An X-shaped solution-processible oligomer having an anthracene unit as a core: A new organic light-emitting material with high thermostability and efficiency

Article abstract of DOI:10.1016/j.orgel.2009.09.030

A new X-shaped solution-processible oligomer with a glass transition temperature of higher than 200 °C based on an anthracene derivative was prepared, and it showed good hole-transporting ability in organic light-emitting diodes (OLED). Such oligomer was also employed as the emitting layer to give the devices showing blue emission.

Full text of DOI:10.1016/j.orgel.2009.09.030

Organic Electronics 11 (2010) 74–80
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
An X-shaped solution-processible oligomer having an anthracene unit as a
core: A new organic light-emitting material with high thermostability and
efficiency
Jing Sun a, Hongliang Zhong a, Erjian Xu a, Danli Zeng a, Jianhua Zhang b, Hongguang Xu c,
Wenqing Zhu b, Qiang Fang a,
*
a Key Laboratory of Organofluorine Chemistry and Laboratory for Polymer Materials Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, PR China
b Key Laboratory of Advanced Display and System Applications, Shanghai University, Ministry of Education, 149 Yanchang Road, Shanghai 200072, PR China
c R&D Center for Flat Panel Display Technology, SVA Company, Shanghai 200081, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new X-shaped solution-processible oligomer with a glass transition temperature of
higher than 200 °C based on an anthracene derivative was prepared, and it showed good
hole-transporting ability in organic light-emitting diodes (OLED). Such oligomer was also
employed as the emitting layer to give the devices showing blue emission.
Ó 2009 Elsevier B.V. All rights reserved.
Received 13 July 2009
Received in revised form 17 September
2009
Accepted 30 September 2009
Available online 20 October 2009
Keywords:
Organic light-emitting diodes
p-Conjugated oligomers
Anthracene derivatives
Optical properties
Electrochemical properties
1. Introduction
materials in non-linear optical device [4,5], light-emitting
diodes [7,11], organic field-effect transistors [12] and
In the recent years, there has been an increasing inter-
est in the synthesis and characterization of the -conju-
chemical sensors [2,3].
p
Compared with the linear conjugated molecules, the
conjugated cruciform compounds show spatially separated
energy levels of HOMO and LUMO, and each branch of the
molecules possess independent properties, which are suit-
able for the applications of chemical sensors [2]. It is more
gated molecules with topologically special structure such
as cruciform or X-shaped [1–12]. Those molecules show
highly electrochemical and photoluminescent activities,
and easily form amorphous glass in the solid state. More-
over, some of them display sensitivity to proton and metals
[2,3]. Such interesting properties of the molecules are de-
sired for the application in optoelectronic field as the
important the
p-aggregation is depressed in the conju-
gated cruciform molecules due to the special 3D structure
of the molecules [13], and such properties probably are
useful for fabrication of the optoelectronic devices with
excellent properties. Undoubtedly, investigation on the
relationship between the chemical structure of the cruci-
form compounds and properties of the molecules is very
necessary.
* Corresponding author. Tel./fax: +86 21 5492 5337.
E-mail address: qiangfang@mail.sioc.ac.cn (Q. Fang).
1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2009.09.030

J. Sun et al. / Organic Electronics 11 (2010) 74–80
75
It is noted that there are few building models for the cru-
2.3. LED device fabrication
ciform compounds. E.g., the reported cruciform molecules
are mainly formed by the models that each branch of the
molecules is extended from a benzene (or a thiophene) ring
[14,15]. Thus, exploring and developing new building mod-
On a cleaning ITO glass, a layer PEDOT-PSS (50 nm,
Bayer AG) was applied. A layer of 11 (10 mg/mL in CHCl3
for the application of hole-transporting layer, 20 mg/mL
in CHCl3 for the application of light-emitting layer) was
then spin-coated on the substrate. After dried 2 h in a glove
box at 60 °C, Alq3 (30 nm), LiF, Ca (120 nm) and Al
(100 nm) were deposited on the compound film by ther-
els of the
are desired.
