Indeno[2,1-c]fluorene-based blue fluorescent oligomers and polymers: Synthesis, structure, photophysical and electroluminescence properties

Article abstract of DOI:10.1016/j.polymer.2013.03.044

An alkylated skeleton of indeno[2,1-c]fluorene, viewed as a positional isomerism of indeno[1,2-b]fluorene, was developed to construct several conjugated oligomers and polymers through a Pd-catalyzed Suzuki coupling reaction for organic light-emitting diodes. Single crystal analysis indicated that the structure of indeno[2,1-c]fluorene was twisted due to close H - -H contacts in the crowded region of molecules. P1-P3 showed good solubility in common organic solvents as well as facile film forming properties. In comparison with linear n-alkyl-substituted oligo(indeno[1,2-b]fluorene)s and poly(indeno[1,2-b]fluorene)s (2,8-PIFs), the absorption and emission features of twisted n-alkyl-substituted oligo(indeno[2,1-c]fluorene)s and poly(indeno[2,1-c]fluorene) (P1) in dilute solution exhibited a correlation to the conjugation length and obviously blue-shifted due to the distorted backbones. All polymers emitted strong blue fluorescence with very narrow full widths at half-maximum (fwhm) (about 50 nm) in dilute solution and in thin film. Electroluminescent (EL) devices with the configuration of ITO/PEDOT:PSS/PVK/ polymers/Ba/Al were fabricated to achieve blue EL emission with maximum brightness up to 2800 cd/m² at a bias of 10 V. The EL devices using P2 as the active materials showed a maximum external quantum efficiency of 1.2% and a maximum luminous efficiency of 1.5 cd/A. Our investigations demonstrate that indeno[2,1-c]fluorene moiety might be regarded as a potential building skeleton to develop the effective blue and UV light-emitting materials.

Full text of DOI:10.1016/j.polymer.2013.03.044

Polymer 54 (2013) 2935e2944
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Indeno[2,1-c]fluorene-based blue fluorescent oligomers and polymers: Synthesis,
structure, photophysical and electroluminescence properties
,
b
b
,
c
Bin Du a, Lei Wang c, Si-Chun Yuan a , Ting Lei , Jian Pei , Yong Cao
**
*
a Food Science and Engineering College, Department of Fundamental Science, Beijing University of Agriculture, Beijing 102206, China
b Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871,
China
c State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640,
China
a r t i c l e i n f o
a b s t r a c t
Article history:
An alkylated skeleton of indeno[2,1-c]fluorene, viewed as a positional isomerism of indeno[1,2-b]fluo-
rene, was developed to construct several conjugated oligomers and polymers through a Pd-catalyzed
Suzuki coupling reaction for organic light-emitting diodes. Single crystal analysis indicated that the
structure of indeno[2,1-c]fluorene was twisted due to close H—H contacts in the crowded region of
molecules. P1eP3 showed good solubility in common organic solvents as well as facile film forming
properties. In comparison with linear n-alkyl-substituted oligo(indeno[1,2-b]fluorene)s and poly(indeno
[1,2-b]fluorene)s (2,8-PIFs), the absorption and emission features of twisted n-alkyl-substituted oli-
go(indeno[2,1-c]fluorene)s and poly(indeno[2,1-c]fluorene) (P1) in dilute solution exhibited a correlation
to the conjugation length and obviously blue-shifted due to the distorted backbones. All polymers
emitted strong blue fluorescence with very narrow full widths at half-maximum (fwhm) (about 50 nm)
in dilute solution and in thin film. Electroluminescent (EL) devices with the configuration of ITO/
PEDOT:PSS/PVK/polymers/Ba/Al were fabricated to achieve blue EL emission with maximum brightness
up to 2800 cd/m2 at a bias of 10 V. The EL devices using P2 as the active materials showed a maximum
external quantum efficiency of 1.2% and a maximum luminous efficiency of 1.5 cd/A. Our investigations
demonstrate that indeno[2,1-c]fluorene moiety might be regarded as a potential building skeleton to
develop the effective blue and UV light-emitting materials.
Received 3 January 2013
Received in revised form
18 March 2013
Accepted 21 March 2013
Available online 4 April 2013
Keywords:
Twisted backbone
Light-emitting polymers
Polycyclic skeleton
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
the methylene bridges to be either on the opposite side (indeno
[1,2-b]fluorene and indeno[2,1-c]fluorene, Scheme 1) or on the
Blue light-emitting polymers and copolymers are drawing wide
interest, notably for applications in large-area flexible and tunable
polymeric light-emitting diodes (PLEDs) [1]. In this respect,
considerable efforts have been devoted to the development of
solution-processable poly(p-phenylene)s (PPPs) [2], polyfluorene
(PF) [3], ladder-type PPPs (LPPPs) [4], poly(tetrahydropyrene)s [5],
poly(2,7- or 3,6-phenanthrylene)s [6], poly(2,8-indenofluorene)s
(2,8-PIFs, Scheme 1) [7], poly(2,7-pyrenylene)s [8] and their de-
rivatives due to the suited HOMOeLUMO energy gaps required for
obtaining blue emission. Of these, linear polycyclic indenofluorene-
based PIFs and their derivatives continue to attract considerable
attention [9]. Indenofluorenes have unique geometries that allow
same side (indeno[2,1-b]fluorene) of the terphenyl core [10].
Polycyclic oligomers and polymers based on indeno[1,2-b]fluo-
rene (IF) are of inherent high fluorescent emission yields with the
maximum emission peaks ranging from 410 to 450 nm [11]. How-
ever, the blue emission of these derivatives in the solid state appears
to be unstable due to the rapid appearance of long-wavelength
emission bands. A variety of strategies toward achieving their spec-
trum stability and color purity have been utilized in the literatures
through copolymerization, appending aryl groups onto methylenes,
functionalization with dispiro-chromophores, and others [12].
Although indeno[1,2-b]fluorene with a planar and long conju-
gated terphenylene moiety has been intensively explored as a
promising building skeleton to develop linear blue light-emitting
oligo(2,8-indenofluorene)s and PIFs with the structures interme-
diate between PFs and totally ladder-type PPPs, to the best of our
knowledge, less was known about its positional isomer indeno[2,1-
* Corresponding author. Tel./fax: þ86 10 62758145.
