Deep-blue light-emitting polyfluorenes with asymmetrical naphthylthio-fluorene as Chromophores

Article abstract of DOI:10.1002/pola.29283

A novel blue polycyclic aromatic compound 2,8-dibromo-14,14-dioctyl-14H-benzo[b]benzo [5,6] fluoreno[1,2-d]thiophene 9,9-dioxide (Br₂NFSO) is designed and synthesized through multistep synthesis, and its structure is confirmed by nuclear magnetic resonance. Based on synthesized polycyclic aromatic compound Br₂NFSO, a series of twisted blue light-emitting polyfluorenes derivatives (PNFSOs) are prepared by one-pot Suzuki polycondensation. Based on the twisted polymer molecular structure resulted from the asymmetric links of 14,14-dioctyl-14H-benzo[b]benzo[5,6]fluoreno[1,2-d]thiophene 9,9-dioxide (NFSO) unit in copolymers and better electron transport ability of NFSO than those of the electron-deficient dibenzothiophene-S,S-dioxide counterpart, the resulting polymers exhibit excellent electroluminescent spectra stability in the current densities from 100 to 800 mA cm^?2, and show blue-shifted and narrowed electroluminescent spectra with the Commission Internationale de L′Eclairage (CIE) of (0.16, 0.07) for PNFSO5, compared to poly(9,9-dioctylfluorene) (PFO) with the CIE of (0.18, 0.18). Moreover, the superior device performance is achieved based on PNFSO5 with the maximum luminous efficiency (LE_max) of 1.96 cd A^?1, compared with the LE_max of 0.49 cd A^?1 for PFO. The results indicate that the twisted polycyclic aromatic structure design strategy has a great potential to tuning blue emission spectrum and improving EL efficiency of blue light-emitting polyfluorenes.

Full text of DOI:10.1002/pola.29283

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Deep-Blue Light-Emitting Polyfluorenes with Asymmetrical
Naphthylthio-Fluorene as Chromophores
Liwen Hu,1,2 Zhonglian Wu,3 Xiaojun Wang,1 Yawei Ma,1 Ting Guo ,1 Lei Ying,1,2
Junbiao Peng,1 Yong Cao1
1Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China
University of Technology, Guangzhou 510640, China
2South China Institute of Collaborative Innovation, Dongguan 523808, China
3School of Materials Engineering, Jiangsu University of Technology, Changzhou 213001, China
Correspondence to: T. Guo (E-mail: mstguo@scut.edu.cn) or L. Ying (E-mail: msleiying@scut.edu.cn)
Received 25 September 2018; accepted 7 November 2018
DOI: 10.1002/pola.29283
ABSTRACT: A novel blue polycyclic aromatic compound 2,8-
dibromo-14,14-dioctyl-14H-benzo[b]benzo [5,6] fluoreno[1,2-d]
thiophene 9,9-dioxide (Br2NFSO) is designed and synthesized
through multistep synthesis, and its structure is confirmed
by nuclear magnetic resonance. Based on synthesized poly-
cyclic aromatic compound Br2NFSO, a series of twisted blue
light-emitting polyfluorenes derivatives (PNFSOs) are pre-
pared by one-pot Suzuki polycondensation. Based on the
twisted polymer molecular structure resulted from the asym-
metric links of 14,14-dioctyl-14H-benzo[b]benzo[5,6]fluoreno
[1,2-d]thiophene 9,9-dioxide (NFSO) unit in copolymers
and better electron transport ability of NFSO than those of
the electron-deficient dibenzothiophene-S,S-dioxide counter-
part, the resulting polymers exhibit excellent electrolumines-
cent spectra stability in the current densities from 100 to
electroluminescent spectra with the Commission Internatio-
nale de L0Eclairage (CIE) of (0.16, 0.07) for PNFSO5, com-
pared to poly(9,9-dioctylfluorene) (PFO) with the CIE of (0.18,
0.18). Moreover, the superior device performance is achieved
based on PNFSO5 with the maximum luminous efficiency
(LEmax
0.49 cd A−1 for PFO. The results indicate that the twisted
polycyclic aromatic structure design strategy has great
potential to tuning blue emission spectrum and improving
EL efficiency of blue light-emitting polyfluorenes. 2018
)
of 1.96 cd A−1
,
compared with the LEmax of
a
©
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2018
KEYWORDS: intromolecular charge transfer; polymer light-
emitting diodes; polycyclic aromatic compound; synthesis;
twisted structure
800 mA cm−2
,
and show blue-shifted and narrowed
functionalization at the C-9 position of the fluorene unit.8,9
However, due to fluorenone defect formation and the poor
electron transport ability,10–12 the color purity of PFs is unsat-
isfactory, and the efficiency of the devices based on such emit-
ters are still low, which limited its extensive application.
INTRODUCTION Depended on flexible display, solid-state light-
ing, low energy consumption, and low driving voltage, poly-
mer light-emitting diodes have attracted great attention in the
past decades.1–5 Among three primary color polymers used in
full-color flat panel displays, the red and green light-emitting
polymers had already developed maturely. On the contrary,
the blue-light emitting polymers still could not meet practical
display applications, for example, the color purity, luminous
efficiency, and spectra stability,6,7 owing to the inherent wide
band-gap nature. As reported, only a limited number of blue
polymers exhibited excellent performance. As one of the
promising blue emitters, polyfluorenes (PFs) are attractive
because of its high photoluminescence quantum efficiency
(PLQY), good thermal and electrochemical stability and facile
To improve color purity of PFs, much effort has been done.
For example, 3,6-linkages fluorene or spirobifluorene were
adopted as a building block to design deep blue-emitting
PFs. Compared with 27-PFs, poly(9,9-dialkylfluorene-3,6-diyl)
(36-PFs) showed a significantly blue-shifted electrolumines-
cence (EL) spectra with maximum emission wavelength at
about 415 nm and stable EL emission resulted from shorter
conjugation length and twisted structure design.13–15 The
chemical stability of PFs was improved remarkably benefiting
Additional supporting information may be found in the online version of this article.