To build a new structure of the cruciform compounds, a
p-conjugated molecules with cruciform structure
better choice is employing anthracene as a starting unit be-
cause an anthracene unit possesses four active positions,
e.g., 2,6- and 9,10-positions, in which the covalent bonds
mal evaporation, respectively, under
a vacuum of
10ꢀ4 Pa. The measurements of the properties of the devices
were operated at room temperature in air. The EL spectra
were recorded on a photometer PR 705.
are easily formed. When an anthracene unit contains
p-
conjugated substituted groups both at 2,6- and 9,10-posi-
tions, those molecules are topologically cross-shaped (or
X-shaped). On the other hand, anthracene-based deriva-
tives have been widely used as organic optoelectronic
materials owing to their high fluorescence quantum yields
[16], implying that studying the derivatives with new
structure is helpful to find new kind of the anthracene-
based materials. Hence, we designed and synthesized a
new anthracene-based oligomer 8, whose structure is
shown in Scheme 1. Such oligomer shows good hole-trans-
porting ability in organic light-emitting diodes. Moreover,
employing the oligomer as the emitting layer gives the
devices showing blue emission. Herein, we report the
details.
2.4. Synthesis of the compounds
2.4.1. Synthesis of (2-bromo-9,9-diethyl-9H-fluoren-7-
yl)trimethylsilane (2)
Under an argon atmosphere, a solution of n-BuLi in hex-
ane (1.6 M, 11.8 mL, 18.8 mmol) was added slowly at ꢀ78 °C
to a solution of 1 in anhydrous THF. The mixture was stirred
for 1 h at ꢀ78 °C before adding chlorotrimethylsilane (6.0 g,
55.3 mmol). The reactants were warmed to room tempera-
ture and stirred for 3 h, poured into water of 200 mL, and ex-
tracted with hexane. The organic layer was washed with
water and dried over Na2SO4. 2 was obtained as a colorless
oil by column chromatography on silica gel with hexane as
the eluent. 6.80 g, yield of 99%. 1H NMR (300 MHz, CDCl3, d
in ppm): 7.68 (d, 1H), 7.69 (d, 1H), 7.52 (d, 1H), 7.46–7.49
(m, 3H), 2.04 (m, 4H), 0.33–0.38 (m, 15H).
2. Experimental
2.1. Materials
2,7-Dibromo-9,9-diethyl-fluorene (1) [17a], 2,6-dib-
romoanthraquinone (4) [17b], pinacol(4-(N,N-diphenyl-
amino)l-phenyl)boronate (6) [17c] were prepared as
previously reported. [Bu4N]ClO4 (Bu = butyl) was an indus-
trial product and purified by recrystallized three times
from a mixture of ethyl acetate and cyclohexane and dried
over P2O5 under vacuum at room temperature for three
days. All solvents were dehydrated and distilled under an
inert atmosphere before use.
2.4.2. Synthesis of (7-(1,3,2-dioxaborinan-2-yl)-9,9-diethyl-
9H-fluoren-2-yl)trimethylsilane (3)
A
solution of n-BuLi in hexane (1.6 M, 12.5 mL,
20.0 mmol) was added slowly to a solution of 2 (6.79 g,
18.2 mmol) in anhydrous THF (80 mL) at ꢀ78 °C. The mix-
ture was stirred for 1 h at ꢀ78 °C before triisopropyl boro-
nate (10.3 g, 54.6 mmol) was added. The reactants were
warmed to room temperature, stirred for 24 h, poured into