** Corresponding author. Tel./fax: þ86 10 80799170.
E-mail addresses: ysc@bac.edu.cn (S.-C. Yuan), jianpei@pku.edu.cn (J. Pei).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.03.044

2936
B. Du et al. / Polymer 54 (2013) 2935e2944
10
2
6
11
3
5
11
6
12
9
4
1
10
12
7
5
4
3
1
1
8
9
2
8
2
5
9
10
11
8
6
4
3
12
7
7
indeno[1,2-b]fluorene
indeno[2,1-b]fluorene
indeno[2,1-c]fluorene
(Isomer2-IF)
(Isomer1-IF)
(IF)
n
R
R
R
R
R
R
R
R
n
R
R
n
R= Alkyl or Aryl
R
R
R
R
2, 8-Poly(indeno[1,2-b]fluorene)s
2, 8-Poly(indeno[2,1-b]fluorene)s
3, 10-Poly(indeno[2,1-c]fluorene)s
(2,8-PIFs)
(3,10-PIFs)
(2,8-PIIFs)
Scheme 1. Chemical structures of polyphenylene-based polymers.
c]fluorene (Isomer 2-IF, Scheme 1) [13], and the photophysical
properties of indeno[2,1-c]fluorene-based polymers in the appli-
cations of optoelectronic devices has not been reported. Indeno
[2,1-c]fluorene, by virtue of its unique zigzag topology, is consid-
ered as an attractive building skeleton and functionalized at C-3, C-
5, C-6, C-7, C-8, and C-10 positions via readily available reactions. A
polycyclic polymer containing the indeno[2,1-c]fluorene moiety
can be expected to offer two major advantages: (1) the attachment
of alkyl groups at the 6,7-positions enhances solubility without
disturbing the conjugation along the chain; (2) the interruption of
the planarity attributed to the disordered skeleton within the
stepladder polymers would reduce the unwanted low energy
emission band, which has been observed for IF-based PLEDs.
Herein, we wish to report the synthesis of indeno[2,1-c]fluo-
rene-based polymers, which can be viewed either as PIF or PF
analogs. To the best of our knowledge, no report has described the
approach for the synthesis of poly(indeno[2,1-c]fluorene)s. Indeno
[2,1-c]fluorene is polymerized through a Pd-catalyzed Suzuki
coupling reaction from a 3,10-substituted monomer to generate a
kinked PIF analog. Additionally, the synthesis of 3,10-linked
the polymers was determined with a Waters GPC 2410 instrument
in tetrahydrofuran (THF) by use of a calibration curve of poly-
styrene standards. The single crystals of compound 7 was per-
formed at 20 ꢁC on a Rigaku RAXIS RAPID IP diffractometer using
graphite-monochromated Mo K
a
radiation (
l
¼ 0.71073 Å). The
determination of crystal class and unit cell parameters was carried
out by the Rapid-AUTO (Rigaku 2000) program package. The
structures were solved by use of SHELXTL program.
2.2. Device fabrication
Device configurations are ITO/PEDOT:PSS/polymers/Ba/Al and
ITO/PEDOT:PSS/PVK/polymers/Ba/Al, respectively. The fabrication
of electroluminescent devices was carried out by the standard
procedure below.
A 50 nm thick layer of poly(ethylene
dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS, Baytron P
4083, purchased from Bayer AG) was spin-coated on the pre-
cleaned ITO-glass substrates, which was followed by drying in a
vacuum oven at 80 ꢁC for 8 h. A thin film of the polymers was spin-
coated on the top of PEDOT in chlorobenzene. A 50 nm PVK film
was placed between the PEDOT and the emitting layer. The film
thicknesses of the active layers were around 75e80 nm, measured
by an Alfa Step 500 surface profiler (Tencor). A layer of Ba (4e5 nm),
and a layer of Al (150 nm) were subsequently vacuum-evaporated
onto the top of the EL polymer layer under vacuum. Device fabri-
cation was carried out in a controlled dry-box (Vacuum Atmo-
sphere Co.) in N2 circulation. Current density (J)evoltage (V)e
luminance (L) data was collected using a Keithley 236 source
measurement unit and a calibrated silicon photodiode. External EL
quantum efficiencies (EQE) were obtained by measuring the total
light output in all directions in an integrating sphere (IS-080, Lab-
sphere). The luminance (cd/m2) was measured by a Si photodiode,
and calibrated by a PR-705 SpectraScan Spectrophotometer (Photo
Research). Photoluminescence quantum yields were obtained by
nanolog infrared fluorescence test system (HORIBA J. Y.). Photo-
luminescence (PL) spectra and electroluminescence (EL) spectra
were recorded by a CCD spectrophotometer (Instaspec 4, Oriel).
bis(indeno[2,1-c]fluorene)s
and
tris(indeno[2,1-c]fluorene)s
(trimer), which serve as model compounds for the spectroscopic
characterization of the corresponding polymers, is described.
2. Experimental section
2.1. Measurement and characterization
1H and 13C NMR spectra were collected on a Bruker DRX 300 and
400 spectrometer in deuterated chloroform solution, with tetra-
methylsilane (TMS) as internal standard in all cases. Elemental
analyses were carried out on an Elementar Vario EL (Germany). EI-
MS were recorded on a LCQ DECA XP Liquid ChromatographeMass
Spectrometer (Thremo Group). UV-vis spectra were recorded on
PerkineElmer Lambda 35 UV-vis spectrometer and PL spectra were
carried out on a PerkineElmer LS55 Luminescence spectrometer.
Cyclic voltammetry was carried out on a CHI660A electrochemical
workstation in
a
solution of tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) (0.1 M) in CH3CN at a scan rate of
10 mV sꢀ1 at room temperature under argon atmosphere. A Pt wire
was used as the counter electrode, and a saturated calomel elec-
trode was used as the reference electrode. The molecular weight of
2.3. Materials
All manipulations involving air-sensitive reagents were per-
formed under an atmosphere of dry argon. All reagents, unless

B. Du et al. / Polymer 54 (2013) 2935e2944
2937
otherwise specified, were obtained from Aldrich and Beida Syn
Chem Co, and were used as received.
pressure, the residue was purified by column chromatography us-
ing petroleum ether as eluant to afford 5 as a white solid (yield:
86%). 1H NMR (CDCl3, 300 MHz, ppm):
d 8.40e8.30 (m, 2H, AreH),
2.3.1. Diethyl-40,50-dipropyl-[1,10:20,100-terphenyl]-30,60-
dicarboxylate (2)
7.38e7.28 (m, 6H, AreH), 2.86e2.31 (m, 4H, CH2), 2.29e2.02 (m,
4H, CH2), 1.99e1.63 (m, 4H, CH2), 1.50e1.40 (m, 4H, CH2), 1.12e0.65
Diethoxycarbonyldiphenylcyclopentadienone
1
(3.76 g, 10.0
(m, 26H, CH2CH3). 13C NMR (CDCl3, 75 MHz, ppm):
d 152.1, 145.7,
mmol) and 4-octyne (1.65 g, 15.0 mmol) were dissolved in mesitylene
(20 mL), and the mixture was then heated at 180 ꢁC for 24 h under
argon atmosphere. The mixture was allowed to cool down to room
temperature. After removal of mesitylene under reduced pressure, the
residue was purified by silica gel column chromatography with pe-
troleum ether and ethyl acetate as eluents to afford 2 as a white solid
141.2, 136.8, 134.8, 126.1, 125.7, 122.1, 121.5, 56.5, 42.4, 30.7, 25.0,
16.9, 14.8, 14.1. MS (EI), m/z: 506 (Mþ, 100%). Elemental analysis:
Anal. Calcd for C26H26: C, 90.06; H, 9.94. Found: C, 90.64; H, 9.79.