L. W. Hu and Z. L. Wu contributed equally to this work.
© 2018 Wiley Periodicals, Inc.
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from the spiro-linkage at the 9-position of fluorene, which
increased a large steric hindrance of fluorene unit warping in
polymer backbone and restrained the appearance of fluore-
none defect. What is more, this large steric hindrance of the
vertical configuration of spirobifluorene can suppress their
aggregation in the solid state.16–18 Unfortunately, they still
suffer from poor device efficiency compared with PFs due to
the unbalanced charge carrier transportation.
Materials
Commercially available reagents were used without further
purification, unless stated otherwise. Toluene was purified by
routine procedures and distilled in dry argon atmosphere
before used.
Synthesis
2-Bromodibenzo[b,d]thiophene (1), 2-(dibenzo [b,d]thiophen-
2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2), methyl 1-
bromo-2-naphthoate (3), 2,7-bis(4,4,5,5-tetramethyl-1,3,2
-dioxaborolan-2-yl)-9,9-dioctylfluorene (M2), 2,7-dibromo-
9,9-dioctylfluorene (M3) were synthesized according to previ-
ous literatures.27–30
PF was famous p-type organic semiconductor polymer, so that
it exhibited poor electron transport ability and unbalanced
charge carrier injection and transportation.18 To enhance its
electron transport ability, much electron-withdrawing groups,
such as sulfonyl, cyano, oxadiazole, and triazole, were intro-
duced into polymer backbones or side chains, respectively.
These results demonstrated that the incorporation of these
electron-deficient units into PFs could enhance the electron
injection and balance the charge carrier transportation, and
further improve the device performances.19–22 However, the
charge transfer (CT) effect from such electron-rich units to
electron-withdrawing groups was inevitably appeared, which
result in the color impurity for blue emission. For example,
poly[(fluorene)-co-dibenzothiophene-S,S-dioxide] (PFSO) was
one of the most efficient blue light-emitting polymers ever
reported. Whereas the electron-deficient dibenzo-thiophene-
S,S-dioxide (SO) unit brought about the photoluminescent
(PL) and electroluminescent spectra bathochromic shift inevi-
tably caused by CT effect.23,24 Subsequently, our team
designed a series of PFs linked SO unit in alkyl side chain to
keep a certain distance between the fluorene donor in main
chain and SO acceptor in side chain. A blue-shift together with
narrowing EL spectra was obtained compared with that of
polymers containing SO unit in main chain, which was attrib-
uted to both improved electron injection/transportation and
weak CT effect.25 Nevertheless, the low luminous efficiency of
1.17 cd A−1 with CIE coordinates of (0.18, 0.11) was achieved,
which may be due to the long distance between electron-
deficient SO unit in side chain and polymer main chain.
Methyl 1-(Dibenzo[b,d]Thiophen-2-yl)-2-Naphthoate (4)
In a 250 mL two-necked flask, Pd(PPh3)4 (1.82 g, 1.57 mmol)
was added to
a mixture of compound (3) (10.00 g,
37.72 mmol), compound (2) (9.75 g, 31.43 mmol), 50 wt%
Na2CO3 aqueous solution (16.66 g/17.0 mL deionized water,
157.17 mmol), tetrabutyl-ammonium bromide (4.0 mg,
12.4 μmol), 100 mL toluene under N2 atmosphere and reacted
at 90 ꢀC for 12 h. The cooled solution was extracted with tolu-
ene, and the organic layer was washed with brine. The crude
product was purified by column chromatography on silica gel
with dichloromethane/petroleum ether (1:5). Yield: 80%. 1H
NMR (500 MHz, CDCl3) δ (ppm): 8.10 (d, J = 7.10 Hz, 2H),
7.99 (d, J = 5.95 Hz, 2H), 7.96–7.93 (m, 2H), 7.90 (d,
J = 7.20 Hz, 1H), 7.61 (d, J = 8.60 Hz, 1H), 7.57 (t, J = 7.48 Hz,
1H), 7.48 (t, J = 7.50 Hz, 1H), 7.45–7.39 (m, 3H), 3.59 (s, 3H).
13C NMR (125 MHz, CDCl3) δ (ppm): 168.65, 141.25, 139.89,
138.51, 135.49, 138.51, 135.49, 135.37, 135.32, 134.80,
132.91, 128.52, 128.29, 127.94, 127.86, 127.55, 126.81,
126.73, 125.49, 124.39, 122.87, 122.51, 122.22, 121.77, 52.03.
MS (APCI): m/z (%): 368.1 [M+].
9-(1-(Dibenzo[b,d]Thiophen-2-yl)Naphthalen-2-yl)
Heptadecan-9-ol (5)
In a 500 mL two-necked flask, compound (4) (10.0 g,
27.1 mmol) was dissolved in 300 mL dry THF and octylmag-
nesium bromide (C8H17MgBr) in THF(59.0 g, 271.4 mmol)
was added slowly. The reaction was stirred and refluxed for
16 h, then it was quenched by deionized water and extracted
by ethyl acetate. After removing the organic phase under
reduced pressure, the crude product was used for the next
step directly.