a large mount of water (200 mL), and extracted with ethyl
acetate. The organic layer was washed with brine and dried
over Na2SO4. After removal of the solvent under reduced
pressure, the residue was obtained as the boric acid
(6.05 g, yield of 98.4%) as white solid. To a solution of the
boric acid (6.05 g, 17.9 mmol) in toluene (150 mL), 1,3-pro-
panediol (4.15 g, 54.6 mmol) was added, and the mixture
was stirred at 110 °C for 10 h. After being cooled to room
temperature, the solution was washed with water, dried
over Na2SO4, and evaporated to remove the solvent. The
obtained crude product was purified by column chroma-
tography using a mixture solvent of hexane and ethyl ace-
tate (8:1, v/v) as the eluent to afford a white solid 3 (6.30 g,
yield of 93.1%). 1H NMR (300 MHz, CDCl3, d in ppm): 7.69–
7.78 (m, 4H), 7.47 (m, 2H), 4.21 (t, 4H), 2.06 (m, 6H), 0.27–
0.32 (m, 15H). 13C NMR (100 MHz, CDCl3, d in ppm):
149.56, 149.13, 143.90, 142.11, 139.38, 132.46, 131.76,
127.91, 127.54, 119.22, 118.96, 62.02, 55.89, 32.60, 27.46,
8.59. FT-IR (KBr pellets, cmꢀ1): 2960, 2852, 1608, 1482,
2.2. Instrumentation
Elemental analysis was taken with a Carlo Erba 1106
elemental analyzer. 1H NMR spectra were carried out on
a Bruker DRX 300 spectrometer and 13C NMR spectra were
carried out on a Bruker DRX 400 spectrometer. UV–Vis
absorption and photoluminescence spectra were measured
with a Hitachi U-2910 spectrophotometer and a Hitachi F-
4500 fluorescence spectrophotometer, respectively. Cyclic
voltammetry (CV) for the cast films were measured in an
acetonitrile solution of [Bu4N]ClO4 (0.10 M, Bu = butyl) at
a
scan rate of 100 mV/s under argon using (0.10 M
AgNO3)/Ag and platinum wire as reference and counter
electrodes, respectively. A CHI 600B analyzer was used
for the measurements. DSC was determined with TA
Instrument DSC Q200 at a heating and cooling rate of
10 °C minꢀ1 under nitrogen flow.

76
J. Sun et al. / Organic Electronics 11 (2010) 74–80
O
i)
ii)
Br
Br
TMS
Br
TMS
B
O
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
1
2
3
I
O
O
O
NH2
Br
iv)
v)
iii)
C2H5
C2H5
3
H2N
C2H5
Br
C2H5
O
O
O
4
5
I
N
O
O
C2H5
O
vi)
C2H5
C2H5
N
B
O
C2H5
5
6
7
N
N
Br
N
C2H5
C2H5
N
C2H5
C2H5
vii)
C2H5
viii)
6
7
C2H5
C2H5
C2H5
1
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
N
N
Br
8
7a
N
Scheme 1. Synthetic procedure of the oligomer 8. Reagents and conditions: (i) (1) n-BuLi, THF, ꢀ78 °C, (2) TMSCl, ꢀ78 °C to room temperature; (ii) (1) n-
BuLi, THF, ꢀ78 °C, (2) (i-PrO)3B, ꢀ78 °C to room temperature; (3) 1,3-propanediol, toluene, 110 °C; (iii) t-BuONO, CuBr2, CH3CN, 65 °C; (iv) Pd(PPh3)4, aq.
K2CO3, toluene, 80 °C; (v) ICl, CHCl3, 0 °C to room temperature; (vi) Pd(PPh3)4, aq. K2CO3, toluene, 80 °C; (vii) 1) n-BuLi, THF, (2) KI, NaH2PO2ꢁH2O, AcOH,
120 °C; (viii) Pd(PPh3)4, K2CO3, THF, 80 °C.
1426, 1313, 1277, 1250, 1230, 1156, 1266, 857, 841, 820,
754, 684. MS (EI): 378 (M+).
washed with water, CH3CN, and benzene. Finally, the more
pure product was obtained by washing with benzene.
(3.75 g, yield of 84%).
