.46 mmol), tetra-n-buty-
lammonium bromide (184.75 mg, 573.10
mmol) was added into
C
reaction vessel, then NaOH (2.29 g, 57.31 mmol) aqueous solution
was added into mixture and stirring at room temperature for 2 h, 2-
d
135.66, 127.79, 126.67, 124.34, 123.00, 122.63, 121.63, 121.29, 36.12,
34.07, 32.95, 31.99, 29.47, 29.30, 28.72, 28.19.
(8-bromooctyl)dibenzo[b,d]thiophene
11.46 mmol) was added into mixture and stirring for 24 h at room
temperature. The reaction was extracted times by using
5,5-dioxide
(4.67
g,
3
4.1.1.3. 2-(8-Bromooctyl)dibenzo[b,d]thiophene-5,5-dioxide
(4).
dichloromethane. After removing the organic phase under reduced
2-(8-Bromo-octyl)dibenzo[b,d]thiophene (5.2 g, 15 mmol) and
acetic acid (40 mL) was added into reaction vessel, then H2O2
(8 mL) was added into reaction vessel under stirring at 120 ꢁC. The
reaction was extracted 3 times by using dichloromethane. After
removing the organic phase under reduced pressure, the crude
product was purified by column chromatography with a yield of
pressure, the crude product was purified by column chromatog-
raphy with a yield of 87%. 1H NMR (500 MHz, CDCl3)
d (ppm) 7.81
(d, J ¼ 7.6 Hz,1H), 7.77 (d, J ¼ 7.7 Hz,1H), 7.71 (d, J ¼ 7.9 Hz,1H), 7.63
(m, 1H), 7.55 (d, J ¼ 0.8 Hz, 1H), 7.54e7.49 (m, 3H), 7.46e7.42 (m,
4H), 7.29 (m, 1H), 2.83e2.50 (m, 2H), 1.95e1.84 (m, 4H), 1.58 (m,
2H), 1.23 (m, 16H), 0.83 (t, J ¼ 7.2 Hz, 5H), 0.61e0.53 (m, 4H). 13
C
85%. 1H NMR (500 MHz, CDCl3)
d(ppm) 7.80 (m, 2H), 7.73 (d,
NMR (125 MHz, CDCl3) d (ppm) 152.66, 150.04, 139.23, 138.42,
J ¼ 7.9 Hz, 1H), 7.63 (m, 1H), 7.58 (s, 1H), 7.52 (m, 1H), 7.33 (d,
J ¼ 7.8 Hz, 1H), 3.40 (t, J ¼ 6.8 Hz, 2H), 2.77e2.69 (m, 2H), 2.10 (s,
2H), 1.93e1.80 (m, 2H), 1.49e1.27 (m, 8H).
135.31, 133.85, 131.99, 130.68, 130.35, 130.32, 126.30, 122.30, 122.20,
121.62, 121.57, 121.30, 55.83, 40.31, 40.21, 36.32, 31.90, 31.25, 29.99,
29.86, 29.34, 29.31, 29.29, 29.12, 23.75, 23.67, 22.74, 14.21.
4.1.1.4. 9-(8-Bromooctyl)-9H-carbazole (5). 9H-Carbazole (5 g,
29.9 mmol), K2CO3 (20.66 g, 149.52 mmol) were solved in 60 mL N,
N-dimethylformamide, then the mixture was stirred for 2 h at
110 ꢁC. Then 1,8-dibromooctane (24.40 g, 89.71 mmol) was added
into reaction vessel, and stirring at 110 ꢁC for 12 h. The mixture was
extracted 3 times by using dichloromethane. After removing the
organic phase under reduced pressure, the crude product was pu-
rified by column chromatography with a yield of 80%. 1H NMR
4.1.2. Synthesis of polymers
General procedures of Suzuki copolymerization, taking PFO-SO
as an example.
Under an argon atmosphere, a solution of M3 (321.28 mg,
0.5 mmol), M4 (268.73 mg, 0.49 mmol), M2 (7.63 mg, 0.01 mmol)
and toluene was added to palladium acetate (Pd(OAc)2) (3.4 mg,
0.015 mmol) and tricyclohexyl-phosphine (PCy3) (8.4 mg,
0.03 mmol). The reaction mixture was stirred and heated up to
80 ꢁC. After the mixture became clear, tetraethyl ammonium hy-
droxide (Et4NOH) (20% aq, 2 mL) was added. The temperature was
kept in the range of 80e85 ꢁC, and the solution was allowed to stir
vigorously for 36 h. The reaction was end-capped by adding phe-
nylboronic acid (0.05 g, 0.4 mmol) and was allowed to stir for 12 h.
