Blue light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide unit in alkyl side chain

Article abstract of DOI:10.1007/s11426-017-9100-0

Blue light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide (SO) unit in alkyl side chain (PF-FSOs and PF-CzSOs) were synthesized. All the polymers show high thermal stability with the decomposition temperatures over 400 °C. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the copolymer slightly decrease with the increase of SO content in side chain. PL spectra of the polymers show slightly red shift and broadening with the increase of solvent polarities, indicating unremarkable intramolecular charge transfer (ICT) effect in the polymers containing SO unit in alkyl side chain. EL spectra of the polymers are almost unchanged in the current densities from 100 to 400 mA cm^?2, indicating the superb EL stability of the resulted polymers. The EL spectra of the copolymers exhibit obvious blue-shift and narrowing with the CIE of (0.18, 0.11) for PF-FSO10 and (0.17, 0.11) for PF-CzSO10, respectively, compared with PF-SO10 containing SO unit in main chain with the CIE of (0.16, 0.17) and PFO with the CIE of (0.18, 0.18). The superior device performances were obtained with the luminous efficiency (LE_max) of 1.17 and 0.68 cd A^?1 for PF-FSO15 and PF-CzSO20, respectively, compared with the LE_max of 0.37 cd A^?1 for PFO. The results indicate that linking SO unit to alkyl side chain of the polyfluorene is a promising strategy for efficient blue light-emitting polymers.

Full text of DOI:10.1007/s11426-017-9100-0

SCIENCE CHINA
Chemistry
doi: 10.1007/s11426-017-9100-0
• ARTICLES •
Blue light-emitting polyfluorenes containing
dibenzothiophene-S,S-dioxide unit in alkyl side chain
Yiwen Fang, Jie Liu, Yuhao Zhang, Ting Guo*, Fei Huang, Wei Yang* & 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
Received March 24, 2017; accepted June 15, 2017; published online September 13, 2017
Blue light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide (SO) unit in alkyl side chain (PF-FSOs and
PF-CzSOs) were synthesized. All the polymers show high thermal stability with the decomposition temperatures over
400ꢀ°C. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels
of the copolymer slightly decrease with the increase of SO content in side chain. PL spectra of the polymers show slightly red
shift and broadening with the increase of solvent polarities, indicating unremarkable intramolecular charge transfer (ICT) effect
in the polymers containing SO unit in alkyl side chain. EL spectra of the polymers are almost unchanged in the current densities
from 100 to 400ꢀmAꢀcm−2, indicating the superb EL stability of the resulted polymers. The EL spectra of the copolymers exhibit
obvious blue-shift and narrowing with the CIE of (0.18, 0.11) for PF-FSO10 and (0.17, 0.11) for PF-CzSO10, respectively,
compared with PF-SO10 containing SO unit in main chain with the CIE of (0.16, 0.17) and PFO with the CIE of (0.18, 0.18).
The superior device performances were obtained with the luminous efficiency (LEmax) of 1.17 and 0.68ꢀcdꢀA−1 for PF-FSO15 and
PF-CzSO20, respectively, compared with the LEmax of 0.37ꢀcdꢀA−1 for PFO. The results indicate that linking SO unit to alkyl side
chain of the polyfluorene is a promising strategy for efficient blue light-emitting polymers.
dibenzothiophene-S,S-dioxide, alkyl side chain, polyfluorene, blue emission, polymer light-emitting diode (PLED)
Citation:
Fang Y, Liu J, Zhang Y, Guo T, Huang F, Yang W, Cao Y. Blue light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide unit in alkyl side
chain. Sci China Chem, 2017, 60, doi: 10.1007/s11426-017-9100-0
1 Introduction
ficiency and color stability [8–10]. As one of the most ex-
tensively investigated blue light-emitting polymers, polyflu-
orene (PF) has been regarded as a potential blue-emitting
polymers owing to the high photoluminescent quantum yield
(PLQY), good charge transport property, outstanding thermal
stability, and versatile molecular structures modifying strate-
gies [11–13]. However, the light emitting diodes fabricated
from polyfluorene always suffer from a degradation of the
device under thermal annealing or upon long-term operation,
which induces color impurity [14,15]. And the efficiency is
limited by the intrinsic disadvantage of unbalanced transport
of electrons and holes. Many efforts have been made to ad-
dress such drawbacks [16,17]. One effective way is to in-
corporate various electron-deficient moieties into the polymer
Recently, polymer light-emitting diodes (PLEDs) have at-
tracted considerable attention due to their great potential ap-
plication in full-color flat-panel displays and solid-state light-
ings based on large area flexible substrates [1–4]. Among
the currently available conjugated polymers as the emissive
layer for PLEDs, the application of red and green light-emit-
ting polymers has reached the industrial production standard
[5–7]. Then blue light-emitting polymers remain a great chal-
lenge for full-color display in terms of lower quantum ef-
*Corresponding authors (email: mstguo@scut.edu.cn; pswyang@scut.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2017
chem.scichina.com link.springer.com

2
Fang et al. Sci China Chem
backbone, which can improve the efficiency and color purity
of blue light-emitting polymers [18–20].
benzo [b,d] thiophene 5,5-dioxide (5)
2,7-Dibromo-9-octyl-9H-fluorene (5ꢀg, 11.46ꢀmmol), tetra-
n-butylammonium bromide (184.75ꢀmg, 573.10ꢀmmol) and
dimethyl sulphoxide (DMSO) (50ꢀmL) were added into
reaction vessel, then NaOH (2.29ꢀg, 57.31ꢀmmol) aqueous
solution was added into mixture and stirring at room tem-
perature for 2ꢀh, 2-(8-bromohexyl) dibenzo [b,d] thiophene
5,5-dioxide (4.35ꢀg, 11.46ꢀmmol) was added into mixture
and stirring for 24ꢀh at room temperature. 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 87%.