2.4.3. Synthesis of 2,6-dibromoanthraquinone (4)
2,6-Diaminoanthraquinone (2.9 g, 12 mmol) was added
to a solution of t-BuONO (3.62 mL, 2.8 g, 27 mmol) and
CuBr2 (6.0 g, 27 mmol) in CH3CN (50 mL) at 65 °C. The mix-
ture was stirred for 3 h and then slowly poured into 20%
aqueous HCl solution (200 mL). The precipice was filtered,
2.4.4. Synthesis of 2,6-bis(9,9-diethyl-7-iodo-9H-fluoren-2-
yl) anthraquinone (5)
Under an argon atmosphere, a mixture of 4 (1.85 g,
5.07 mmol),
3 (4.31 g, 11.4 mmol), Pd(PPh3)4 (0.29 g,
0.25 mmol) and K2CO3 (3.22 g, 23.3 mmol) in THF

J. Sun et al. / Organic Electronics 11 (2010) 74–80
77
(80 mL) and distilled water (40 mL) was stirred for 48 h at
80 °C. After being cooled to room temperature, the mixture
was extracted with CHCl3. The organic layer was washed
with water and dried over with anhydrous Na2SO4. After
removal of the solvents, the residue was recrystallized
with a mixture solvent of toluene and ethanol to yield a
yellow solid, which was dissolved in anhydrous CHCl3
(3.02 g in 60 mL), and to the obtained solution was added
a mixture of ICl in methylene chloride solution (1.0 M,
8.4 mL, 8.4 mmol) under an argon atmosphere at 0 °C. After
warmed to room temperature, the reactants were stirred
for 4 h, then poured into aqueous NaHSO3 (5% w/w,
200 mL). The mixture was extracted with CHCl3 and the or-
ganic layer was washed with water before being dried over
with Na2SO4. After evaporated to remove the solvent, the
residue was purified by recrystallization with toluene to
give 5 as a crude product with a yield of 69%. 1H NMR
(300 MHz, CDCl3, d in ppm): 8.64 (s, 2H), 8.45 (d, 2H),
8.12 (d, 2H), 7.83 (d, 2H), 7.69–7.77 (m, 8H), 7.53 (d, 2H),
2.11 (m, 8H), 0.37 (t, 12H). 13C NMR (100 MHz, CDCl3, d
in ppm): 183.11, 152.75, 150.46, 147.25, 141.95, 140.36,
138.14, 134.12, 132.36, 132.28, 132.16, 128.18, 126.61,
125.51, 121.73, 121.62, 120.45, 93.26, 56.70, 32.73, 8.56.
FT-IR (KBr pellets, cmꢀ1): 2958, 1672, 1592, 1453, 1399,
1313, 969, 816, 746, 479. MS (MALDF-TOF): 900.3 (M+).
added to the mixture. The reactants were allowed to
slowly warm to room temperature, stirred for 24 h and
poured to water of 200 mL. The mixture was extracted
with ethyl acetate, and the obtained organic layer was
washed with brine and dried over Na2SO4. Column chro-
matography on silica gel with hexane:CHCl3:ethyl acetate
(30:10:4) as the eluent afford an intermediate (901 mg,
yield of 93.8%) as an orange glass. 1H NMR (300 MHz,
CDCl3, d in ppm):7.86 (s, 2H), 7.41–7.72 (m, 32H), 7.25–
7.30 (m, 8H), 7.16 (m, 12H), 7.04 (m, 4H), 3.36 (s, 2H),
1.96–2.11 (m, 16H), 0.27–0.41 (m, 24H). 13C NMR
(100 MHz, CDCl3,
d in ppm): 152.76, 151.03, 149.60,
147.92, 147.32, 145.96, 142.06, 141.77, 141.10, 140.44,
140.05, 139.88, 139.81, 139.19, 135.75, 130.33, 129.51,
129.33, 128.03, 127.30, 126.50, 126.21, 125.83, 124.57,
124.31, 123.13, 121.75, 121.42, 121.35, 121.28, 121.21,
120.35, 120.08, 119.55, 76.94, 56.85, 56.44, 32.97, 8.88,
8.65. FT-IR (KBr pellets, cmꢀ1): 3543, 3033, 2963, 2920,
2875, 2852, 1592, 1514, 1493, 1464, 1377, 1313, 1279,
1006, 908, 815, 753, 734, 697. MS (MALDF-TOF): 1738.3
(M+).
Under an argon atmosphere, the above mentioned
intermediate (901 mg, 0.52 mmol), was added to a mixture
of potassium iodide (861 mg, 5.2 mmol) and sodium hypo-
phosphite hydrate (1.10 g, 10.4 mmol) in acetic acid
(50 mL), and the mixture was stirred for 19 h at 120 °C.