In this manuscript, a novel polycyclic aromatic compound
2,8-dibromo-14,14-dioctyl-14H-benzo[b]benzo[5,6]fluoreno
[1,2-d]thiophene 9,9-dioxide (Br2NFSO) was designed and syn-
thesized based on following consideration, (a) polycyclic aro-
matic compound had higher fluorescence quantum efficiency,24
therefore the benzene moiety of fluorene unit was replaced by
naphthalene moiety, (b) electron-deficient SO unit was rear-
ranged into polycyclic aromatic compound because of better
electron transport ability of SO unit, and SO unit can also
increase the molecular weight of resulting monomer, (c) asym-
metric molecular structure can disturb the effective conjugated
length and improve the blue emission purity. A series of PF
derivatives containing the NFSO unit were also prepared and
studied in detail. The resulting copolymers exhibited weak CT
effect, balanced charge carrier injection/transportation, superior
EL efficiency and deep blue emission with respect to PFO, which
can be attributed to indirect conjugation of SO moiety and the
twisted polycyclic aromatic structure in copolymer backbone.26
14,14-Dioctyl-14H-Benzo[b]Benzo[5,6]Fluoreno [1,2-d]
Thiophene (6)
After compound (5) was dissolved in 150 mL dichloromethane
completely, 1.0 mL BF3 etherate was injected into the reaction
solution with stirring for 4 h. Then it was quenched by deionized
water and extracted by dichloromethane. The organic phase was
removing under reduced pressure and the crude product was
purified by column chromatography on silica gel with petroleum
1
ether. Yield: 35%. H NMR (500 MHz, CDCl3) δ (ppm): 8.76 (d,
J = 8.50 Hz, 1H), 8.59 (d, J = 8.45 Hz, 1H), 8.41 (d, J = 8.15 Hz,
2
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1H), 8.01 (d, J = 7.60 Hz, 2H), 7.97 (t, J = 8.05 Hz, 2H), 7.77–7.72
(m, 2H), 7.62–7.59 (m, 3H), 2.75 (t, J = 7.55 Hz, 3H). 13C NMR
(125 MHz, CDCl3) δ (ppm): 152.36, 150.26, 140.48, 139.54,
138.00, 133.71, 132.83, 129.53, 129.42, 129.04, 128.57, 128.48,
127.13, 126.82, 126.71, 126.36, 126.00, 125.15, 124.96, 124.36,
123.68, 121.92, 116.73, 56.14, 35.37, 30.93, 30.59, 28.71, 28.56,
28.36, 28.31, 27.96, 27.80, 26.21, 24.62, 22.71, 21.65, 21.42,
13.11, 12.95. MS (APCI): m/z (%): 547.4 [M+].
0.062 mmol), palladium acetate (Pd(OAc)2) (2.8 mg,
0.0125 mmol), and tricyclohexyl-phosphine (PCy3) (6.98 mg,
0.249 mmol) were dissolved in 8 mL toluene under argon
atmosphere, until the reaction was stirred clearly, tetraethyl
ammonium hydroꢀxide (Et4NOH) (20% aq, 1.5 mL) was added
and stirred at 80 C for 24 h. The reaction was end-capped by
phenylboronic acid (0.05 g, 0.4 mmol) for 12 h stirring.
Finally, bromobenzene (0.125 g, 0.8 mmol) was added fol-
lowed by stirring for another 12 h. After the reaction was
cooled to room temperature, the mixture was precipitated
with methanol and filtered. The crude product was further
purified by Soxhlet extraction by methanol, acetone, and THF.
The tetrahydrofuran component was repurified by column
chromatography, and concentrated under reduced pressure,
followed by reprecipitation methanol, drying under vacuum.
Yield: 70%. 1H NMR (500 MHz, CDCl3) δ (ppm): 7.73–7.83 (br,
ArH), 7.71–7.67 (br, ArH), 2.13 (br, CH2), 1.22–1.14 (br, CH2),
0.85–0.77 (br, CH3). 13C NMR (125 MHz, CDCl3) δ (ppm):
151.84, 140.52, 126.19, 121.52, 120.00, 64.47, 55.35, 40.37,
31.80, 30.05, 29.23, 25.37, 22.61, and 14.08.
2,8-Dibromo-14,14-Dioctyl-14H-Benzo[b]Benzo[5,6]
Fluoreno[1,2-d]Thiophene (7)
Compound (6) (1.00 g, 1.83 mmoL) and two piece of I2 were
dissolved in 50 mL CHCl3, a mixture Br2 (642.91 mg,
4.02 mmol) in CHCl3 was added slowly. It was stirred for 24 h
in the dark and quenched by sodium bisulfite aqueous solu-
tion. The crude product was purified by column chromatogra-
phy with petroleum ether. Yield: 70%. 1H NMR (500 MHz,
CDCl3) δ (ppm): 8.60 (d, J = 8.45 Hz, 1H), 8.54 (s, 1H), 8.40 (d,
J = 8.45 Hz, 1H), 8.30 (d, J = 8.15 Hz, 1H), 7.91 (d, J = 7.65 Hz,
1H), 7.82 (s, 1H), 7.75 (t, J = 8.05 Hz, 1H), 7.70 (t, J = 7.50 Hz,
1H), 7.65 (t, J = 7.48 Hz, 1H), 7.57 (t, J = 7.50 Hz, 1H),
2.68–2.62 (m, 2H), 2.12–2.06 (m, 2H), 1.08–0.84 (m, 24H),
0.47–0.31 (m, 6H). 13C NMR (125 MHz, CDCl3) δ (ppm):
151.99, 149.20, 144.84, 137.59, 133.20, 132.64, 130.78,
130.65, 129.92, 129.40, 128.90, 128.68, 128.11, 127.79,
127.05, 126.18, 125.92, 125.54, 123.57, 122.47, 121.87,
116.40, 56.20, 35.39, 30.92, 30.62, 28.69, 28.56, 28.34, 28.30,
27.97, 27.82, 26.19, 24.58, 22.74, 21.66, 21.41, 13.11, 12.98.
MS (APCI): m/z (%):705.2 [M+].
PNFSO2
M2 (200 mg, 0.311 mmol), M3 (163.87 mg, 0.299 mmol), and
M1 (8.84 mg, 0.012 mmol). Yield: 71%. 1H NMR (500 MHz,
CDCl3) δ (ppm): 7.85–7.80 (br, ArH), 7.71–7.63 (br, ArH), 2.12
(br, CH2), 1.24–1.14 (br, CH2), 0.83–0.80 (br, CH3). 13C NMR
(125 MHz, CDCl3) δ (ppm): 151.80, 140.48, 140.01, 126.14,
121.47, 119.95, 55.33, 40.37, 31.78, 30.30, 30.03, 29.21,
23.90, 22.59, and 14.05.