1H NMR (500ꢀMHz, CDCl3) δ (ppm): 7.81 (dd, 2H), 7.71 (d,
1H), 7.66 (t, 1H), 7.61–7.45 (m, 8H), 7.24 (d, 1H), 2.63 (t,
2H), 1.92 (m, 4H), 1.49 (m, 2H), 1.29–1.20 (br, 14H), 0.83
(m, 3H), 0.60 (br, 4H).13C NMR (125ꢀMHz, CDCl3) δ (ppm):
150.02, 144.18, 140.06, 137.65, 134.86, 134.26, 133.53,
130.91, 130.78, 130.63, 129.64, 128.67, 128.0, 122.53,
122.45, 121.70, 43.82, 36.05, 31.26, 30.21, 29.25, 24.46.
(2) 2-(6-(3,6-Dibromo-9H-carbazole-9-yl)hexyl) dibenzo
[b,d] thiophene 5,5-dioxide (7)
The dibenzothiophene-S,S-dioxide (SO) unit is topologi-
cally similar to fluorene and carbazole moiety by replacing
carbon or nitrogen atom with a –SO2– group, turning it into
an electron-deficient moiety with high electron affinity and
high fluorescent efficiency [21–23]. Recently, SO derivatives
were incorporated into small molecules and polymers to im-
prove the electron injection/transportation ability and simul-
taneously achieve high-efficiency blue light emitter [24–26].
Specially, the fluorene-based polymers embodying SO unit
have obtained high device efficiencies [27–32]. For example,
the spectral stability and luminous efficiency of the blue-emit-
ting polyfluorenes were significantly improved via incorpo-
rating of SO isomer (3,7-diyl or 2,8-diyl) into the polyflu-
orene backbone [24]. The solubility and the color purity
were improved via introducing 2,8-dioctyldibenzothiophene-
S,S-dioxide (DOSO) unit in the polyfluorene backbone [25].
While the intramolecular charge transfer (ICT) effect existed
in the polymers containing SO unit in the main chain cause
the red shift and broadening PL spectra, and further result in
the color impurity in blue emission [26].
3,6-Dibromocarbazole (3.72ꢀg, 11.46ꢀmmol), tetra-n-buty-
lammonium bromide (184.75ꢀmg, 573.10ꢀmmol) and
DMSO (50ꢀmL) were added into reaction vessel, then
NaOH (2.29ꢀg, 57.31ꢀmmol) aqueous solution was
added into mixture and stirring at room temperature for
2ꢀh, 2-(8-bromohexyl) dibenzo [b,d] thiophene 5,5-dioxide
(4.35ꢀg, 11.46ꢀmmol) was added into mixture and stirring
for 24ꢀh at room temperature. 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 90%. 1H NMR
(500ꢀMHz, CDCl3) δ (ppm): 8.45 (d, 2H), 7.82 (d, 1H),
7.66–7.60 (dd, 2H), 7.63 (m,1H), 7.56–7.52 (m, 4H), 7.27
(d, 3H), 4.27–4.22 (t, 2H), 2.68–2.63 (t, 2H), 1.87–1.82
(br, 2H), 1.63–1.58 (br, 2H), 1.36–1.26 (m, 4H). 13C NMR
(125ꢀMHz, CDCl3) δ (ppm): 149.30, 139.18, 138.11, 135.19,
133.69, 131.80,131.59, 130.33, 130.19, 128.96, 123.35,
123.18, 122.02, 121.99, 121.97, 121.38, 121.30,111.92,
110.31, 43.14, 35.91, 30.79, 28.74, 28.67, 26.96.
Up to date, the reported light-emitting polymers containing
SO unit were almost based on the SO unit in main chain, al-
though the β-phase was induced by grafting the SO unit of 1
mol% in the side chain of poly(9,9-dioctylfluorene) [30]. The
polymers linked SO unit in alkyl side chain keep a certain dis-
tance between the backbone donor and SO acceptor, which
probably could restrain the ICT effect. Herein, the polyflu-
orenes containing SO unit linked to the alkyl side chain of
fluorene and carbazole moieties were synthesized and inves-
tigated, respectively.
2 Experimental
2.1 Materials
Solvents (THF and toluene) were purified and freshly distilled
prior to use according to the reported procedure. All reagents,
unless otherwise specified, were obtained from Aldrich
(USA), Acros (Belgium), or TCI Chemical Co. (Japan) and
used as received. All manipulations involving air-sensitive
reagents were performed under an atmosphere of dried argon.
2-Bromo-dibenzo [b,d] thiophene (1), 2-(8-bromohexyl)
dibenzo [b,d] thiophene (2), 2-(8-bromohexyl) dibenzo [b,d]
thiophene-5,5-dioxide (3), 2,7-dibromo-9-octyl-9H-fluorene
(4) and 3,6-dibromocarbazole (6), 2,7-bis(4,4,5,5-tetram-
2.1.2 Synthesis of the polymers
General procedures of Suzuki copolymerization, taking
PF-FSO5 as an example.
Under
(321.3ꢀmg, 0.5ꢀmmol),
an
argon
atmosphere,
(246.8ꢀmg, 0.45ꢀmmol),
monomer
8
5
9
(36.6ꢀmg, 0.05ꢀmmol), palladium acetate (Pd(OAc)2)
(3.4ꢀmg, 0.015ꢀmmol) and tricyclohexylphosphine (PCy3)
(8.4ꢀmg, 0.03ꢀmmol) were added to toluene solvent. The
solution was stirred and heated up to 80ꢀ°C. After the solution
became clear, tetraethyl ammonium hydroxide (Et4NOH)
(20% aq, 4ꢀmL) was added. The temperature was kept in
the range of 80–85ꢀ°C, and the solution was allowed to stir
ethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene
(8),
2,7-dibromo-9,9-dioctylfluorene (9) and 3,7-dibromo-diben-
zothiophene-S,S-dioxide (10) were synthesized according to
the reported procedures [33–35].
2.1.1 Synthesis of monomers
(1) 2-(6-(2,7-Dibromo-9-octyl-9H-fluoren-9-yl)hexyl)di-

Fang et al. Sci China Chem
3
vigorously for 36ꢀh. The reaction was end-capped by adding
phenylboronic acid (0.05ꢀg, 0.4ꢀmmol) and allowed to stir for
12ꢀh. Then bromobenzene (0.125ꢀg, 0.8ꢀmmol) was added
followed by stirring for another 12ꢀh. After cooling, the mix-
tures were precipitated into methanol (150ꢀmL) and filtered.