After being cooling to room temperature, the mixture
was filtered and washed with water. The residue was puri-
fied by column chromatography on silica gel using a mix-
ture of petroleum ether and CHCl3 (2:1, v/v) as the eluent
to give 7a as a green–yellow powder, 658 mg, yield of
74.5%. 1H NMR (300 MHz, CDCl3, d in ppm): 8.06 (s, 2H),
7.98 (d, 2H), 7.90 (d, 2H), 7.64–7.79 (m, 8H), 7.50–7.59
(m, 20H), 7.25–7.31 (m, 8H), 7.13–7.17 (m, 12H), 7.04
(m, 4H), 1.97–2.14 (m, 16H), 0.53 (m, 12H), 0.28–0.38
(m, 12H). 13C NMR (100 MHz, CDCl3, d in ppm): 152.42,
150.87, 150.81, 149.78, 147.67, 147.05, 139.83, 139.54,
138.53, 137.57, 135.51, 130.40, 130.22, 129.60, 129.24,
127.74, 127.57, 126.33, 125.57, 125.45, 124.31, 124.05,
122.86, 121.48, 121.37, 121.17, 120.96, 119.96, 119.89,
56.72, 56.14, 32.82, 32.69, 8.70, 8.58. FT-IR (KBr pellets,
cmꢀ1): 2962, 2918, 2974, 2851, 1592, 1541, 1493, 1455,
1377, 1313, 1278, 883, 812,752, 754, 696. MS (MALDF-
TOF): 1702.8 ((M-1)+). Elemental analysis: Calcd for
C118H98Br2N2: C, 83.18; H, 5.80; N, 1.64; Br, 9.38. Found:
C, 82.93; H, 6.10; N, 1.61; Br, 9.32.
The obtained green–yellowpowder (200 mg, 0.12 mmol),
mentioned above was added to a mixture of 6 (96 mg,
0.26 mmol), Pd(PPh3)4 (14 mg, 0.012 mmol) and K2CO3
(250 mg, 1.81 mmol) in THF (40 mL) containing distilled
water (15 mL). The reactants were stirred for 48 h at 80 °C.
After being cooled to room temperature, the mixture was ex-
tractedwith CHCl3. The organiclayer was washed with water
and dried over with anhydrous Na2SO4. After removal of the
solvent, the residue was purified by column chromatography
on silica gel using a mixture solvent of petroleum ether and
CH2Cl2 (3:2, v/v) as the eluent to give 8 as a yellow–green
powder, 190 mg, yield of 79.5%. 1H NMR (300 MHz, CDCl3,
d in ppm): 8.12 (s, 2H), 8.02 (d, 2H), 7.94 (m, 4H), 7.78 (d,
2H), 7.61–7.71(m, 14H), 7.49–7.58 (m, 14H), 7.23–7.33 (m,
18H), 7.12–7.22 (m, 22H), 7.05 (m, 8H), 2.17 (m, 8H), 1.99
2.4.5. Synthesis of 2,6-bis(7-(4-(diphenylamino)phenyl)-9,9-
diethyl-9H-fluoren-2-yl)-anthraquinone (7)
Under an argon atmosphere, a mixture of 5 (1.88 g,
2.08 mmol),
6 (1.70 g, 4.58 mmol), Pd(PPh3)4 (0.12 g,
0.10 mmol) and K2CO3 (1.32 g, 9.57 mmol) in THF
(50 mL) and distilled water (25 mL) was stirred for 48 h
at 80 °C. After being cooled to room temperature, the mix-
ture was extracted with CHCl3. The organic layer was
washed with water and dried over anhydrous Na2SO4.