2.3.5. 3,10-Dibromo-5,5,6,7,8,8-hexapropyl-5,8-indeno[2,1-c]
fluorene (6)
(yield: 50%).1H NMR (CDCl3, 300 MHz, ppm):
H), 3.94e3.87 (m, 4H, CH2), 2.66e2.61 (m, 4H, CH2), 1.73e1.63 (m, 4H,
CH2),1.04e0.85 (m,12H, CH3).13C NMR (CDCl3, 75 MHz, ppm):
169.3,
d
7.11e6.99 (m,10H, Are
Compound 5 (5.1 g, 0.010 mol) was dissolved in tetrachloro-
methane, and 20 g of Al2O3/CuBr2 (2:1) was added with a vigorous
stirring. The mixture was kept at reflux overnight. The resultant was
cooled to room temperature and filtered off. After removal of the
solvents under reduced pressure, the residue was purified by column
chromatography using petroleum as eluant to afford 6 as awhite solid
d
138.3,136.9,136.7,136.3,130.1,127.1,126.6, 60.73, 32.8, 24.8,14.8,13.5.
MS (EI), m/z: 458 (Mþ, 100%). Elemental analysis: Anal. Calcd for
C30H34O4: C, 78.57; H, 7.47. Found: C, 78.95; H, 7.63.
(yield: 95%). 1H NMR (CDCl3, 300 MHz, ppm):
d 8.13e8.11 (m, 2H, Are
2.3.2. 6,7-Dipropyl-indeno[2,1-c]fluorene-5,8-dione (3)
H), 7.47e7.42 (m, 4H, AreH), 2.86e2.28 (m, 4H, CH2), 2.21e1.99 (m,
4H, CH2), 1.97e1.89 (m, 4H, CH2), 1.39e1.37 (m, 4H, CH2), 1.12e0.58
A mixture of 2 (4.58 g, 10.0 mmol) and polyphosphoric acid
(40 mL) was stirred vigorously for 2 h at 100 ꢁC. The mixture was
poured slowly into ice water and yellow precipitation was filtered
off. The resulting solid was purified by a flash column chromatog-
raphy using dichloromethane as eluant to afford 3 as a yellow-
(m, 26H, CH2CH3). 13C NMR (CDCl3, 75 MHz, ppm):
d 154.9, 145.9,
140.1, 137.7, 134.0, 129.1, 125.2, 123.4, 120.4, 57.1, 42.4, 30.9, 25.2, 17.1,
15.0, 14.2. MALDI-TOF MS, m/z: 664.1 (Mþ). Elemental analysis: Anal.
Calcd for C38H48Br2: C, 68.67; H, 7.28. Found: C, 68.25; H, 7.19.
orange solid (yield: 60%). 1H NMR (CDCl3, 300 MHz, ppm):
d 8.10e
8.07 (m, 2H, AreH), 7.76e7.74 (m, 2H, AreH), 7.64e7.57 (m, 2H, Are
H), 7.41e7.36 (m, 2H, AreH), 3.15e3.10 (m, 4H, CH2), 1.59e1.52 (m,
4H, CH2), 1.13e1.08 (m, 6H, CH3). 13C NMR (CDCl3, 75 MHz, ppm):
2.3.6. 3,10-Bis(40,40,50,50-tetramethyl-10,30,20-dioxaborolan-20-yl)-
5,5,6,7,8,8-hexapropyl-5,8-indeno[2,1-c]fluorene (7)
To a stirred solution of 3,10-dibromo-5,5,6,7,8,8-hexapropyl-
indeno[2,1-c]fluorene (0.66 g, 1.0 mmol) in DMSO (10 mL),
4,4,40,40,5,5,50,50-octamethyl-2,20-bi(1,3,2-dioxaborolane) (0.25 g,
2.0 mmol), KOAc (0.90 g, 8.0 mmol) and Pd(dppf)Cl2 (0.06 g,
0.06 mmol) were added. The mixture was heated to 80 ꢁC overnight
with vigorous stirring under argon atmosphere. After removal of
the solvents under reduced pressure, the residue was extracted
with chloroform and the combined organic phase was dried over
anhydrous MgSO4. After removal of the solvents under reduced
pressure, the residue was purified by flash chromatography to
afford 7 as a white solid (yield: 80%). 1H NMR (CDCl3, 300 MHz,
d
193.3, 145.7, 142.6, 136.7, 135.6, 135.1, 134.4, 128.9, 124.2, 123.3,
28.3, 24.2, 14.6. MS (EI), m/z: 366 (Mþ, 100%). Elemental analysis:
Anal. Calcd for C26H22O2: C, 85.22; H, 6.05. Found: C, 85.54; H, 6.43.
2.3.3. 6,7-Dipropyl-5,8-dihydroindeno[2,1-c]fluorene (4)
With a vigorous stirring, 3 (3.66 g, 10.0 mmol) was dissolved in
2,20-oxydiethanol (50 mL) and hydrazine (14.5 g, 40.0 mmol) was
added to the mixture. The mixture was heated at 70 ꢁC for 1 h.
NaOH (4.0 g, 40.0 mmol) was added to the mixture and then the
mixture was refluxed overnight. The resultant was poured into 2 M
HCl solution and extracted with chloroform. The combined organic
phase was dried over anhydrous MgSO4. After removal of the sol-
vents under reduced pressure, the residue was purified by column
chromatography using petroleum ether and dichloromethane as
eluents to afford 4 as a white solid (yield: 45%). 1H NMR (CDCl3,
ppm):
d 8.36e8.34 (m, 2H, AreH), 7.79e7.76 (m, 4H, AreH), 2.83e
2.81 (m, 4H, CH2), 2.21e2.05 (m, 8H, CH2), 1.41 (s, 24H, CH3), 1.39e
1.37 (m, 4H, CH2), 1.13e0.53 (m, 26H, CH2CH3). 13C NMR (CDCl3,
75 MHz, ppm):
d 151.5, 146.6, 144.3, 137.7, 135.2, 132.9, 127.8, 121.7,
83.6, 56.8, 42.3, 30.9, 25.2, 24.9, 17.1, 15.0, 14.3. MS (ESI), m/z: 759.40
(Mþ, 100%). Elemental analysis: Anal. Calcd for C50H72B2O4: C,
79.15; H, 9.56. Found: C, 79.35; H, 9.18.