2,8-Dibromo-14,14-Dioctyl-14H-Benzo[b]Benzo[5,6]
PNFSO5
Fluoreno[1,2-d]Thiophene 9,9-Dioxide (M1)
M2 (200 mg, 0.311 mmol), M3 (153.63 mg, 0.280 mmol), and
M1 (22.93 mg, 0.031 mmol). Yield: 73%. 1H NMR (500 MHz,
CDCl3) δ (ppm): 7.85–7.80 (br, ArH), 7.71–7.67 (br, ArH), 2.13
(br, CH2), 1.23–1.14 (br, CH2), 0.84–0.70 (br, CH3). 13C NMR
(125 MHz, CDCl3) δ (ppm): 151.82, 140.52, 140.04, 126.17,
121.51, 119.98, 55.35, 40.39, 31.80, 30.05, 29.30, 29.23,
25.37, 23.92, 22.61, and 14.07.
Compound (7) (1.00 g, 1.42 mmol) was dissolved in 20 mL
acetic acid, then 3.0 mL hydrogen peroxide was added and
refluxed for 2 h. The reaction was quenched by deionized
water and extracted by methylene dichloride. The product
was purified by column chromatography on silica gel with
dichloromethane/petroleum ether (1:1) and recrystallized
1
with THF/Methanol. Yield: 50%. H NMR (500 MHz, CDCl3) δ
(ppm): 8.68 (d, J = 8.45 Hz, 1H), 8.60 (s, 1H), 8.47 (d,
J = 8.45 Hz, 1H), 8.37 (d, J = 8.15 Hz, 1H), 7.98 (d, J = 7.60 Hz,
1H), 7.90 (s, 1H), 7.82 (t, J = 7.65 Hz, 1H), 7.76 (t, J = 8.38 Hz,
1H), 7.72 (t, J = 8.10 Hz, 1H), 7.64 (t, J = 7.48 Hz, 1H),
2.75–2.69 (m, 2H), 2.19–2.12 (m, 2H), 1.13–0.89 (m, 24H),
0.55–0.41 (m, 6H). 13C NMR (125 MHz, CDCl3) δ (ppm):
152.99, 150.19, 145.84, 138.59, 134.21, 133.64, 131.77,
131.64, 130.93 130.40, 129.89, 129.68, 129.11, 128.78,
128.05, 127.17, 126.93, 126.54, 124.57, 123.47, 122.87,
111.39, 57.18, 36.36, 31.60, 29.56, 29.48, 28.97, 28.82, 23.73,
23.69, 22.44, 13.98. MS (APCI): m/z (%): 736.2 [M+].
PFO
M2 (200 mg, 0.311 mmol) and M3 (200 mg, 0.311 mmol).
Yield: 75%. 1H NMR (500 MHz, CDCl3) δ (ppm): 7.85–7.83 (br,
ArH), 7.70–7.67 (br, ArH), 2.13 (br, CH2), 1.22–1.14 (br, CH2),
0.83–0.80 (br, CH3). 13C NMR (125 MHz, CDCl3) δ (ppm):
151.84, 140.52, 140.03, 126.18, 121.51, 119.98, 55.35, 40.38,
31.80, 30.04, 29.23, 23.92, 22.61, and 14.07.
Instrumentation
Nuclear magnetic resonance (NMR) spectra were performed
on Bruker Avance 500 spectrometer (operating at 500 MHz
for 1H NMR and 125 MHz for 13C NMR), using deuterated
chloroform as solution and the tetramethylsilane as the refer-
ence. The molecular weight and polydispersity index (PDI)
were measured by gel permeation chromatography (GPC)
using Waters Associates 515 GPC with polystyrene as stan-
dard and THF as eluent at room temperature, the concentra-
General Procedures for Suzuki Polycondensation, Taking
PNFSO10 as an Examples
2,20-(9,9-dioctyl-9H-fluorene-2,7-diyl) bis(4,4,5,5-tetramethyl-
1,3,2-dioxa-borolane) (M2) (200 mg, 0.311 mmol), 2,7-
dibromo-9,9-dioctyl-9H-fluorene (M3) (136.56 mg, 0.249
mmol), 2,8-dibromo-14,14-dioctyl-14H-benzo [b]benzo[5,6]
fluoreno[1,2-d] thiophene 9,9-dioxide (M1) (45.86 mg,
tion of the polymer sample is
Thermogravimetric analysis (TGA) was recorded on a Netzsch
2 × g .
10−3 mL−1
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TG 209 at a heating rate of 20 ꢀC min−1 with nitrogen flow.
The differential scan calorimetry (DSC) was carried out by a
Netzsch DSC 204 under N2 atmosphere at a heating rate of
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
Ba followed by 100 nm 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 × 10−4 Pa to form the cathode.
−1
ꢀ
10 C min UV–vis absorption spectra were recorded with a
.
UV–vis–NIR light source (DT-MINI-2-GS, Ocean Optics)
equipped with a QE65 Pro detector (Ocean Optics). Photolu-
minescence (PL) spectra were performed on a Spex Fluorolog-
3 spectrofluorometer. Cyclic voltammetry (CV) were tested
out by a CHI630E electrochemical work station, equipped
with a glassy carbon working electrode, a saturated calomel
electrode as the reference electrode, and a Pt wire counter
electrode. The electrolyte solution was used as anhydrous ace-
tonitrile tetrabutylammonium hexafluorophosphate in acetoni-
trile (0.1 M) under N2 atmosphere. The density functional
theory (DFT) calculation results were obtained by using
Gaussian at the B3LYP/6-31G(d) level. Fluorescence quantum
yields were measured using a Hamamatsu absolute PL quan-
tum yield spectrometer C11347 Quantaurus-QY. The current
density(J)-voltage (V)-luminance(L) characteristics of poly-
mers were measured on a Keithley 236 source measurement
unit. The luminance was calibrated by using a PR-705 Spectra
Scan Spectro-photometer (Photo Research), with simultaneous
acquisition of the EL spectra and CIE coordinates, driven by a
Keithley model 2400 voltage–current source. Atomic force
microscopy (AFM) measurements were performed on a Bru-
ker Multimode 8 microscope in Scan Asyst in air mode. The
films for AFM measurement were spin-coated on the top of
the ITO/PEDOT:PSS surface.