The collected solids were re-dissolved in dichloromethane
and washed 3 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
ArH), 7.79–7.57 (brm, ArH), 2.16 (br, CH2), 1.28–1.04
(br, CH2), 0.84 (br, CH3). Elemental analysis calcd (%) for
[(C29H40)0.95(C30H25O2NS)0.05]: C 88.99, H 10.02, S 0.41;
found: C 88.31, H 11.62, S 0.57.
PF-CzSO10: monomer 8 (321.3ꢀmg, 0.5ꢀmmol), 9
(219.4ꢀmg, 0.40ꢀmmol) and 7 (62.3ꢀmg, 0.10ꢀmmol), yield:
1
68%. H NMR (500ꢀMHz, CDCl3) δ (ppm): 8.54 (s, ArH),
7.89 (brm, ArH), 7.79–7.55 (brm, ArH), 2.16 (br, CH2),
1.28–1.05 (br, CH2), 0.84 (br, CH3). Elemental analysis calcd
(%) for [(C29H40)0.90(C30H25O2NS)0.10]: C 88.29, H 9.73, S
0.81; found: C 87.26, H 10.14, S 1.15.
1
after drying under vacuum with a yield of 65%. H NMR
PF-CzSO15: monomer 8 (321.3ꢀmg, 0.5ꢀmmol), 9
(192.0ꢀmg, 0.35ꢀmmol) and 7 (93.5ꢀmg, 0.15ꢀmmol), yield:
(500ꢀMHz, CDCl3) δ (ppm): 7.88 (br, ArH), 7.74 (br, ArH),
2.15 (br, CH2), 1.28–1.05 (br, CH2), 0.84 (t, CH3). Elemental
analysis calcd (%) for [(C29H40)0.95(C39H42O2S)0.05]:C 89.10,
H 10.09, S 0.40; found: C 88.62, H 10.73, S 0.71.
PFO: monomer 8 (321.3ꢀmg, 0.5ꢀmmol) and 9 (274.2ꢀmg,
0.5ꢀmmol), yield: 76%. 1H NMR (500ꢀMHz, CDCl3) δ
(ppm): 7.85–7.84 (br, ArH), 7.71–7.68 (br, ArH), 2.21 (br,
CH2), 1.22–1.44 (br, CH2), 0.83–0.80 (br, CH3). Elemental
analysis calcd (%) for [(C29H40)]: C 89.69, H 10.31; found:
C 88.34, H 10.13.
PF-SO10: monomer 8 (321.3ꢀmg, 0.5ꢀmmol), 9 (219ꢀmg,
0.40ꢀmmol) and 10 (37.4ꢀmg, 0.10ꢀmmol), yield: 68%.
1H NMR (500ꢀMHz, CDCl3) δ (ppm): 8.12 (s, Ar–H),
7.88 (br, 2H), 7.68 (br, 4H), 2.14 (br, 4H), 1.16–1.01
(m, 24H), 0.78 (br, 6H); elemental analysis calcd (%) for
[(C29H40)0.90(C12H6O2S)0.10]: C 88.40, H 9.88, S 0.86; found:
C 87.66, H 9.51, S 0.73.
1
75%. H NMR (500ꢀMHz, CDCl3) δ (ppm): 8.54 (s, ArH),
7.89 (brm, ArH), 7.79–7.71 (brm, ArH), 7.64–7.53 (brm,
ArH), 2.16 (br, CH2), 1.28–1.05 (br, CH2), 0.84 (br, CH3). El-
emental analysis calcd (%) for [(C29H40)0.85(C30H25O2NS)0.15]:
C 87.61, H 9.45, S 1.20; found: C 87.33, H 10.08, S 1.29.
PF-CzSO20: monomer 8 (321.3ꢀmg, 0.5ꢀmmol), 9
(164.5ꢀmg, 0.3ꢀmmol) and 7 (124.6ꢀmg, 0.2ꢀmmol), yield:
1
70%. H NMR (500ꢀMHz, CDCl3) δ (ppm): 8.54 (s, ArH),
7.86–7.71 (brm, ArH), 7.64–7.57(brm, ArH), 2.74 (br,
CH2), 2.16 (br, CH2), 1.71 (br, CH2), 1.50 (br, CH2), 1.17
(br, CH2), 0.84(br, CH3). Elemental analysis calcd (%)
for [(C29H40)0.80(C30H25O2NS)0.20]: C 83.97, H 9.18, S 1.59;
found: C 86.51, H 9.53, S 1.97.
2.2 Measurements
PF-FSO10: monomer 8 (321.3ꢀmg, 0.5ꢀmmol),
9
Nuclear magnetic resonance (NMR) spectra were recorded on
a Bruker DRX 500 spectrometer (operating at 500ꢀMHz for
1H NMR, and 125ꢀMHz for 13C NMR, Germany) in deuter-
ated chloroform solution, tetramethylsilane was used as the
reference. Elemental analysis was performed on a Vario EL
Elemental Analysis Instrument (Elementar Co., Germany).
Number-average (Mn) molecular weights were determined
by using a Waters GPC 2410 instrument (USA) in tetrahydro-
furan (THF) via a calibration curve of polystyrene standards.