300 MHz, ppm): d 8.55e8.48 (m, 2H, AreH), 7.62e7.59 (m, 2H, Are
H), 7.46e7.41 (m, 2H, AreH), 7.38e7.32 (m, 2H, AreH), 3.98e3.95 (s,
4H, CH2), 2.85e2.79 (m, 4H, CH2), 1.75e1.54 (m, 4H, CH2), 1.15e1.10
(m, 6H, CH3). 13C NMR (CDCl3, 75 MHz, ppm):
d
144.0, 142.8, 142.7,
2.3.7. 3-Bromo-5,5,6,7,8,8-hexapropyl-indeno[2,1-c]fluorene(8)
To a mixture of 5,5,6,7,8,8-hexapropyl-indeno[2,1-c]fluorene
(0.5 g, 1.0 mmol), chloroform (10 mL) and acetic acid (10 mL), NBS
(0.18 g,1.0 mmol) in chloroform (15 mL) was added dropwise with a
vigorous stirring. The reaction mixture was stirred overnight. The
organic phase was extracted with chloroform and the combined
organic phase was dried over anhydrous MgSO4. After removal of
the solvents under reduced pressure, the residue was purified by
flash column chromatography to afford 8 as a white solid (yield:
135.3, 134.7, 126.4, 126.0, 125.0, 123.3, 43.6, 32.4, 23.8, 15.0. MS (EI),
m/z: 338 (Mþ, 100%). Elemental analysis: Anal. Calcd for C26H26: C,
92.26; H, 7.74. Found: C, 92.55; H, 7.49.
2.3.4. 5,5,6,7,8,8-Hexapropyl-5,8-indeno[2,1-c]fluorene (5)
With a vigorous stirring, n-BuLi (10 mL, 2.5 M in hexane,
0.025 mol) was added dropwise to a suspension of 4 (3.38 g,
0.010 mol) in anhydrous THF under argon at 0 ꢁC. The mixture was
stirred at 0 ꢁC for 1 h, followed by adding 1-bromopropane (3.0 g,
0.025 mol) dropwise, and then the mixture was stirred for another
1 h. n-BuLi (10 mL, 2.5 M in hexane, 0.025 mol) was added dropwise
at 0 ꢁC and the mixture was stirred for 1 h. Finally, 1-bromopropane
(3.0 g, 0.025 mol) was added dropwise. The mixture was allowed to
cool down to room temperature slowly and stirred overnight. The
resultant was poured into saturated aqueous NH4Cl and extracted
with dichloromethane and then the combined organic phase was
dried with MgSO4. After removal of the solvents under reduced
95%). 1H NMR (CDCl3, 300 MHz, ppm):
d 8.37e8.35 (m, 2H, AreH),
7.81e7.75 (m, 2H, AreH), 7.36e7.30 (m, 3H, AreH), 2.87e2.80 (m,
4H, CH2), 2.25e2.02 (m, 8H, CH2), 1.35e1.30 (m, 4H, CH2), 1.14e1.10
(m, 26H, CH2CH3). MS (EI), m/z: 585, (Mþ, 100%).
2.3.8. 3-(40,40,50,50-Tetramethyl-10,30,20-dioxaborolan-2-yl)-
5,5,6,7,8,8-hexapropyl-indeno[2,1-c]fluorene(9)
To a stirred solution of 8 (0.58 g, 1.0 mmol) in DMSO (10 mL),
4,4,40,40,5,5,50,50-octamethyl-2,20-bi(1,3,2-dioxaborolane) (0.25 g,

2938
B. Du et al. / Polymer 54 (2013) 2935e2944
1.0 mmol), KOAc (0.45 g, 4.0 mmol) and Pd(dppf)Cl2 (0.03 g,
0.03 mmol) was added. The mixture was heated to 80 ꢁC overnight
with vigorous stirring under argon atmosphere. The mixture was
cooled to room temperature. After removal of the solvents under
reduced pressure, the residue was extracted with chloroform and
the combined organic phase was dried over anhydrous MgSO4.
After removal of the solvents under reduced pressure, the residue
was purified by flash chromatography to give 9 as a white solid
chloroform was added dropwise a solution of bromine (1.05 mL,
20.00 mmol) in 10 mL of chloroform at 0 ꢁC over 1 h. The mixture
was then allowed to return to room temperature. After stirring
24 h, the mixture was washed with saturated sodium thiosulfate
solution and brine to remove excess bromine. The precipitate was
filtered and washed with water for three times. The filtrate was
washed with brine and extracted with chloroform. Removal of the
solvent gave a yellow residue, which was then combined with the
white precipitate. Recrystallization of the combined solid from
EtOH afforded M3 as a white solid (yield: 96%). 1H NMR (CDCl3,
(yield: 87%): 1H NMR (CDCl3, 300 MHz, ppm):
d 8.37e8.35 (m, 2H,
AreH), 7.81e7.75 (m, 2H, AreH), 7.36e7.30 (m, 3H, AreH), 2.87e
2.80 (m, 4H, CH2), 2.25e2.02 (m, 8H, CH2), 1.41 (s, 12H, CH3), 1.14e
1.10 (m, 6H). 0.55e0.48 (m, 24H). 13C NMR (CDCl3, 75 MHz, ppm):
300 MHz, ppm):
d 8.10e8.08 (m, 1H, AreH), 8.01e7.94 (m, 2H, Are
H), 7.35e7.14 (m, 7H, AreH), 2.78e2.58 (m, 6H, CH2), 1.86e1.74 (m,
6H, CH2), 0.75e0.60 (m, 36H, CH2), 0.41e0.34 (m, 18H, CH2), 0.27e
0.20 (m, 12H, CH3).
d
152.4, 151.4, 146.5, 145.9, 144.4, 141.4, 137.7, 135.1, 135.4, 134.8,
132.9, 127.7, 126.4, 125.9, 122.4, 121.8, 121.6, 83.6, 56.8, 56.7, 42.6,
42.4, 30.9, 30.9, 25.2, 25.0, 17.2, 17.1, 15.1, 14.3, 14.3. Elemental
analysis: Anal. Calcd for C44H61BO2: C, 83.52; H, 9.72. Found: C,
83.41; H, 9.56.