After removal of the solvents, the residue was purified by
column chromatography on silica gel using a mixture sol-
vent of petroleum ether and CH2Cl2 (1:1, v/v) as the eluent
to obtain 7 as an orange powder (1.98 g, yield of 83.8%). 1H
NMR (300 MHz, CDCl3, d in ppm): 8.67 (d, 2H), 7.46 (d, 2H),
8.14 (d, 2H), 7.23–7.88 (m, 8H), 7.27–7.32 (m, 8H), 7.17 (m,
12H), 7.05 (m, 4H), 2.16 (m, 8H), 0.42 (t, 12H). 13C NMR
(100 MHz, CDCl3,
d in ppm): 138.41, 151.50, 151.31,
147.91, 147.68, 147.45, 142.55, 140.41, 139.79, 137.82,
135.62, 134.37, 132.51, 132.29, 129.53, 128.39, 128.06,
126.74, 125.89, 125.67, 124.62, 124.27, 123.19, 121.68,
121.31, 120.60, 120.50, 56.74, 33.14, 8.91. FT-IR (KBr pel-
lets, cmꢀ1): 3032, 2962, 2874, 1673, 1591, 1514, 1492,
1464, 1377, 1310, 1281, 817, 747, 695, 497. MS (MALDF-
TOF): 1136.0 (M+). Elemental analysis: Calcd for
C84H68N2O2: C, 88.86; H, 5.86; N, 2.47. Found: C, 88.77;
H, 6.00; N, 2.13.
2.4.6. Synthesis of 2,6,9,10-tetra(7-(4-
(diphenylamino)phenyl)-9,9-diethyl-9H-fluoren-2-yl)-
anthracene (8)
A solution of n-BuLi (1.6 M, 1.38 mL, 2.21 mmol) in hex-
ane was added slowly to
a solution of 1 (839 mg,
2.21 mmol) in anhydrous THF (30 mL) at ꢀ78 °C, and the
mixture was stirred for 1 h at the same temperature. Then
a solution of 7 (627 mg, 0.55 mmol) in anhydrous THF was

78
J. Sun et al. / Organic Electronics 11 (2010) 74–80
(m, 8H), 0.54–0.60 (m, 12H), 0.28–0.38 (m, 12H). 13C NMR
(100 MHz, CDCl3, in ppm): 150.91, 150.82, 150.33,
365 nm
491 nm
8 in toluene-UV-vis
8 in film-UV-vis
8 in toluene-PL
8 in film-PL
474 nm
d
1
0.5
0
147.72, 147.68, 147.13, 147.03, 140.87, 140.62, 140.36,
139.92, 139.75, 139.65, 139.53, 137.81, 137.68, 135.62,
135.84, 130.51, 130.25, 129.71, 129.29, 127.84, 127.75,
126.18, 126.05, 125.74, 125.56, 125.36, 124.53, 124.34,
124.17, 124.08, 122.90, 122.86, 121.51, 121.05, 120.97,
120.10, 119.52, 119.91, 119.77, 56.48, 56.16, 33.12, 33.01,
8.84, 8.58. FT-IR (KBr pellets, cmꢀ1): 3036, 2961, 2917,
2873, 2851, 1592, 1513, 1492, 1467, 1376, 1313, 1279,
1177, 883, 813, 752, 695. MS (MALDF-TOF): 2033.0 (M+). Ele-
mental analysis: Calcd for C154H126N4: C, 91.00; H, 6.25; N,
2.76. Found: C, 90.52; H, 6.56; N, 2.30.
300
400
500
600
700
Wavelength (nm)
3. Results and discussion
Fig. 2. UV–Vis and PL spectra of 8 in toluene and in the solid state.
3.1. Synthesis
spectra are displayed in Fig. 2. The oligomer exhibits a max-
imum absorption peak at 365 nm in both toluene and in the
solid state, whereas the onset position of its absorption
band in the solid state has a red-shift, suggesting the exis-
Oligomer 8 was built from 2,6-dibromoanthraquinone,
fluorene boric ester and triphenylamine boric ester, and
the synthesis procedure is shown in Scheme 1, and the de-
tailed preparation and characterization of the oligomer are
listed in experimental section. 8 was an amorphous pow-
der and can easily form a film on a glass plate by using a
spin-coating method, suggesting that the oligomer pos-
sesses good solution-processability, which is desire for
the fabrication of the optoelectronic devices.
tence of part
p-stacking structure in the oligomer. It is
noted that a compound composed of a fluorene unit and
two triphenylamine units showed an absorption peak at
about 370 nm [20], thus, the maximum absorption peak
at 365 nm in Fig. 2 is attributed to the electronic transition
(p–p*) along the backbone of a fluorene unit linked with a
triphenylamine group. The weak absorption peaks at 400–
450 nm are also observed in Fig. 2, which are probably de-
rived from the anthracene unit in 8, the similar absorption
peaks were found in our previous work concerning a poly-
mer containing anthracene unit as side chain [21].