Differential scanning calorimetry (DSC) measurements
were carried out on a Netzsch DSC 204 (Germany) at a
heating rate of 10ꢀ°Cꢀmin−1 under N2. Thermal gravimetric
analysis (TGA) was carried out in a Netzsch TGA-209
thermal analyzer. All measurements were carried out under
N2 with a heating rate of 20ꢀ°Cꢀmin−1. Cyclic voltammetry
(CV) was measured by using a CHI660A electrochemical
work-station. The measurement was performed at a scan rate
of 50ꢀmVꢀs−1 at room temperature under the inert atmosphere
of Ar. The tetra-(n-butyl) ammonium hexafluorophosphate
(n-Bu4NPF6, 0.1ꢀM) in acetonitrile solution was selected as
the electrolyte. UV-Vis absorption spectra of polymers in
both dilute toluene solution and as thin films were recorded
with a HP 8453 spectrophotometer (USA). Photolumines-
cence (PL) spectra were recorded with a Spex Fluorolog-3
(219.4ꢀmg, 0.4ꢀmmol) and 5 (73.3ꢀmg, 0.10ꢀmmol), yield:
69%. 1H NMR (500ꢀMHz, CDCl3) δ (ppm): 7.89 (br,
ArH), 7.74–7.50 (br, ArH), 2.15 (br, CH2), 1.57–1.05 (br,
CH2), 0.84 (t, CH3). Elemental analysis calcd (%) for
[(C29H40)0.90(C39H42O2S)0.10]: C 88.54, H 9.89, S 0.79; found:
C 88.13, H 10.43, S 1.02.
PF-FSO15: monomer 8 (321.3ꢀmg, 0.5ꢀmmol),
9
(192.0ꢀmg, 0.35ꢀmmol) and 5 (109.9ꢀmg, 0.15ꢀmmol), yield:
62%. 1H NMR (500ꢀMHz, CDCl3) δ (ppm): 7.88 (br,
ArH), 7.74–7.49 (br, ArH), 2.61 (br, CH2), 2.16 (br, CH2),
1.57–1.17 (br, CH2), 0.84 (t, CH3). Elemental analysis calcd
(%) for [(C29H40)0.85(C39H42O2S)0.15]: C 88.00, H 9.69, S 1.15;
found: C 87.30, H 11.09, S 1.27.
PF-FSO20: monomer 8 (321.3ꢀmg, 0.5ꢀmmol),
9
(164.5ꢀmg, 0.3ꢀmmol) and 5 (146.5ꢀmg, 0.2ꢀmmol), yield:
1
73%. H NMR (500ꢀMHz, CDCl3) δ (ppm): 7.88 (br, ArH),
7.72–7.50 (br, ArH), 2.61 (br, CH2), 2.16 (br, CH2), 1.57 (br,
CH2), 1.18 (br, CH2), 0.84 (t, CH3). Elemental analysis calcd
(%) for [(C29H40)0.80(C39H42O2S)0.20]: C 87.49, H 9.50, S 1.51;
found: C 86.85, H 11.01, S 1.94.
PF-CzSO5: monomer 8 (321.3ꢀmg, 0.5ꢀmmol),
(246.8ꢀmg, 0.45ꢀmmol) and 7 (31.2ꢀmg, 0.05ꢀmmol), yield:
63%. 1H NMR (500ꢀMHz, CDCl3) δ (ppm): 7.88 (brm,
9

4
Fang et al. Sci China Chem
spectrofluorometer (Japan). Photoluminescence (PL) ef-
ficiencies were determined in an IS080 integrating sphere
(Labsphere, USA) with 325ꢀnm excitation of a HeCd laser
(Mells Griot, USA). PL and EL spectra were recorded on
an Instaspec IV CCD spectrophotometer (Oriel Co., UK).
The morphology of polymer was characterized by a tapping
mode atomic force microscopy (AFM), which consists a
Seiko SPA 400 (Japan) equipped with an SPI 3800 probe
station. The films were spin-coated from xylene solution
on the top of the prefabricated poly(styrenesulfonic acid)
(PEDOT:PSS) layer.
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 zinc oxide (40ꢀnm) to replace the
PEDOT:PSS film, then a thick copolymer layer (80ꢀnm) was
spin-cast from p-xylene solution on top of the zinc oxide
layer and annealed at 100ꢀ°C for 20ꢀmin. Profilometry
(Tencor Alfa-Step 500) was used to determine the thickness
of the films. Finally, 1.5ꢀnm of CsF followed by 100ꢀnm of
aluminum (thickness monitored with a STM-100/MF Sycon
quartz crystal) were thermally evaporated through a shadow
mask at a base pressure of 3.0×10−4ꢀPa to form the cathode.
2.3 Device fabrication and characterization
3 Results and discussion
The obtained polymers were dissolved in p-xylene and fil-
tered with a 0.45ꢀmm PTFE filter. Patterned ITO coated glass
substrates were cleaned with acetone, detergent, distilled
water and 2-propanol followed by ultra-sonication. After
treatment with oxygen plasma, 50ꢀnm of poly (3,4-ethylene-
dioxythiophene) doped with PEDOT:PSS (Batron-P 4083,
Bayer AG, Germany) was spin-coated onto the cleaned ITO
substrates followed by drying on a hot plate at 120ꢀ°C for
20ꢀmin. The films of copolymers were coated on the top
of the prefabricated PEDOT:PSS layer by spin-casting in a
nitrogen-filled glove box. The film thickness of the active
layers was around 75–80ꢀnm, as estimated by an Alfa Step
500 surface profiler (Tencor, USA). Finally, 1.5ꢀnm of CsF
followed by 120ꢀnm of aluminum (thickness monitored with
a STM-100/MF-Sycon quartz crystal) were subsequently
evaporated on the top of the light-emitting copolymer layer
in a vacuum of 3×10−4ꢀPa.
3.1 Synthesis and characterization
The detialed synthetic routes of monomers are shown
in Scheme 1. Compound 2-bromo-dibenzo [b,d] thio-
phene (1) was synthesized by bromination using liquid
bromine as bromine source. 2-(8-Bromohexyl) dibenzo
[b,d] thiophene (2) was synthesized based on the alkylation
reaction of monomer 1 and 1,6-dibromohexyl at ultralow
temperature. Compound 2-(8-bromohexyl) dibenzo [b,d]
thiophene-5,5-dioxide (3) was prepared in a high yield
of 89% based on an oxidizing reaction using hydrogen
peroxide as the oxidizer. 2-(6-(2,7-Dibromo-9-octyl-9H-flu-
oren-9-yl)hexyl) dibenzo [b,d] thiophene 5,5-dioxide (5)
was prepared based on the reaction of monomer 3 and 4
under air atmosphere in DMSO solution with a yield of 87%.