2.3.12. General procedures for the synthesis of polymers
With P1 as an example 3,10-Bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-5,5,6,7,8,8-hexapropyl-indeno[2,1-c]fluorene
7
2.3.9. 5,5,50,50,6,60,7,70,8,8,80,80-Dodecapropyl-3,30-bis(indeno[2,1-
c]fluorene)s (dimer)
(759 mg,1.0 mmol), 3,10-dibromo-5,5,6,7,8,8-hexapropyl-indeno[2,1-
c]fluorene 6 (664 mg, 1.0 mmol), and tetrakis(triphenylphosphine)
palladium (5 mg) were dissolved in toluene/THF (10 mL, 3:1) and
stirred for 0.5 h, and then 2 M Na2CO3 aqueous solution (4 mL) was
added. The mixture was heated to 100 ꢁC and stirred for 2 d under
argon atmosphere. Then the resultant was poured into stirred meth-
anol (200 mL)to generate plentyof light-yellow precipitates. The solid
was collected by filtration and then dissolved in toluene and washed
twice with dilute NaHCO3 solution. The careful reprecipitation pro-
cedure in acetone/methanol was repeated several times. The polymer
was further purified by washing with acetone at reflux in a Soxhlet for
2 days. The light-yellow solid was dried in vacuum at room temper-
Compound 8 (5.85 g, 0.010 mol), 9 (6.32 g, 0.010 mol), and
Pd(PPh3)4 (0.34 g, 0.30 mmol) were dissolved in a mixture of THF
(100 mL) and aqueous 2 M Na2CO3 (20 mL). The mixture was
refluxed with vigorous stirring for 2 d under argon atmosphere.
After the mixture was cooled to room temperature, it was extracted
with ethyl acetate and the combined organic phase was dried over
anhydrous MgSO4. After removal of the solvents under reduced
pressure, the residue was purified by column chromatography us-
ing petroleum ether as eluant to afford white solids, which was
dissolved in chloroform and precipitated in methanol to afford
dimer as a white solid (yield: 70%). 1H NMR (CDCl3, 400 MHz, ppm):
ature (yield: 52%). 1H NMR (CDCl3, 400 MHz, ppm):
d 8.57e8.55 (m,
d
8.49e8.44 (m, 3H, AreH), 7.73e7.71 (m, 3H, AreH), 7.59e7.58 (m,
2H), 7.79e7.73 (m, 4H), 2.89 (m, 4H), 2.35 (m, 4H), 2.15 (m, 4H),1.53e
1H, AreH), 7.37e7.35 (m, 1H, AreH), 7.34e7.24 (m, 6H, AreH),
2.85e2.80 (m, 8H, CH2), 2.29e2.02 (m, 16H, CH2), 1.47e1.45 (m, 8H,
CH2), 1.16e0.64 (m, 52H, CH2CH3). 13C NMR (CDCl3, 100 MHz, ppm):
1.50 (m, 4H), 1.18e1.15 (m, 8H), 0.88e0.72 (m, 18H). 13C NMR (CDCl3,
100 MHz, ppm):
d 153.3, 146.4, 140.8, 139.4, 137.2, 134.7, 128.8, 127.1,
125.0,122.7,120.1, 57.0, 42.8, 31.9, 31.0, 29.8, 29.7, 29.4, 25.3, 22.7,17.3,
15.1, 14.4, 14.4, 14.1. Elemental analysis: Anal. Calcd for C39H50: C,
90.29; H, 9.71. Found: C, 89.69; H, 9.58. Mn: 6968, PDI: 1.57.
d
153.2, 152.4, 146.3, 146.0, 141.5, 140.7, 139.3, 137.2, 137.1, 135.0,
134.7, 128.8, 127.1, 126.4, 126.0, 124.9, 122.7, 122.4, 121.9, 121.8,
120.0, 56.9, 56.8, 56.7, 42.8, 42.7, 31.0, 29.7, 25.3, 17.3, 17.2, 15.1, 14.4,
14.3. MALDI-TOF MS, m/z: 1012.8 (Mþ).
P2 (yield: 58%): 1H NMR (CDCl3, 400 MHz, ppm):
d 8.42e8.40
(m, 2H), 7.90e7.88 (m, 2H), 7.77e7.56 (m, 8H), 7.32e7.24 (m, 4H),
6.85e6.78(m, 4H), 3.94e3.92 (m, 4H), 2.85 (m, 4H), 2.28 (m, 4H),
2.06 (m, 4H), 1.78e1.73 (m, 4H), 1.55 (m, 8H), 1.32e1.25 (m, 4H),
1.14e1.12 (m, 10H), 0.88e0.66 (m, 26H). 13C NMR (CDCl3, 100 MHz,
2.3.10. 5,5,50,50,500,500,6,60,600,7,70,700,8,8,80,80,800,800-Octadecapropyl-
3,30:100,300-ter(indeno[2,1-c]fluorene)s (trimer)
Compound 6 (6.64 g, 0.010 mol), 9 (6.32 g, 0.010 mol), and
(PPh3)4Pd (0.34 g, 0.30 mmol) were dissolved in a mixture of THF
(80 mL) and aqueous 2 M Na2CO3 (20 mL). The mixture was
refluxed with vigorous stirring for 2 d under argon atmosphere.
After the mixture was cooled to room temperature, it was extracted
with ethyl acetate and the combined organic phase was dried over
anhydrous MgSO4. After removal of the solvents under reduced
pressure, the residue was purified by column chromatography us-
ing petroleum ether as eluant to afford white solids, which was
dissolved in chloroform and precipitated in methanol for two times
to afford trimer as a white solid (yield: 63%). 1H NMR (CDCl3,
ppm): d 158.0, 153.1, 152.9, 146.3, 141.0, 140.8, 139.2, 138.8, 138.0,
137.2, 134.6, 129.4, 129.3, 128.7, 127.2, 126.5, 125.2, 124.6, 122.6,
120.4, 120.1, 114.2, 67.9, 64.5, 42.7, 31.9, 31.6, 29.7, 29.6, 29.4, 29.3,
25.7, 25.3, 22.7, 22.6, 17.2, 15.1, 14.4, 14.3, 14.1, 14.0. Elemental
analysis: Anal. Calcd for C76H90O2: C, 88.15; H, 8.76. Found: C, 88.49;
H, 9.27. Mn: 9124, PDI: 1.67.