3.2. Thermal stability and optical properties
Thermal properties of 8 were characterized by differen-
tial scanning calorimetry (DSC), and the result is shown in
Fig. 1. As can be seen from Fig. 1, 8 shows a glass transition
temperature, Tg, of 223 °C, which is higher than those of
most of the well-known small molecule materials used in
optoelectronic fields [18], and even higher than those of
some polymeric materials [19], indicating that the oligo-
mer possesses high thin film morphological stability.
Moreover, no obvious crystallization peak was observed
during a scanning from 50 to 300 °C, demonstrating that
8 has a limited tendency to crystallize.
The photoluminescent (PL) spectrum of 8 in toluene
shows a main peak at 474 nm with a quantum yield of
62% (estimated with a 0.5 M H2SO4 solution of quinine as
a reference), similar to the PL spectra of poly(9,9-alkylfluo-
rene-alt-triphenylamine) [22]. In the solid state, 8 gives a
maximum emission peak at 491 nm with an absolute PL
quantum yield of 24%, measured by using a calibrated inte-
grating sphere. Here, the main emission peak of 8 has a
red-shift of 17 nm relative to that in the solution, indicat-
ing the existence of the interaction between the molecules
in the solid state, similar phenomena were also observed in
poly(9,9-alkylfluorene-alt-triphenylamine) [23] and the
fluorene–anthracene compounds [24].
The basic photophysical properties of 8 were character-
ized by UV–Vis and photoluminescent (PL) spectra. The
3.3. Electrochemical properties
The electrochemical behavior of 8 was measured by
cyclic voltammetry (CV) of its cast film on a Pt plate, and
the result is shown in Fig. 3. The electrochemical oxidation
(or p-doping) of 8 starts at about 0.60 V versus Ag+/Ag, and
gives a p-doping peak at 1.02 V. The corresponding reduc-
tion peak appears at 0.18 V. According to the relationship
Tg = 223 o
C
between the oxidation onset potential (Eooxnset) and HOMO
onset
level value as ꢀ(E
þ 4:4) [24], the HOMO energy level
ox
of 8 is estimated as ꢀ(0.60 + 4.4) = ꢀ5.00 eV. Relative to
the triphenylamine derivatives with HOMO value of about
5.7 eV, that of 8 is close to the work function of ITO glass
(4.5–4.8 eV) [25]. This result implies that 8 is very suitable
for OLEDs as a hole-transport layer (HTL). Consequently,
150
50
100
200
250
300
Temperature (oC)
Fig. 1. DSC trace of 8.

J. Sun et al. / Organic Electronics 11 (2010) 74–80
79
1.2
0.8
A
1500
1
8
1.02 V
8/ETL
0.5
0
0.4
0
400
500
600
700
1000
500
onset = 0.60 V
8/ETL
8
-0.4
-0.8
0.18 V
0
5
10
15
0
0.5
1
Voltage (V)
E (V) vs Ag/Ag+
B
Fig. 3. CV curve of a cast film of 8 on a Pt electrode (wire) with a sweep
rate of 100 mV sꢀ1 in an acetonitrile solution of 0.10 M [Bu4N]ClO4. The
applied potential is 0.0–1.5 V.
8
1500
1000
500
0
8/ETL
the LUMO value of 8 is calculated by subtraction of the
optical bandgap Eopt (eV) from the HOMO value, given as
ꢀ2.41 eV.