The same synthetic method was used for the preparation
of 2-(6-(3,6-dibromo-9H-carbazole-9-yl) hexyl) dibenzo
[b,d] thiophene 5,5-dioxide (7) with a yield of 90%, the
related 1H NMR spectra are shown in Figure S1 (Supporting
Information online).
The hole-/electron-only devices were used as ITO/PE-
DOT:PSS (40ꢀnm)/polymer (80ꢀnm)/MoO3 (10 nm)/Al and
ITO/ZnO (40ꢀnm)/polymer (80ꢀnm)/CsF (1.5ꢀnm)/Al, re-
spectively. To fabricate the hole-only device, a layer of MoO3
(10ꢀnm) instead of cesium fluoride was thermally deposited
The detailed synthetic routes of polymers are shown in
Scheme 1 Synthetic routes of the monomers.

Fang et al. Sci China Chem
5
Scheme 2. Through a palladium catalyzed Suzuki polymer-
3.2 Thermal properties
ization with molar feed ratios of monomer 8:9:5 of 50:50:0,
50:45:5, 50:40:10, 50:35:15, and 50:30:20, the correspond-
ing polymers were afforded and denoted as PFO, PF-FSO5,
PF-FSO10, PF-FSO15, and PF-FSO20, respectively. Re-
placing monomer 5 by 7 and with the molar feed ratio of
monomer 8:9:7 of 50:45:5, 50:40:10, 50:35:15, 50:30:20,
copolymers that denoted as PF-CzSO5, PF-CzSO10,
PF-CzSO15, and PF-CzSO20 were afforded in moderate
yields, respectively.
All the resulting copolymers can be easily dissolved in
common organic solvents, such as tetrahydrofuran (THF),
chloroform (CHCl3), chlorobenzene and dichlorobenzene at
room temperature. The number average molecular weights
(Mn) are in the range of 87.9–171.7ꢀkDa with a polydispersity
index (PDI) of 1.35–1.90 for PF-FSOs and PF-CzSOs. The
content of SO unit in copolymers were calculated from the
results of elemental analysis and listed in Table 1. It can be
seen that the actual contents of SO unit in polymers are more
than the feed ratios, which means that the monomers 5 and
7 own higher reactivity.
Thermal properties of the polymers were evaluated by the
thermal gravimetric analysis (TGA) measurement. TGA re-
veals excellent thermal stability of polymers under nitrogen.
As shown in Table 1, the decomposition temperature (5%
weight loss) of the homopolymer PFO is 385ꢀ°C, while those
of the copolymers are over 400ꢀ°C (Figure S2). Thermally
induced phase transition behavior of the polymers was in-
vestigated by the differential scanning calorimetry (DSC).
Figure 1 shows the heating curves of polymers PF-FSOs (a)
and PF-CzSOs (b). The glass transition temperatures (Tg)
of PF-FSOs are in the range of 79–94ꢀ°C, higher than that
of PFO. The phase transition peaks from partially crystalline
state to liquid crystalline (TCr-LC) appear around 142ꢀ°C for
PFO, and 139–155ꢀ°C for PF-FSOs (Figure 1(a)). From
Figure 1(b), it can be seen that no obvious phase transition
peak occurred, neither the glass transition nor the partially
crystalline state, which indicate the higher degree of unreg-
ularity of PF-CzSOs backbone, due to the meta-linking car-
bazole unit in the polymer main chain [36].
Scheme 2 Synthetic routes of the polymers.
Table 1 Molecular weight and thermal properties of the polymers
SO (mol%) in
TLC
polymer
Mn (×104)
PDI
Td (oC)
Tg (oC)
(oC)
a)
feed ratio
polymer
0
PFO
3.05
3.55
2.31
1.82
1.64
1.35
1.45
1.68
1.55
1.90
1.47
1.48
0
72
–
140
156
155
150
148
139
–
385
397
411
410
411
412
413
411
405
408
PF-SO10
10
5
8.5
–
PF-FSO5
10.25
15.62
14.93
11.18
12.39
8.79
8.9
PF-FSO10
PF-FSO15
PF-FSO20
PF-CzSO5
PF-CzSO10
PF-CzSO15
PF-CzSO20
10
15
20
5
13.1
16.6
26.5
7.0
79
83
94
–
–
–
10
15
20
14.3
16.1
25.0
–
–
16.66
17.17
–
–
a) Calculated from the elemental analysis data.

6
Fang et al. Sci China Chem
3.3 Electrochemical properties
measured to be 0.49ꢀV. It is assumed that the redox po-
tential of Fc/Fc+ has an absolute energy level of 4.8ꢀeV to
vacuum [37]. Therefore, the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels of the copolymers are calculated
according to the equation of EHOMO=−e(Eox+4.80−0.49) (eV)
and ELUMO=−e(Ered+4.80−0.49) (eV), where Eox and Ered are
the onset of the oxidation potentials and reduction potentials
Electrochemical properties of the copolymers were ex-
amined by cyclic voltammetry (CV). The p-doping and
n-doping traces of copolymers are shown in Figure 2, and
the detailed electrochemical data are summarized in Table 2.
The oxidation potentials of polymers were calibrated by
ferrocene/ferrocenium redox couple (Fc/Fc+), which was
Figure 1 DSC curves of the polymers (color online).
Figure 2 CV curves for PF-FSOs (a) and PF-CzSOs (b) (color online).
Table 2 Photophysical and electrochemical properties of the polymers
b)
sample
SO
Eox (V)
1.6
HOMO (eV)
−6.00
Ered (V)
−1.74
−2.42
−2.19
−2.23
−2.26
−2.28
−2.36
−2.30
−2.25
−2.34
−2.21
LUMO (eV)
−2.57
−1.89
−2.12
−2.08
−2.05
−2.03
−1.95
−2.08
−2.06
−1.97
−2.10
Egopt a) (eV)
–
λabs,f (nm)
–
λPL,f (nm)
–
QPL (%)
–
PFO
1.28
1.44
1.38
1.46
1.40
1.46
1.44
1.42
1.46
1.37
−5.68
3.00
2.90
2.93
2.93
2.93
2.93
2.94
2.96
2.96
2.96
388
390
384
385
385
385
382
382
379
375
425450
446468
426447
423447
424,449
423,445
423,439
423,448
422,447
424,447
32
42
48
40
51
59
34
32
36
66
PF-SO10
PF-FSO5
PF-FSO10
PF-FSO15
PF-FSO20
PF-CzSO5
PF-CzSO10
PF-CzSO15
PF-CzSO20
−5.75
−5.69
−5.77
−5.71
−5.77
−5.75
−5.73
−5.77
−5.68
a) Calculated from the absorption spectra threshold; b) measured in film.