P3 (yield: 42%): 1H NMR (CDCl3, 400 MHz, ppm):
d 8.61e8.43
(m, 4H), 7.86e7.77 (m, 8H), 7.44e7.24 (m, 4H), 3.05e2.81 (m, 16H),
2.37e2.10 (m, 20H), 1.53e1.43 (m, 4H), 1.01e0.94 (m, 46H), 0.73e
0.63 (m, 38H). 13C NMR (CDCl3, 100 MHz, ppm):
d 154.5, 154.4,
153.6, 153.3, 146.4, 145.2, 145.1, 144.9, 140.9, 140.4, 139.6, 139.3,
139.1, 138.5, 138.1, 137.3, 134.4, 127.1, 125.1, 125.0, 57.0, 55.9, 55.8,
42.8, 37.1, 31.6, 31.5, 29.7, 29.6, 29.5, 24.0, 22.4, 22.3, 22.2, 17.3, 15.1,
14.4, 14.0, 13.9. Elemental analysis: Anal. Calcd for C102H138: C,
89.71; H, 10.29. Found: C, 90.25; H, 9.68. Mn: 11,575, PDI: 1.92.
300 MHz, ppm): d 8.56e8.44 (m, 6H, AreH), 7.73e7.61 (m, 8H, Are
H), 7.38e7.31 (m, 6H, AreH), 2.87 (m,12H, CH2), 2.32e2.02 (m, 24H,
CH2),1.55e1.48 (m,12H, CH2).1.16e0.66 (m, 78H, CH2CH3). 13C NMR
(CDCl3, 75 MHz, ppm):
d 153.3, 153.2, 152.4, 146.4, 146.3, 146.0,
141.5, 140.7, 139.4, 139.3, 137.2, 137.1, 135.0,134.7, 126.3, 125.9, 125.0,
122.7, 120.0. 57.0, 56.9, 56.8, 42.8, 42.7, 31.9, 31.0, 29.7, 29.4, 25.3,
22.7, 17.3, 17.2, 15.1, 14.4, 14.3, 14.1. MS MALDI-TOF MS, m/z: 1515.9
(Mþ).
3. Results and discussion
3.1. Synthesis and characterization
2.3.11. 2-Bromo-5,5,10,10,15,15-hexahexyl-truxene (M3)
To a mixture of 5,5,10,10,15,15-hexahexyltruxene (10.00 g,
11.81 mmol) and 20 mg of anhydrous FeCl3 as catalyst in 60 mL of
The synthetic approaches for monomers and polymers P1eP3
are illustrated in Schemes 2 and 3. Compound 1 was prepared ac-
cording to the typical procedures [14] by the condensation reaction

B. Du et al. / Polymer 54 (2013) 2935e2944
2939
Scheme 2. The synthetic routes of the compounds 1e9.
between 1,3-diethoxycarbonaylacetone and benzil in the presence
of NaOH, followed by an acid-promoted dehydration using the
concentrated H2SO4 in 45% yield. The subsequent DielseAlder re-
action between 1 with 4-octyne afforded diester 2 in 50% yield.
Compound 2 underwent an intramolecular double FriedeleCrafts
acylation to form 6,7-dipropyl-indeno[2,1-c]fluorene-5,8-dione 3
with 60% yield in hot polyphosphoric acid. A WolffeKishner
catalyzed Suzuki coupling polymerization was employed to pro-
duce the desired polymers (P1, P2, and P3) in moderate yields. The
crude polymers were washed with methanol, water, and methanol
again, successively, and were placed in a Soxhlet apparatus and
extracted with acetone for 48 h. The structures of the dimers, tri-
mers, and the resultant polymers were confirmed by 1H and 13C
NMR characterization, as well as elemental analysis. All polymers
were readily soluble in common organic solvents such as chloro-
form, tetrahydrofuran, and toluene. The number average molecular
weights (Mn) of P1eP3 were 6968, 9124 and 11,575, respectively,
determined by gel permeation chromatography (GPC) using THF as
the eluant and polystyrene as standard.
reduction of compound
3 afforded 6,7-dipropyl-5,8-dihydro-
indeno[2,1-c]fluorene 4 in 45% yield. An alkylation of 4 with n-BuLi
and 1-bromopropane led to 5,5,6,7,8,8-hexapropyl-indeno[2,1-c]
fluorene 5 in 86% yield. Subsequent alumina-supported CuBr2
bromination afforded 3,10-dibromo-5,5,6,7,8,8-hexapropyl-indeno
[2,1-c]fluorene 6. A Miyaura coupling between 6 and two equiva-
lent of bis(pinacolato)diboron catalyzed by Pd(dppf)Cl2 produced
boronic diester 7 in 80% yield. Monobromination of 5 with NBS
generated bromide 8 in 95% yield. Boronic ester 9 was obtained
from 8 through a Miyaura reaction in 87% yield. To more precisely
assess the mode of conjugated polymer, indeno[2,1-c]fluorene-
based dimer and trimer were prepared. Compound 8 was coupled
with boronic ester 9 to give dimer under the standard Suzuki re-
action condition in 70% yield. The Suzuki coupling reaction be-
tween 6 and 9 afforded trimer in 63% yield. The palladium-
To elucidate molecular structure, single crystal of compound 7
was obtained by slow diffusion of methanol into a solution of the
sample in dichloromethane (Fig. 1). There are two molecules in the
unit cell with different configurations. Close H—H contacts occur in
the crowded region of molecules, and two kinds of distances of the
ꢀ
ꢀ
hydrogen atoms are observed (1.944 A and 2.041 A). The distances
are significantly shorter than twice the van der Waals radius of the
ꢀ
hydrogen atom (2.4 A). Therefore, the strong repulsive interaction
between the hydrogen atoms leads to the spatial distortion of the
conjugated indeno[2,1-c]fluorene
p-system which provides

2940
B. Du et al. / Polymer 54 (2013) 2935e2944
Scheme 3. Synthetic routes of dimer, trimer and polymers.
significant molecular stabilization [15]. The dihedral angles be-
tween two fluorenyl planes (through the central phenyl ring) are
16.37ꢁ (between the plane {C10, C11, C12, C13, C14, C15, C16, C17,
C5, C6, C7, C8, C9} and plane {C5, C6, C7, C8, C16, C17, C18, C19, C20,
C1, C2, C3, C4}), 19.06ꢁ (between the plane {C51, C52, C53, C54, C55,
C56, C57, C58, C65, C66, C67, C68, C69} and the plane {C55, C56, C57,
C58, C100, C59, C60, C61, C62, C63, C64, C65, C66}), respectively.