RESULTS AND DISCUSSION
Synthesis of Monomers and Polymers
The synthetic route toward the monomers and the target
polymers were outlined in Scheme 1. 2-(dibenzo[b,d]thio-
phen-2-yl)-4,4,5,5-tetra-methyl-1,3,2-dioxaborolane (2) was
synthesized with 2-bromodibenzo[b,d] thiophene and
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane as start-
ing materials at −78 ꢀC. Methyl 1-bromo-2-naphthoate
(3) was obtained via esterification reaction in a good yield of
89%. Then methyl 1-(dibenzo[b,d]thiophen-2-yl)-2-naphthoate
(4) was prepared by palladium-catalyzed Suzuki coupling
reaction with a yield of 60%. Addition of the excess Grignard
reagent n-octyl magnesium bromide to compound (4) pro-
duced the 9-(1-(dibenzo [b,d]thiophen-2-yl) naphthalene-2-yl)
heptadecan-9-ol (5), and then it was ring-closed to generate
14,14-dioctyl-14H-benzo[b]benzo[5,6]fluoreno[1,2-d]
thio-
phene 9,9-dioxide (6). The bromination of monomer 6 with
bromine in chloroform gave the 2,8-dibromo-14,14-dioctyl-
14H-benzo[b]benzo[5,6]fluoreno [1,2-d]thiophene 9,9-dioxide
(7) in high yields. Finally, resulting monomer 2,8-dibromo-
14,14-dioctyl-14H-benzo[b]benzo[5,6]fluoreno[1,2-d]thio-
phene 9,9-dioxide (M1) was synthesized through oxidizing
reaction with hydrogen peroxide as oxidizer. The accurate
assignment of aromatic protons of monomer M1 relied on the
1H–1H homonuclear correlation (COSY) experiments and
1H-13C heteronuclear multiple bond correlation of NMR spec-
troscopy with corresponding spectra shown in Figure 1(c,d).
Device Fabrication and Characterization
The structure of device was ITO/PEDOT:PSS/EML/Ba/Al. The
sheet resistance of ITO glass substrates was 15–20 Ω/square.
PEDOT:PSS layer with 40 nm was spin-coated on the top of
the ITO glass substrate, which was treated by oxygen plasma,
and the PEDOT:PSS film was heated at 180 ꢀC for 10 min.
Then the emission material PNFSOs were dissolved in p-xylene
and the solution with concentration of 1.5 × 10−2 g mL−1 was
deposited with a thickness of 60 nm. Finally, the device was
finished after an 4 nm thick interface of Ba and an 200 nm
thick cathode of Al. Excepted the deposition of PEDOT:PSS
layer, all of the other procedures were carried out in the
nitrogen filled glovebox. The thickness of each layer was mea-
sured by the thickness monitored using a Tencor Alpha-step
500 Surface Profilometer. The multilayer cathode was ther-
mally deposited in a vacuum coating unit at base pressures
<10−5 mbar.
1
On the basis of peak split and the six group H-1H correlation
signal in NMR spectrum [Fig. 1(b,c)], and correlation signal
between singlet of δ = 7.90 ppm and quaternary carbon of
δ = 57.2 ppm, and between singlet of δ = 8.61 ppm and qua-
ternary carbon of δ = 117.4 ppm, δ = 134.24 ppm, and
δ = 150.2 ppm [Fig. 1(d)], the exact bromination position of
resulting monomer M1 can be confirmed and the result is our
designed structure. The 13C NMR and 1H-13C heteronuclear
singular quantum correlation spectra were summarized in
Supporting Information Figure S1.
The target polymers were synthesized through a palladium-
catalyzed Suzuki polymerization, which were carried out by
the molar feed ratios of M2:M3:M1 = 50:50:0, 50:48:2,
50:45:5, and 50:40:10, with relevant polymers denoted as
PFO, PNFSO2, PNFSO5, and PNFSO10, respectively. The con-
trast polymers PFSO5 containing 5% dibenzothiophene-S,S-
dioxide (SO) unit in polymer main chain and PF-FSO5 linked
5% SO moiety by long alkyl chain in polymer side chain were
also prepared (Supporting Information Scheme S1). All poly-
mers were easily dissolved in common organic solvents, such
as tetrahydrofuran, chlorobenze, chloroform, and dichloro-
methane at room temperature, and showed superb film-
The hole-/electron-only devices were fabricated as ITO/PE-
DOT:PSS (40 nm)/polymer (80 nm)/MoO3 (10 nm)/Al and
ITO/ZnO (40 nm)/polymer (80 nm)/Ba (1.5 nm)/Al, respec-
tively. 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 zinc oxide (40 nm) to replace the PEDOT:PSS
film, then a thick copolymer layer (80 nm) was spin-cast from
p-xylene solution on top of the zinc oxide layer and annealed
4
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SCHEME 1 Synthetic routes for monomers and polymers.
forming ability by solution spin-coating processing. The num-
ber average molecular weight (Mn) of polymers was from 25.8
to 42.8 kDa, with the PDI in the range of 1.91–2.34. Detailed
molecular weight data were summarized in Table 1.
according to the equations of EHOMO = −(Eox + 4.80–0.39) eV
and ELUMO = −(Ered + 4.80–0.39) eV. Thus, the EHOMOS and ELU-
MOS of polymers were estimated to be in the range of −5.75 eV to −5.77 eV
and −2.26 to −2.16 eV, respectively. The EHOMO have no obvious change
and the ELUMO gently decreased with the increase of NFSO
unit content in polymer, indicating the better electron trans-
port ability of resulting polymers.