Fang et al. Sci China Chem
7
relative to Ag/Ag+. The EHOMO and ELUMO of PF-FSOs
and PF-CzSOs are in the range of −5.69–−5.77ꢀeV and
−5.68–−5.75ꢀeV, −1.95–−2.08ꢀeV and −1.97–−2.10ꢀeV, re-
spectively, which slightly decrease with the increase of SO
content in side chain.
peaks for PF-FSOs, indicating the SO moiety in alkyl side
chain have no contributions to the PL emission of poly-
mers (Figure 3(a)). The detailed photophysical results are
summarized in Table 2. In Figure S4(a), it can be seen that
the PF-FSOs and PF-CzSOs containing SO unit in side
chain showed the same spectra with respect to that of PFO.
Compared with the PF-SO10 containing SO unit in main
chain, the PF-FSOs and PF-CzSOs containing SO unit in
side chain exhibited blue-shifted absorption and narrowing
emission spectra (Figure S4(b)).
PL spectra of the PF-FSO20 and PF-CzSO20 in differ-
ent polar solution are shown in Figure 4, respectively. It
can be seen that the PL spectra of copolymers are kept un-
changed in toluene, THF and CHCl3 solution, neither obvi-
ously bathochromic shift nor broadening trends. This fact is
remarkably different from the polymers containing SO unit
in backbone, which show stronger ICT effect [25–28], indi-
cating the unremarkable ICT effect existed in the polymers
containing SO unit in alkyl side chain.
3.4 Photophysical properties
Figure 3 shows UV-Vis absorption and PL spectra of the
polymers in toluene solution. A distinct absorption peak at
about 388ꢀnm is attributed to the π-π* transition of the conju-
gated fluorene backbone. It should be noted that the absorp-
tion spectra of PF-FSOs are unchanged with the increasing
SO content in side chain, indicating the absorption spectra
of polymer are independent on the SO content in side chain
(Figure 3(a)). While, the PF-CzSOs reveal a weak blue-shift
absorption due to the meta-linking carbazole unit in the poly-
mer backbone (Figure 3(b)).
All polymers show well-structured PL spectra with the
emission peaked at about 416ꢀnm and a shoulder peaked at
about 440ꢀnm in toluene solution, respectively. With the
increasing SO content in side chain, the PL emission has
no obvious changes, in addition a slight decreased shoulder
Figure 5 shows UV-Vis absorption and PL spectra of the
copolymers in film. It can be seen that with the increasing of
FSO and CzSO content, the absorption spectra display un-
changed for PF-FSOs and a weak blue shift for PF-CzSOs,
Figure 3 UV-Vis absorption and PL spectra of the polymers PF-FSOs (a) and PF-CzSOs (b) in toluene solution (color online).
Figure 4 PL spectra of the polymers PF-FSO20 (a) and PF-CzSO20 (b) in different polar solution (color online).

8
Fang et al. Sci China Chem
respectively. PL spectra of the copolymers show the well-
structured local excited (LE) emission, and also no broaden-
ings and redshifts for PF-FSOs and PF-CzSOs. In Figure
S5(a), the PF-FSO10 and PF-CzSO10 containing SO unit in
side chain show the same spectra with respect to that of PFO.
Compared with the PF-SO10 with SO unit in main chain, the
PF-FSO10 and PF-CzSO10 containing SO unit in side chain
exhibit obvious blue-shifted and narrowing emission spectra
(Figure S5(b)). The photoluminescence (PL) efficiencies of
the polymers are in the range of 40%–59% for PF-FSOs, and
32%–66% for PF-CzSOs, which are higher than that of PFO
homopolymer (Table 2).
ing SO unit to side chain, PF-FSO10 and PF-CzSO10 ex-
hibit a slight blue shift with respect to that of PFO, but a re-
markable blue shift and narrowing compared with that of the
PF-SO10 containing SO unit in main chain, which could be
attributed to both strong steric effect and weak ICT effect on
the EL emission existed in the polymers containing SO unit
in side chain.
The EL spectra of PF-FSO15 and PF-CzSO15 with the
variation of current densities are shown inFigure 8. It can be
seen that EL spectra of the polymers are almost unchanged
in the range of current densities from 100–400ꢀmAꢀcm−2. It
also should be noted that the unexpected low-energy emis-
sion band corresponding to the excimers or fluorenone-de-
fects cannot be discerned even at a high current density of
400ꢀmAꢀcm−2, indicating the superb EL stability of the de-
vices based on the resulted copolymers.
The single layer device performances are summarized in
Table 3. It can be seen that the device efficiency of PFO ex-
hibits as low as LEmax of 0.37ꢀcdꢀA−1. In contrary, the effi-
ciencies of the copolymers have a substantial enhancement
with the increasing content of SO unit in side chain, and
PF-FSO15 and PF-CzSO20 show superior performances, in
terms of the LEmax of 1.17 and 0.68ꢀcdꢀA−1 with the CIE
3.5 Device performances
Electroluminescence of the polymers were evaluated based
on the device with a configuration of ITO/PEDOT:PSS/poly-
mer (80ꢀnm)/CsF (1.5ꢀnm)/Al (80ꢀnm). EL spectra of the
polymers are shown in Figure 6. It can be seen that the
EL spectra of polymers all show a weak hypsochromic shift
with the increasing SO unit in side chain. EL spectra of the
copolymers compared with PFO and PF-SO10 are shown in
Figure 7. It can be seen that EL spectra of the polymers link-
Figure 5 UV-Vis absorption and PL spectra of the polymers PF-FSOs (a) and PF-CzSOs (b) in film (color online).