with a shoulder at about 450 nm. In general, the presence of well
defined vibronic structures in the emission spectra indicates that
the polymers have rigid and well-defined backbone structures. In
comparison with P1, the emission maximum (lmax) of P3 showed
a blue-shift (in web version) of 21 nm. The obvious blue-shift
maybe attributed to the meta-conjugation of truxene cores. All
polymers show large stokes shift (ca. 50 nm) between absorption
and emission maximum, indicating large structural differences
between the ground states and excited states [7]. Furthermore, the
absorption and emission behaviors of P1 in chloroform were
different with those of n-alkyl-substituted poly(indeno[1,2-b]flu-
orene)s (2,8-PIFs, Scheme 1) [7]. For 2,8-PIFs, the absorption and
emission maxima peaked at 416 and 432 nm, respectively, with
small Stokes shift of 16 nm, which is similar to ladder-type poly-
mers. The differences might be caused by enhanced torsion angle
of the twisted-indenofluorene repeating units in polymer back-
bone, leading to the decrease of conjugation length. The absorp-
tion (lmax ¼ 385 nm) and emission peaks (lmax ¼ 428 nm) of P1
red-shifted (in web version) in comparison with those of PF
3.2. Absorption and photoluminescence properties
Normalized absorption and emission spectra of P1, P2, and P3
in chloroform (10ꢀ6 M) are shown in Fig. 2. The photophysical
properties are summarized in Table 1. All polymers showed very
strong
pep* transition attributed to the conjugated backbone,
which progressively blue-shifted (in web version), indicating the
decrease of the effective conjugation length. The absorption
maximum (lmax) peaked at 385 nm for P1, 381 nm for P2, and
364 nm for P3, implicating that the incorporation of truxene unit
into polymer backbone can decrease the electronic conjugation
degree in comparison with fluorene and twisted indeno[2,1-c]
fluorene counterparts. P1 and P2 showed almost identical pho-
toluminescent (PL) behaviors, which peaked at around 428 nm
(
lmax ¼ 383 nm, lmax ¼ 415 nm), implying enhanced conjugation.
The VPL of polymers P1, P2, and P3 in THF are 0.46, 0.69 and 0.50,
respectively.

B. Du et al. / Polymer 54 (2013) 2935e2944
2941
appeared slightly blue-shifted (in web version) compared to those
in solutions, suggesting that the aggregation of the chromophores
was suppressed in solid state. However, the PL spectra of the
polymers behaved differently when came from solution to film. As
shown in Fig. 3, the peaks of the zigzag polymers did not show
much red-shift (in web version), but the shoulder peaks decreased
significantly, even disappeared in solid state. P1 and P2 showed
almost identical emission features, which peaked at around 450 nm
without the vibronic structure. In the case of P1, it did not exhibit a
long-wavelength emission peaked at around 530 nm, which was
frequently observed in straight-alkyl-substituted 2,8-PIFs in solid
states, probably attributing to the aggregation of 2,8-PIFs [15,16]. It
is also indicative that incorporating kinked indeno[2,1-c]fluorene is
probably effective to suppress the aggregation in the solid state.
To determinate the effective conjugation of P1, the absorption
and emission features of dimer, trimer and homopolymer P1 in
chloroform were investigated. As shown in Fig. 4, the electronic
absorption behaviors of twisted oligo(indeno[2,1-c]fluorene)s
exhibited a direct correlation to the conjugation length, i.e., the
absorption maximum continuously red-shifted (in web version) as
the number of the indeno[2,1-c]fluorene units increased. Dimer,
trimer, and P1 exhibited maximum absorptions at 354, 378, and
385 nm, respectively. Their PL features revealed typical character-
istics of conjugated oligoindenofluorenes derivatives and exhibited
maximum emissions at 410 nm for dimer, 424 nm for trimer, and
428 nm for P1, with well-defined vibronic features. For these
values, the effective conjugation length of P1 is estimated to be
about 4 units of indeno[2,1-c]fluorene.
3.3. Electrochemical properties
Electrochemical properties of the polymers were investigated
by cyclic voltammetry (CV). As shown in Fig. 5, all polymers except
P3 exhibited reversibility in both n-doping and p-doping processes.
For P1, it is observed that upon cathodic sweeping of the polymer,
the onset of the reduction occurred at ꢀ2.15 V with a cathodic peak
at ꢀ2.48 V. In the anodic scan, the oxidation (p-doping) process
gave a sharp peak at 1.60 V with the onset of the oxidation occurred
at 1.25 V. Accordingly, the HOMO and LUMO energy levels were
estimated to be ꢀ5.65 eV and ꢀ2.25 eV, respectively. The electro-
chemical properties of P2 showed only slightly differences and a
smaller bandgap than that of P1. For P3, by insertion of the truxene
unit into the polymer main chain, a decrease in oxidation potential
was expected initially because of the decrease in effective conju-
gation of the polymer main chain together with the steric hin-
drance of the alkyl groups. However, P3 showed almost the same
oxidation peak potential as that of P1. On the other hand, there was
an obvious difference in the onset potential of the n-doping process
between the two polymers, which caused a decrement of 0.4 eV of
LUMO energy level for P3 in comparison with P1. The reason of this
inconsistency between absorption spectra and electrochemical
results is unknown, possibly due to some dynamic problems in CV
measurement.
Fig. 1. Single crystal structure of compound 7: (a) wireframe representation and (b)
ORTEP views of the crystal structure with thermal ellipsoids at the 50% probability
level (hydrogen atoms have been omitted for clarity).
Normalized absorption and emission spectra of polymers P1, P2,
and P3 in films are shown in Fig. 3. The structured absorption bands
of polymers observed in solution were also exhibited in the solid
state. The UV-vis absorption maxima of the polymers in thin films
1.2
1.0
1.2
1.0
P1
P2
P3
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
3.4. Electroluminescent properties
Initially, single-layer devices with the configuration of ITO/
PEDOT:PSS(50 nm)/polymers(75 nm)/Ba(4 nm)/Al(150 nm) were
fabricated to investigate their EL properties. As depicted in Fig. 6, EL
features showed blue (in web version) emission maxima peaked at
430 nm for P1, 432 nm for P2, and 426 nm for P3, respectively. A
maximum external quantum efficiency of 0.08% and a luminous
efficiency of 0.10 cd/A were achieved from P2, with the EL onset at
about 5.4 V in the forward bias direction. The low efficiencies were
attributed to the imbalance of electron and hole injection and
300
350
400
450
500
550
Wavelength (nm)
Fig. 2. Absorption and emission spectra of polymers in chloroform (1 ꢂ 10ꢀ6 M).

2942
B. Du et al. / Polymer 54 (2013) 2935e2944
Table 1
The photophysical and electrochemical properties of polymers.