3.4. Electroluminescence properties
To evaluate the performance of 8 as a HTL in OLEDs,
three non-optimized OLED devices were fabricated with
0
500
Current density (mA/cm2)
1000
A
Fig. 5. B–V (A) and B–I curves (B) of a device based on 8.
5000
PEDOT/Alq
4000
8/Alq
the following configuration: (1) ITO/PEDOT:PSS/Alq/LiF/
Ca/Al (device PEDOT/Alq); (2) ITO/8/Alq/LiF/Ca/Al (device
8/Alq) and (3) ITO/PEDOT:PSS/8/Alq/LiF/Ca/Al (device PED-
OT/8/Alq). Here, PEDOT-PSS was employed as a hole-injec-
tion material that had been widely used in OLEDs.
Moreover, a commercially available electron transport
material, Alq, was used as both electron transport layer
and emitting layer. All devices showed green emission de-
rived from Alq, illuminating that the holes from the anode
(ITO) and the electrons from the cathode (Ca/Al) combine
in the Alq layer. The B–V (brightness versus operation volt-
age) and B–I (brightness versus current density) character-
istics of the devices shows in Fig. 4. As can be seen form
Fig. 4, device 8/Alq shows a maximum brightness of about
3600 cd/m2 at 10 V, which is much higher than that of the
device PEDOT/Alq only with PEDOT as a hole-injection
layer, suggesting that oligomer 8 shows good hole-trans-
port ability. Interestingly, when 8 was used together with
PEDOT-PSS, the device (PEDOT/8/Alq) shows higher bright-
ness and luminous efficiency. Its maximum brightness was
4549 cd/m2 at 11 V and the maximum current efficiency
reached 2.32 cd/A. Moreover, the maximum external quan-
tum efficiencies (Max EQE) of the devices showed an order
of PEDOT/Alq (0.11%) < 8/Alq (0.26%) < PEDOT/8/Alq
(0.57%). Those results indicate that a combination of 8
and a commercial PEDOT-PSS may obtain a better perfor-
mance in OLEDs, and that oligomer 8 could be used as a
new component in OLEDs.
PEDOT/8/Alq
3000
2000
1000
0
4
6
8
10
12
Voltage (V)
B
5000
4000
3000
2000
1000
0
PEDOT/Alq
8/Alq
PEDOT/8/Alq
0
200
400
600
800
1000
Current density (mA/cm2)
Fig. 4. B–V (A) and B–I curves (B) of the three devices.

80
J. Sun et al. / Organic Electronics 11 (2010) 74–80
(c) W.W. Gerhardt, A.J. Zucchero, J.N. Wilson, C.R. South, U.H.F. Bunz,
The high absolute PL quantum yield of 8 inspired us to
M. Weck, Chem. Commun. (2006) 2141.
investigate its application in OLED as a light-emitting layer.
A device was fabricated with the configuration of ITO/PED-
OT:PSS/8/LiF/Ca/Al (device 8). The B–V curve and efficiency
of the device are shown in Fig. 5. As shown in Fig. 5, device
8 shows light-blue emission with a maximum emission
peak of 478 nm, which has a slight blue-shift relative to
that of its PL emission in the solid state. Device 8 exhibits
a low turn-on voltage of 3.5 V, whereas its efficiency is very
low (maximum current efficiency = 0.1 cd/A, and Max
EQE = 0.043%). This may be due to the unbalance between
the hole-transport and the electron transport in the device,
derived from the strong hole-transporting ability of 8. Con-
sidering the good electron transport of a bitriazine deriva-
tive developed by our group [26], we designed a new
configuration of the device using the bitriazine derivative
as electron transport layer (ETL) to adjust the balance be-
tween the electron transport and the hole-transport in
the device. The new device has a configuration of ITO/PED-
OT:PSS/8/ETL/LiF/Ca/Al (device 8/ETL), and shows the same
emission color as that of device 8 with a maximum emis-
sion peak of 474 nm (Fig. 5, the insert), but its efficiency
is much higher than that of device 8 (for device 8/ETL,
maximum current efficiency = 0.6 cd/A and Max
EQE = 0.29%), implying that 8 also shows good emitting
properties.
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