Figure 6 EL spectra of the polymers PF-FSOs (a) and PF-CzSOs (b) (color online).

Fang et al. Sci China Chem
9
Figure 7 EL spectra of the polymers compared with PFO (a) and PF-SO10 (b) (color online).
Figure 8 EL spectra of the polymers PF-FSO15 (a) and PF-CzSO15 (b) with the variation of current densities from 100 toꢀ400ꢀmAꢀcm−2 (color online).
Table 3 Device performances of the polymersa)
b)
polymer
PFO
Von (V)
3.4
LEmax (cd A−1)
0.37
EQEmax (%)
0.42
Lmax (cd m−2)
1934
CIE (x,y)
(0.18, 0.18)
(0.16, 0.17)
(0.17, 0.13)
(0.18, 0.11)
(0.18, 0.11)
(0.17, 0.10)
(0.17, 0.12)
(0.17, 0.11)
(0.17, 0.10)
(0.17, 0.10)
PF-SO10
3.2
2.04
1.46
4296
PF-FSO5
PF-FSO10
PF-FSO15
PF-FSO20
PF-CzSO5
PF-CzSO10
PF-CzSO15
PF-CzSO20
3.8
0.74
0.77
3911
4.6
0.80
0.83
3095
4.8
1.17
1.10
2002
4.4
0.70
0.99
1288
4.2
0.54
0.56
2837
3.8
0.22
0.23
1143
3.6
0.51
0.52
1764
4.1
0.68
0.71
2075
a) The device structure: ITO/PEDOT:PSS/EL/CsF/Al; b) measured at 12ꢀmAꢀcm−2 current density.
coordinates of (0.18, 0.11) and (0.17, 0.10), respectively.
The improved performances could be attributed to the in-
creased PL quantum efficiencies, and might be correlated to
more balanced charge carrier transport in the emissive layer
(Figure 9(b)).
The single charge carrier devices were fabricated and the
current density-electric field characteristics are shown in
Figure 9. It is realized that the PFO exhibits the imbalanced
hole/electron fluxes, as the hole mobility is distinctly higher
than that of electron mobility in a range of the applied elec-
tric fields, which may lead to inferior device performances
at the high current densities. In contrast, the well-bal-
anced hole/electron fluxes can be realized for copolymer
PF-FSO15, that the hole current decreases 30 times and the
electron current increases 15 times at a driving electric field
of 2×105 (V cm−1). And PF-FSO15 shows the more balanced

10
Fang et al. Sci China Chem
Figure 9 Current densities as a function of electric fields for single carrier devices of PFO (a), PF-FSO15 (b), and PF-CzSO20 (c) (color online).
Figure 10 Current density (J)-voltage (V) curves of the PF-FSOs (a) and PF-CzSOs (b) with a configuration of ITO/PEDOT:PSS/EL/CsF/Al (color online).
charge carrier than PF-CzSO20, which could illustrate that
PF-FSO20 has a highest PLQY, but with the middle device
performances. The fact indicated that linking SO unit to the
alkyl side chain of the polyfluorene effectively resulted in the
balanced charge carriers across the emissive layer, which can
in turn lead to the improved device performances.
In Figure 10, it can be seen that the current densities ob-
viously decrease with the increasing content of SO unit in
side chain of the polymers. As we all know, the contact be-
tween the hole transporting layer and the emissive layer will
be non-ohm contact if the energy gap of HOMO level go be-
yond 0.3ꢀeV [38]. Considering that the bigger HOMO level
Figure 11 Energy levels of the materials used in the devices (color online).
gap between PEDOT:PSS (−5.2ꢀeV) and PFO (−5.68ꢀeV), the
hole injection should be non-ohm injection. In Table 2, the
HOMO level of PF-CzSOs and PF-FSOs are even deeper
than that of PFO. Therefore, the PF-CzSOs and PF-FSOs
have bigger injection barrier compared with PFO, which will
lead to higher turn-on voltages [39]. Furthermore, the pack-
ing of polymer chain is disordered and incompact due to the
huge steric hindrance introduced by long side chain with SO
unit, which decreases the carrier mobility (Figure 9). The de-
crease of carrier mobility also results in higher turn-on volt-
age.
can be understood as that the attached electron-deficient SO
unit can effectively facilitate electron injection from the cath-
ode, and further to PFO backbone, which allow to inject more
electrons from the cathode to the polymer main chain and
leads to more balanced charge carriers in the device [40].
3.6 Film morphology
AFM measurement was used and the relevant images are
shown in Figure 12. We can see that PFO exhibits quite
smooth morphology with root-mean-square (RMS) value of
0.53ꢀnm. In contrast, the PF-FSO15 consisting SO unit in
The energy levels of materials used in the devices are
shown in Figure 11. It can be seen that the SO moiety owns
low ELUMO and EHOMO compared with PFO backbone, which

Fang et al. Sci China Chem
11
7
Qiao Z, Cao DR, Liu QL, Weng JN, He ZC, Han SH, Peng JB. Sci
China Ser B-Chem, 2009, 52: 2038–2042
8
9
Zhu M, Yang C. Chem Soc Rev, 2013, 42: 4963–4976
Jin G, Xia L, Liu Z, Lin H, Ling J, Wu H, Hou L, Mo Y. J Mater
Chem C, 2016, 4: 905–913
10 Wang M, Li Y, Xie ZY, Wang LX. Sci China Chem, 2011, 54: 656–665
11 Liu C, Li Y, Li Y, Yang C, Wu H, Qin J, Cao Y. Chem Mater, 2013,
25: 3320–3327
12 Cook JH, Santos J, Al-Attar HA, Bryce MR, Monkman AP. J Mater
Chem C, 2015, 3: 9664–9669
13 Knaapila M, Monkman AP. Adv Mater, 2013, 25: 1090–1108
14 Gong X, Iyer PK, Moses D, Bazan GC, Heeger AJ, Xiao SS. Adv
Funct Mater, 2003, 13: 325–330
Figure 12 AFM topography (3ꢀμm×3ꢀμm) of PFO (a) and PF-FSO15 (b)
(color online).