Thermal Properties
DSC and TGA characteristics were conducted to assess the
thermal properties of the resulting polymers as shown in the
Figure 2 and Table 1. The copolymers presented the melting
temperature (Tm) over 130 C and the decomposition temper-
ature (Td) (corresponding to 5 wt % weight loss) over 420 C,
ꢀ
Photophysical Properties
ꢀ
Figure 4 displayed the UV–vis absorption and PL spectra in
toluene solution and as thin film. In toluene, the absorption
profiles of all polymers exhibited a strong absorption peaked
at about 380 nm, which ascribed to the π-π* transition of the
conjugated backbone.33 The absorption spectra for the films
concisely correspond to those in solution, other than minor
red-shifts and broadening of peaks. But for the PL spectra in
toluene, all polymers showed a well-structured PL emission.
And with the increase of NFSO unit, the PL intensity peaked
at 416 nm kept unaltered, and that of the shoulder peaked
around 436 nm remarkably enhanced, attributing to the CT
effect from fluorene donor to NFSO acceptor unit. The PL
spectrum of polymers in film exhibited weak redshift and
broadening with increased NFSO unit. The maximum emis-
sion peak red shifted from 437 nm for PNFSO2 to 447 nm
for PNFSO10, and the full width at half maximum (FWHM)
changed from 45 nm for PNFSO2 to about 66 nm for
PNFSO10. The optical band gap (Egopt) was calculated from
the UV–vis absorption band edge in film by the equation
indicating the good thermal stability of polymers. This is
important and desirable for optoelectronic applications. The
Tds of all copolymers are slightly lower than that of homopol-
ymers PFO, indicating that the incorporation of NFSO unit into
PFO increased the irregularities of polymer backbone attrib-
uted to the twisted-structure of monomer M1.26 In addition, it
can be seen that the Tm of copolymers increased gently as the
fraction of NFSO unit increases, due to fused large polycyclic
aromatic of NFSO unit. There was no obvious glass transition
temperature during the heating process.
Electrochemical Properties
The electrochemical properties of the polymers were exam-
ined by CV. The reversible p-doping and n-doping traces of
polymers were shown in Figure 3 and the detailed CV data
were summarized in Table 1. With respect to all polymers, the
oxidation potentials (EOX) referred to ferrocene/ferrocenium
(FC/FC+) was measured to be 0.39 V. It was assumed that the
opt
opt
+
Eg
= 1240/λedge. Hence, the Eg
were 2.92, 2.91, and
redox potential of Fc/Fc was 4.80 eV relative to the vacuum
energy level,31,32 therefore, the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels of the polymers were calculated
2.90 eV for polymer PNFSO2, PNFSO5, and PNFSO10, respec-
tively. The detailed photophysical parameters are summa-
rized in Table 2.
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(a)
(b)
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
ppm
j
e
(c)
(d)
i
h
f
d
c
b g
a
ppm
ppm
7.4
60
70
Hj↔Cm
7.6
7.8
8.0
8.2
8.4
8.6
8.8
Hb↔Hg
Hf↔Hb
Hc↔Hh
80
Ha↔Hc
90
Hd↔Hh
100
110
120
130
140
Hc↔Ch
He↔Cq
Hf↔Cn
Hg↔Ci
Hi↔Hg
Ha↔Ck
Hd↔Ch Hg↔Cb
Hh↔Cc
Hf↔Cb
He↔Cs
Ha↔Cc
He↔Cr
150
9.0
9.0
8.8
8.6
8.4
8.2
8.0
7.8
7.6
7.4
ppm
8.9 8.8 8.7 8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5
ppm
FIGURE 1 Molecular structure of monomer (a), 1H NMR spectrum (b), 1H-1H COSY spectrum (c), and 1H-13C HMBC spectrum (d).
[Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1 Molecular Weight and Thermal Properties of Polymers
a
Polymer
Mn (kDa)
PDI
T
m (ꢀC)
Td (ꢀC)
Eox (V)
Ered (V)
HOMO (eV)
LUMO (eV)
Egopt,b (eV)
PFO
25.8
35.8
34.7
42.8
2.26
2.34
1.93
1.91
148
132
135
139
424
423
420
424
1.34
1.35
1.36
1.36
b
−2.25
−2.22
−2.16
−2.15
−5.75
−5.76
−5.77
−5.77
−2.16
−2.19
−2.25
−2.26
2.93
2.92
2.91
2.90
PNFSO2
PNFSO5
PNFSO10
a
Corresponding to 5 wt % weight loss.
Calculated from the UV–vis absorption band edge of the molecule film
by the equation, Eg opt = 1240/λedge
.
To evaluate the effect of NFSO unit on optical properties of
polymer, we also made a contrastive study about the UV–vis
absorption and PL properties for polymer PNFSO5 and PFSO5.
The PL spectra of PNFSO5 exhibited significant blue-shift in
toluene and as film state compared with that for PFSO5, which
determined by the weak CT effect attributed to the SO unit in
side chain for PNFSO5 (Supporting Information Fig. S2).
Figure 5 showed the UV–vis absorption and PL spectra of the
6
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110
(b)
(a)
PNFSO2
100
PNFSO5
PNFSO2
PNFSO10
90
PNFSO5
80
70
PNFSO10
60
50
40
100
200
300
400
500
600
700
800
40
60
80
100
120
140
160
180
200
Temperature (oC)
Temperature (oC)
FIGURE 2 DSC and TGA curves of polymers. [Color figure can be viewed at wileyonlinelibrary.com]
PNFSO5 and PFSO5 in different polar solution. The absorption
spectra of polymers in solution with different polarity are
almost identical; however, the PL spectra gradually became
broadened from toluene, THF to CHCl3 solution and batho-
chromic shift by about 26 nm for PFSO5. On the contrary, the
main peak close to 416 nm was almost unchanged, and the
shoulder peak was red-shifted from 437 to 442 nm for
PNFSO5, respectively, indicating that the CT character of the
polymers in the excited state and the stronger CT effect for
PFSO5. In same solution, the stokes shifts of PNFSO5 was
smaller than that of PFSO5, which can be illustrated that the
asymmetric structure from NFSO unit may induce a twisted
polymeric backbone, and the SO unit in the side chain could
weaken CT effect. The photoluminescence quantum efficien-
cies (QPL) of the polymers are in the range of 46%–51%,
which are higher than that of PFO homopolymer (Table 2).