Then bromobenzene (0.125 g, 0.8 mmol) was added followed by
stirring for another 12 h. After cooling, the mixtures were precip-
itated into methanol (150 mL) and filtered. The collected solids
were re-dissolved in dichloromethane and washed three times
with de-ionized water. The organic phase was concentrated under
reduced pressure, followed by re-precipitation in methanol. The
crude product was further purified by Soxhlet extraction by
methanol and acetone successively. The target polymer was
collected after drying under vacuum with a yield of 60%. 1H NMR
(500 MHz, CDCl3)
d
(ppm) 8.11 (d, J ¼ 7.7 Hz, 2H), 7.47 (m, 2H), 7.41
(d, J ¼ 8.2 Hz, 2H), 7.25e7.20 (m, 2H), 4.31 (t, J ¼ 7.2 Hz, 2H), 3.38 (t,
J ¼ 6.8 Hz, 2H), 1.94e1.73 (m, 4H), 1.34 (m, 8H).
4.1.1.5. 2,7-Dibromo-9-octyl-9H-fluorene (6).
2,7-Dibromo-9H-fluorene (10 g, 30.86 mmol), KOH (860.80 mg,
15.43 mmol) and octyl alcohol (19.94 g, 154.32 mmol) were added
into 250 mL reaction vessel, and stirring the mixture at room
temperature for 24 h. The mixture was cooled to room temperature
and vacuum distilled to remove excessive octyl alcohol. Then the
mixture was extracted 3 times by using dichloromethane. After
removing the organic phase under reduced pressure, the crude
product was purified by column chromatography with a yield of
78%. 1H NMR (500 MHz, CDCl3)
(s, 1H), 7.42 (s, 1H), 7.36 (m, 2H), 4.03 (m, 1H), 2.02e2.10 (m, 2H),
1.22e1.41 (m, 12H), 0.94 (t, J ¼ 6.9 Hz, 3H).
d
(ppm) 7.81 (d, J ¼ 6.4 Hz, 2H), 7.51
(500 MHz, CDCl3) d (ppm) 7.85e7.84 (br, ArH), 7.71e7.68 (br, ArH),
2.21 (br, CH2), 1.22e1.44 (br, CH2), 0.83e0.80 (br, CH3).
PFO. M3(321.28 mg, 0.5 mmol) andM4(274 mg, 0.5mmol), yield:
4.1.1.6. 9-(8-(2,7-Dibromo-9-octyl-9H-fluoren-9-yl)octyl)-9H-carba-
zole (M1). 2,7-Dibromo-9-octyl-9H-fluorene (5 g, 11.46 mmol),
76%.1H NMR (500 MHz, CDCl3)
d(ppm)7.85e7.84(br, ArH), 7.71e7.68
(br, ArH), 2.21 (br, CH2), 1.20e1.440 (br, CH2), 0.83e0.80 (br, CH3).
PFO-Cz. M3 (321.28 mg, 0.5 mmol), M4 (268.73 mg, 0.49 mmol),
M1 (7.13 mg, 0.01 mmol), yield: 70%. 1H NMR (500 MHz, CDCl3)
tetrabutylammonium bromide (184.75 mg, 573.10
mmol) was
added into reaction vessel, then NaOH (2.29 g, 57.31 mmol)
aqueous solution was added into mixture and stirring at room
d
(ppm) 7.85e7.84 (br, ArH), 7.71e7.68 (br, ArH), 2.23e2.21 (br,
temperature for
11.46 mmol) was added into mixture and stirring for 24 h at room
temperature. The reaction was extracted times by using
2
h, 9-(8-bromooctyl)-9H-carbazole (4.11 g,
CH2), 1.45e1.30 (br, CH2), 0.83e0.80 (br, CH3).
3
4.2. Measurements
dichloromethane. After removing the organic phase under reduced
pressure, the crude product was purified by column chromatog-
Nuclear magnetic resonance (NMR) spectra were recorded on a
Bruker DRX 500 spectrometer (operating at 500 MHz for 1H NMR,
raphy with a yield of 85%. 1H NMR (500 MHz, CDCl3)
d (ppm)

S. Zhao et al. / Organic Electronics 38 (2016) 130e138
137
and 125 MHz for 13C NMR) in deuterated chloroform solution.
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