Polymers
In CHCl3 solution
In filma
Eg (eV)
Eox (eV)
HOMO (eV)
LUMO (eV)
b
labs,max (nm)
lPL,maxnm)
Stokes shift (nm)
FPL
labs,max (nm)
lPL,max (nm)
P1
P2
P3
385
381
364
428
427
418
43
46
54
0.46
0.69
0.50
378
377
361
450
451
436
3.40
3.45
2.90
1.25
1.35
1.15
ꢀ5.65
ꢀ5.75
ꢀ5.55
ꢀ2.25
ꢀ2.30
ꢀ2.65
a
measured as film spin-coated from chloroform solution.
b
measured in THF solution.
0.0008
1.2
1.2
1.0
P1
P2
P3
P1
P2
P3
0.0006
0.0004
0.0002
0
1.0
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
-0.0002
-0.0004
-0.0006
300
350
400
450
500
550
600
Wavelength (nm)
-3
-2
-1
0
1
2
3
Fig. 3. The absorption and emission spectra of polymers in thin film.
Potential (V)
transportation. The detailed device performances are shown in
Table 2. and Fig. 7.
Fig. 5. Cyclic voltammograms of polymers in CH3CN solution.
(150 nm) were fabricated. The maximal external quantum effi-
ciency for P3 was increased to 1.2% and a maximum luminous ef-
ficiency of 1.5 cd/A was achieved with EL onset at about 7 V. The
onset voltage was significantly increased, in accordance with the
In order to improve the device performances, PVK was intro-
duced to balance the electron and hole injection. With a low elec-
tron affinity (ca. ꢀ2.2 eV) and ionization potential (ca. ꢀ5.8 eV),
PVK was used as a hole-transporting and electron-blocking mate-
rial in OLEDs. The multilayed devices with the configuration of ITO/
PEDOT:PSS (50 nm)/PVK(40 nm)/polymers (80 nm)/Ba (4 nm)/Al
1.2
1.2
1.0
0.8
0.6
0.4
0.2
0
1.2
1.0
P1
P2
1.0
Dimer
Trimer
P1
P3
0.8
0.8
0.6
0.4
0.2
0
0.6
0.4
0.2
0
250
300
350
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 4. The absorption and emission spectra of P1 and oligo(indeno[2,1-c]fluorene)s in
chloroform (1 ꢂ 10ꢀ6 M).
Fig. 6. Electroluminescent spectra of polymers in devices.

B. Du et al. / Polymer 54 (2013) 2935e2944
2943
Table 2
probably due to the effective electron-blocking as suggested by
Jenekhe et al. [17].
OLED device performances of P1, P2 and P3.
In comparison, PLED from 2,8-PIFs, showed a greenish-blue (in
Materials Device
Turn-on QEmax LEmax Lmax
CIE (x, y)
configurationa (V)
(%)
(cd/A) (cd/m2)
web version) emission with CIE coordinates (x
¼
0.380,
P1
P2
P3
A
B
A
B
A
B
6.8
5.4
7.7
5.4
6.2
5.6
0.93
0.06
0.73
0.08
1.20
0.06
1.10
0.07
0.87
0.10
1.50
0.07
2780
500
2340
490
1820
330
0.204, 0.150
0.208, 0.227
0.201, 0.157
0.278, 0.204
0.215, 0.149
0.200, 0.187
y ¼ 0.448). However, in above-mentioned devices with twisted
indeno[2,1-c]fluorene-based polymers (P1eP3) as emitting layers,
the green-blue emission was absent, probably due to the differ-
ences in the packing of conjugated chains. The unplanar twisted
indeno[2,1-c]fluorene along the polymer backbone may globally
lead to less dense chain-packing and a subsequent decrease in the
rate of excited-state interchain migration [15e18]. Despite the
enhanced device performances by inserting PVK layer, the effi-
ciencies are not high and the EL stability under thermal annealing
and electric field are still unknown. To solve this problem, uti-
lizing proper thermal annealing treatment to adjust the
morphology, copolymerization with other potential building
blocks, and selection of a good electron injection-cathode could
be good choices. The efforts are in progress and will be reported
in a forthcoming paper.
a
A: ITO/PEDOT:PSS/PVK/polymers/Ba/Al. B: ITO/PEDOT:PSS/polymers/Ba/Al.
general observations that the PVK layer would cause a substantial
increase in the operating voltage, as shown in Fig. 7a. Certain
important device characteristics have been dramatically improved
by inserting a PVK layer between PEDOT:PSS and the emitting layer,
4. Conclusions
A conjugated skeleton, indeno[2,1-c]fluorene, has been devel-
oped to construct new conjugated polymer for organic light-
emitting diodes. Single crystal analysis of its structure shows that
this molecule exhibits interesting twisted structures. A series of
novel conjugated polymers and oligomers containing twisted
indeno[2,1-c]fluorene unit have been synthesized through the Pd-
catalyzed Suzuki coupling polymerization and their electrolumi-
nescent properties have been characterized. In comparison with n-
alkyl-substituted 2,8-PIFs, the absorption and emission behaviors
of alkyl-substituted P1 in solution and in solid film are different,
probably caused by the distorted backbone. The electronic ab-
sorption and emission behaviors of these distorted oligo(indeno
[2,1-c]fluorene)s and polymers exhibit a direct correlation to the
conjugation length, and substantially blue-shifted relative to the
linear oligo(indeno[1,2-b]fluorene)s and 2,8-PIFs. Electrolumines-
cent devices with the configuration of ITO/PEDOT:PSS/PVK/poly-
mers/Ba/Al show blue EL emission spectrum with luminance values
of typically 1800e2800 cd/m2 at a bias of 10 V. The maximal
external device efficiency of 1.2% and a maximum luminous effi-
ciency of 1.5 cd/A are achieved. The formation of long wavelength
emission bands, typically attributed to polymer aggregation in n-
alkyl-substituted 2,8-PIFs in electroluminescent devices, is not
observed in our polymers. Our investigations demonstrate that
twisted indeno[2,1-c]fluorene moiety might be utilized as a po-
tential building skeleton to generate the effective blue light-
emitting materials.
Acknowledgments
This work was supported by the Major State Basic Research
Development Program (No. 2009CB623601) from the Ministry of
Science, Research Improvement Project of BUA (No.1086716078),
Funding Project for Academic Human Resources Development
in Institutions of Higher Learning under the Jurisdiction of
Beijing Municipality (PHR201008420) and the program on Devel-
opment and Their Bioactive Investigation of Functional Foods
(PXM2013_014207_000048).
Appendix A. Supplementary data
Fig. 7. (a) The current densityevoltage curves of devices. (b) The current densitye
efficiency curves of the devices. (c) The energy level diagram of the device
components.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.03.044.

2944
B. Du et al. / Polymer 54 (2013) 2935e2944
(b) Poriel C, Liang JJ, Rault-Berthelot J, Barriere F, Cocherel N, Slawin AMZ,
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