15 Lu HH, Liu CY, Chang CH, Chen SA. Adv Mater, 2007, 19:
2574–2579
side chain exhibit rougher morphology, with RMS values of
1.08ꢀnm. The rougher morphology indicates that SO unit in
alkyl side chain of the polymers causes disordered packing
of the polymer chain, which can potentially facilitate charge
injection from the electrodes.
16 King SM, Perepichka II, Perepichka IF, Dias FB, Bryce MR,
Monkman AP. Adv Funct Mater, 2009, 19: 586–591
17 Schelkle KM, Bender M, Jeltsch K, Buckup T, Müllen K, Hamburger
M, Bunz UHF. Angew Chem Int Ed, 2015, 54: 14545–14548
18 Gopikrishna P, Das D, Iyer PK. J Mater Chem C, 2015, 3: 9318–9326
19 Lee JF, Hsu SLC. Polymer, 2009, 50: 5668–5674
20 Li JY, Ziegler A, Wegner G. Chem Eur J, 2005, 11: 4450–4457
21 Perepichka II, Perepichka IF, Bryce MR, Pålsson LO. Chem Commun,
2005, 11: 3397
4 Conclusions
In conclusions, the blue light-emitting polyfluorenes
(PF-FSOs and PF-CzSOs) containing dibenzothio-
phene-S,S-dioxide (SO) unit in alkyl side chain were
synthesized. The thermal, electrochemical, photophysical
and electroluminescent properties of resulting polymers were
systematically investigated by comparison with that of PFO
and PF-SO10 containing SO unit of 10ꢀmmol% in main
chain. The results indicated that introducing SO unit into
the alkyl side chain of PFO can restrain the intramolecular
charge transfer (ICT) and improve the EL performances. EL
spectra of copolymers show high spectra stability with the in-
crease of current densities in the range of 100–400ꢀmAꢀcm−2.
22 Huang TH, Whang WT, Shen JY, Wen YS, Lin JT, Ke TH, Chen LY,
Wu CC. Adv Funct Mater, 2006, 16: 1449–1456
23 Ye J, Zheng CJ, Ou XM, Zhang XH, Fung MK, Lee CS. Adv Mater,
2012, 24: 3410–3414
24 Romain M, Quinton C, Tondelier D, Geffroy B, Jeannin O,
Rault-Berthelot J, Poriel C. J Mater Chem C, 2016, 4: 1692–1703
25 Hsu CY, Hsieh MT, Tsai MK, Li YJ, Huang CJ, Su YK, Whang TJ.
Tetrahedron, 2012, 68: 5481–5491
26 Hsiao SH, Wu LC. Dyes Pigments, 2016, 134: 51–63
27 Li Y, Wu H, Zou J, Ying L, Yang W, Cao Y. Org Electron, 2009, 10:
901–909
28 Liu J, Hu S, Zhao W, Zou Q, Luo W, Yang W, Peng J, Cao Y.
Macromol Rapid Commun, 2010, 31: 496–501
29 He R, Hu S, Liu J, Yu L, Zhang B, Li N, Yang W, Wu H, Peng J. J
Mater Chem, 2012, 22: 3440–3446
Acknowledgments This work was supported by the National Key Basic
Research and Development Program of China (2015CB655004), the Na-
tional Natural Science Foundation of China (51473054, 91333206), and the
Fundamental Research Funds for the Central Universities, South China of
Technology (2017MS020).
30 Yu L, Liu J, Hu S, He R, Yang W, Wu H, Peng J, Xia R, Bradley DDC.
Adv Funct Mater, 2013, 23: 4366–4376
31 Guo T, Yu L, Zhao B, Ying L, Wu H, Yang W, Cao Y. J Polym Sci
Part A-Polym Chem, 2015, 53: 1043–1051
32 Hu L, Yang Y, Xu J, Liang J, Guo T, Zhang B, Yang W, Cao Y. J
Mater Chem C, 2016, 4: 1305–1312
Conflict of interest
The authors declare that they have no conflict of
33 Zhao S, Liang J, Guo T, Wang Y, Chen X, Fu D, Xiong J, Ying L,
Yang W, Peng J, Cao Y. Org Electron, 2016, 38: 130–138
34 Guo T, Zhong W, Zhang A, Zou J, Ying L, Yang W, Peng J. New J
Chem, 2016, 40: 179–186
interest.
1
2
3
Ying L, Li Y, Wei C, Wang M, Yang W, Wu H, Cao Y. Chin J Polym
Sci, 2013, 31: 88–97
35 Xu J, Yang Y, Hu L, Guo T, Zhang B, Yang W, Cao Y. Dyes Pigments,
2017, 138: 245–254
Liu SJ, Zhong CM, Zhang J, Duan CH, Wang XH, Huang F. Sci China
Chem, 2011, 54: 1745–1749
36 Scherf U, List EJW. Adv Mater, 2002, 14: 477–487
37 Oguzhan E, Bilgili H, Baycan Koyuncu F, Ozdemir E, Koyuncu S.
Polymer, 2013, 54: 6283–6292
Zhang XW, Lei ZF, Chen YH, Chen KY, Xu WD, Hao L, Fan QL, Lai
WY, Huang W. RSC Adv, 2016, 6: 10326–10333
Lai WY, Liu D, Huang W. Sci China Chem, 2010, 53: 2472–2480
Guan R, Xu Y, Ying L, Yang W, Wu H, Chen Q, Cao Y. J Mater Chem,
2009, 19: 531–537
4
5
38 Liaptsis G, Meerholz K. Adv Funct Mater, 2013, 23: 359–365
39 Park J, Lee C, Jung J, Kang H, Kim KH, Ma B, Kim BJ. Adv Funct
Mater, 2014, 24: 7588–7596
6
Guo T, Yu L, Yang Y, Li Y, Tao Y, Hou Q, Ying L, Yang W, Wu H,
Cao Y. J Lumin, 2015, 167: 179–185
40 Huang CW, Tsai CL, Liu CY, Jen TH, Yang NJ, Chen SA.
Macromolecules, 2012, 45: 1281–1287