+
Fc/Fc
PNFSO2
PNFSO5
PNFSO10
-3
-2
-1
0
1
2
Potential(V)
DFT Calculation
FIGURE 3 Cyclic voltammetry characteristics of polymers. [Color
figure can be viewed at wileyonlinelibrary.com]
To understand the frontier molecular orbitals and molecular
backbone conformation of the polymers, DFT calculations
(b)
(a)
1
0.8
0.6
0.4
0.2
0
1
1
0.8
0.6
0.4
0.2
0
1
PNFSO2
PNFSO5
PNFSO10
PNFSO2
PNFSO5
PNFSO10
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
300
350
400
450
500
550
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
FIGURE 4 UV–vis absorption and PL spectra of polymers in toluene solution with concentration of 1 × 10−5 g mL−1 (a) and solid film
(b). [Color figure can be viewed at wileyonlinelibrary.com]
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TABLE 2 Photophysical Properties of Polymers
λmaxUV (nm)
λmaxPL (nm)
QPL
Polymer
Toluene
CHCl3
Film
λonset (nm)
Egopt,a (eV)
Toluene
CHCl3
Film
(%)
PFO
380
381
378
378
376
371
376
377
377
382
379
381
424
424
426
428
2.93
2.92
2.91
2.90
416, 438
416, 437
416, 436
419, 436
416
417
418
443
440
439
445
449
32
46
58
51
PNFSO2
PNFSO5
PNFSO10
(b) PNFSO5
(a) PFSO5
1
0.8
0.6
0.4
0.2
0
1
1
0.8
0.6
0.4
0.2
0
1
Toluene
tol-PL
Toluene
TTHHFF
THF
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
CHCl
CCHHCCll33
3
300
350
400
450
500
550
600
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
FIGURE 5 The UV–Vis absorption spectra and PL spectra in solution for PFSO5 (a) and PNFSO5 (b). [Color figure can be viewed at
wileyonlinelibrary.com]
were performed by using the B3LYP/6-31G(d) in the Gaussian
09.34 All alkyl chains were replaced by methyl groups and
pentamers were used as the main chromophores of each poly-
mer to simplify the calculation. Model molecules of polymers
PFO (FFFFF), PNFSO (FF-NSO-FF) were constructed,
respectively. Figure 6 depicted the calculated molecular con-
formations and frontier molecular orbitals of the model mole-
cules. The molecular conformation of FF-NSO-FF showed clear
distortion attributed to the asymmetric linked NFSO moiety in
pentamers, then that of FFFFF exhibited linear typed
FIGURE 6 Structure and spatial distribution of the HOMO and LUMO in model molecules. [Color figure can be viewed at
wileyonlinelibrary.com]
8
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(b)
(a)
1
100 mA/cm2
1
0.8
0.6
0.4
0.2
0
PFO
200 mA/cm2
300 mA/cm2
400 mA/cm2
800 mA/cm2
PNFSO2
PNFSO5
0.8
PNFSO10
0.6
0.4
0.2
0
400
450
500
550
600
650
400
450
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
FIGURE 7 EL spectra for all polymers (a) and EL spectra with the variation of current densities for PNFSO5 (b). [Color figure can be
viewed at wileyonlinelibrary.com]
105
3
(a)
(b)
PFO
102
100
PFO
2.5
2
PNFSO2
PNFSO5
PNFSO10
104
103
102
10
1
PNFSO2
PNFSO5
PNFSO10
1.5
1
10-2
10-4
10-6
0.5
0
-0.5
0
2
4
6
8
10
12
0
100
200
300
400
500
Current Density (mA cm-2)
Voltage (V)
FIGURE 8 J-V-L curves (a) and LE-J curves (b) of devices based on polymers. [Color figure can be viewed at wileyonlinelibrary.com]
conformation. The HOMO and LUMO were delocalized over
the pentamer (FFFFF), whereas, the molecular orbital distri-
bution of FF-NSO-FF was different from that of the former
completely. The HOMO was mainly localized at fluorene moi-
ety, while its LUMO was mainly distributed on the electron
deficient NFSO unit.35 Such electron distribution endows the
FF-NSO-FF inherent intramolecular CT property. The calcu-
lated EHOMO/ELUMO of FF-NSO-FF was −5.25/−2.11 eV, which
was deeper than −5.08/−1.53 eV of FFFFF, displaying good
agreement with the experimental results measured by the CV.
PNFSO2 upward to 442 nm for PNFSO10 arising from weak
CT effect. The EL spectra of PNFSO5 under increased current
densities from 100 to 800 mA cm−2 was shown in Figure 7
(b). It was suggested that PNFSOs exhibited current-density-
independent EL spectra, implying the excellent stability of the
resulting polymers.
Figure 8 illustrated the current density (J) and luminance (L)
as functions of driving voltage (V) (J-V-L) characteristics and
the luminous efficiencies-current densities (LE-J) curves of
devices. It can be seen that the leakage current density in PFO
device is higher than that in other device, which can be attrib-
uted to a larger electron injection barrier at the cathode/emis-
sive layer interface for PFO device from ELUMO of Table 1 and
the unbalanced hole and electron fluxes of PFO device in
Figure 9. With the increase of NFSO moiety in polymer, the
turn-on voltage gradually decreased, and luminance and cur-
rent density enhanced evidently at same turn-on voltage
[Fig. 8(a)]. Seen from Figure 8(b), in comparison with homo-
polymer PFO, the LE of the copolymers were improved
EL Properties
To evaluate the EL properties of polymers, a single-layer
device with the structure of ITO/PEDOT:PSS/EML(60 nm)/
Ba/Al was fabricated. The EL spectra of polymers were pre-
sented in Figure 7(a). The twisted backbone structure of
copolymers can disrupt the extended π-conjugation, so the EL
profiles of copolymers were in deep-blue emission and blue-
shifted slightly compared to PFO. While increasing the amount
of NFSO unit, the emission peak goes from 432 nm for
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102
101
101
(a) PFO
(c) PNFSO10
101
100
(b) PNFSO5
100
10-1
10-2
10-3
10-4
10-5
100
10-1
10-2
10-3
10-4
10-5
10-6
10-1
10-2
10-3
10-4
hole-only
hole only
hole-only
electron-only
electron-only
electron-only
0
2
4
6
8
10
0
2
4
6
8
10
0
2
4
6
8
10
Voltage(V)
Voltage(V)
Voltage(V)
FIGURE 9 J-V characteristics of the hole-/electron-only devices of PFO (a), PNFSO5 (b) PNFSO10 (c). [Color figure can be viewed at
wileyonlinelibrary.com]
conspicuously, and we can note that all devices showed rela-
tively low roll-off of luminous efficiency with the increasing of
current densities, which was favorable for the long-term
working stabilities of the devices.
TABLE 3 Device Performances of Polymers
a
Von
(V)
LEmax
Lmax
Polymer
(cd A−1
)
(cd m−2
)
CIEb (x,y)
The device performance data were summarized in Table 3. It
can be seen that the PFO exhibited low device efficiency with
LEmax of 0.49 cd A−1 and CIE coordinates of (0.18, 0.18). How-
ever, the PF-FSO5 exhibited better EL performance with LEmax
of 0.77 cd A−1 and CIE coordinates of (0.17, 0.13).25 In con-
trary, the combination property of the copolymers PNFSOs
have a dramatically enhancement with respect to PFO and PF-
FSO5. With varying content of NFSO unit, the EL profiles
showed deep blue light emission, and all copolymers achieved
PFO
4.6
0.49
1.30
1.96
1.65
2691
3573
5481
3279
(0.18,0.18)
(0.16,0.06)
(0.16,0.07)
(0.16,0.08)
PNFSO2
PNFSO5
PNFSO10
a
4.3
4.0
4.0
Corresponding to 1 cd m−2
.
b
Measured at 36 mA cm−2
.
FIGURE 10 AFM images of all polymers. [Color figure can be viewed at wileyonlinelibrary.com]
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superior CIE coordinate with CIEx + CIEy < 0.3 and CIEy < 0.10,
which was benefited from the twisted-structure NFSO unit.
The best device performances were achieved based on
PNFSO5, which exhibited the maximum luminous efficiencies
(LEmax) of 1.96 cd A−1 and the maximum luminance (Lmax) of
5481 cd m−2, associated with the characteristic deep blue
emission with CIE coordinates of (0.16, 0.07). The improved
performance can be attributed to a lower leakage current
(Fig. 8), the balanced hole and electron fluxes (Fig. 9), and
improved PLQY (Table 2).
from the device based on PNFSO5, which showed a low
turn-on voltage of 4.0 V, a maximum luminous efficiency of
1.96 cd A−1, with CIE coordinates of (0.16, 0.07), respec-
tively. The result indicated that our designed strategy effec-
tively weakened CT effect compared with PFSO5 and
improved the EL efficiency and achieved deep-blue light
emission with respect to PF-FSO5, indicating that the
twisted-structure NFSO unit had a great potential for adjust-
ing the polymers blue-light emission and enhancement of EL
efficiency.
To explore the efficiency improvement of PNFSOs, the single
charge carrier devices were fabricated and their current
density-voltage characteristics (J-V) are shown in Figure 9.
The hole-only device of PFO exhibited much higher current
densities than the corresponding electron-only device in the
entire applied voltage range of 0–10 V. The introducing of
electron-withdrawing NFSO unit into polymer enhanced
electron affinity, and thus, increased electron current and
bring about more balanced electron and hole transport and
further improved device performance of PNFSO5. Further-
more, the electron fluxes exceed the hole fluxes in
PNFSO10, which also explains why the device performance
decreased when the NFSO moiety content further increased
from 5% to 10%.
ACKNOWLEDGMENTS
The authors are grateful for the financial support from the
National Key Research and Development Program of
China(No. 2016YFB0401302) funded by MOST, the Science
and Technology Program of Guangdong Province, China (No.
2017A050503002), the National Natural Science Foundation
of China (No. 21704027), and the Natural Science Foundation
of Jiangsu Province (Grant No. BK20141151).
REFERENCES AND NOTES
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Film Morphology
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The root-mean-square (RMS) roughness value recorded for
PFO was 1.35 nm, and the RMS value gradually reduced after
introducing NFSO unit into PFO backbone from 1.16 nm for
PNFSO2 to 0.60 nm for PNFSO10, which demonstrated that
the polymers containing the NFSO moiety have an excellent
film forming ability. There was an inverse correlation between
the content of NFSO unit and the RMS values. The more pro-
portion of NFSO unit accelerated the more distorted structure
of polymer to reduce the π-π stacking of polymer chains,
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thereby enhance the film forming ability.
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In summary,
a
novel polycyclic aromatic compound
2,8-dibromo-14,14-dioctyl-14H-benzo[b]benzo[5,6]fluoreno
[1,2-d] thiophene 9,9-dioxide was designed and synthesized,
and was incorporated into PFO backbone. The resulting poly-
mers exhibited good thermal stability with thermal decompo-
sition temperature over 420 ꢀC and higher luminescent
efficiency with PLQY over 45%. Compared with poly(9,-
9-dialkylfluorene-2,7-diyl)s (PFO), the LUMO energy levels of
the copolymer PNFSOs slightly decreased with the increase
of NFSO content in copolymer backbone from −2.16 eV for
PFO to −2.26 eV for PNFSO10. The PL spectra of copolymers
blue-shifted to acquire deep blue emission due to the suit-
able twisted structure. The copolymers show stable EL spec-
tra with the increase of current densities in the range of
100–800 mA cm−2. The champion performance was achieved
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