Efficient 9-alkylphenyl-9-pyrenylfluorene substituted pyrene derivatives with improved hole injection for blue light-emitting diodes

Article abstract of DOI:10.1039/b607923g

Two efficient blue-light-emitting materials with stable diarylfluorene and hole-transporting pyrene groups, 9-alkoxyphenyl-9-pyrenylfluorene substituted pyrenes (EHOP1 and EHOP2), have been synthesized by Suzuki coupling reaction and have been fully characterized. They have extreme thermal stability in nitrogen with onset decomposition temperature over 430 °C. They are amorphous materials with T_g of 103 and 191 °C for EHOP1 and EHOP2, respectively. They show stable bright blue emission in the solid state. Due to the non-conjugated pyrene groups at C9 of the fluorene moieties, the absorption spectra of the two compounds showed some characteristic peaks and they showed improved hole-injection ability than conjugated fluorene derivatives. A preliminary simple three-layer blue light-emitting diode, i.e. ITO/TCTA (8 nm)/EHOP1 (30 nm)/BCP (45 nm)/Mg:Ag, was obtained without the need for a hole-injection layer, with high luminance of 11620 cd m^-2, turn-on voltage of 4.0 V and current efficiency of 3.04 cd A^-1. Furthermore, the device still had a considerable efficiency of 1.74 cd A^-1, when the luminescence reaches its maximum. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b607923g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Efficient 9-alkylphenyl-9-pyrenylfluorene substituted pyrene derivatives
with improved hole injection for blue light-emitting diodes
Chao Tang,ab Feng Liu,a Yi-Jie Xia,a Ling-Hai Xie,ab Ang Wei,ab Sheng-Biao Li,a Qu-Li Fanab and
Wei Huang*abc
Received 5th June 2006, Accepted 3rd August 2006
First published as an Advance Article on the web 31st August 2006
DOI: 10.1039/b607923g
Two efficient blue-light-emitting materials with stable diarylfluorene and hole-transporting
pyrene groups, 9-alkoxyphenyl-9-pyrenylfluorene substituted pyrenes (EHOP1 and EHOP2),
have been synthesized by Suzuki coupling reaction and have been fully characterized. They have
extreme thermal stability in nitrogen with onset decomposition temperature over 430 uC. They are
amorphous materials with Tg of 103 and 191 uC for EHOP1 and EHOP2, respectively. They show
stable bright blue emission in the solid state. Due to the non-conjugated pyrene groups at C9 of
the fluorene moieties, the absorption spectra of the two compounds showed some characteristic
peaks and they showed improved hole-injection ability than conjugated fluorene derivatives.
A preliminary simple three-layer blue light-emitting diode, i.e. ITO/TCTA (8 nm)/EHOP1
(30 nm)/BCP (45 nm)/Mg:Ag, was obtained without the need for a hole-injection layer, with high
luminance of 11620 cd m22, turn-on voltage of 4.0 V and current efficiency of 3.04 cd A21
.
Furthermore, the device still had a considerable efficiency of 1.74 cd A21, when the luminescence
reaches its maximum.
derivatives have excellent thermal stability. In this paper, we
1. Introduction
report the synthesis and characterization of the two novel
Since the fabrication of the first efficient molecule-based
organic light-emitting diodes (OLEDs), reported by Tang and
Van Slyke,1 intense research and development have brought
OLEDs to the brink of penetrating the market of flat panel
displays,2 which is currently dominated by liquid crystal
materials. Whether to polymer or to small molecule materials,
only the red and green ones have shown sufficient efficiencies
and lifetimes to be of commercial value.3 The performance
of blue electroluminescent emitters is usually inferior to that of
green or red emitters, due to large bandgap energy. Because
of the high photoluminescence (PL) efficiency, much research
into blue-emitting materials has centred on conjugated
fluorene derivatives, and aryl groups were introduced at C9
position of fluorene to improve the stability of the mateials.4
As a large conjugated aromatic ring, pyrene not only has the
advantage of high PL efficiency,5 high carrier mobility, but
also has the much improved hole-injection ability than
oligofluorenes or polyfluorene.6 Very recently, some pyrene
derivatives have been used in OLEDs in order to improve hole-
transporting ability because of its electron-rich property.7
However, the pyrene itself is not suitable to act as blue
emitter in the OLEDs because it is easy to crystallize. On the
other hand, as mentioned above, conjugated diarylfluorene
9-(49-(20-ethylhexyloxyphenyl))-9-pyrenylfluorene substituted
aInstitute of Advanced Materials (IAM), Fudan University, Shanghai
200433, People’s Republic of China. E-mail: wei-huang@njupt.edu.cn/
chehw@nus.edu.sg.; Fax: +86 25 8349 2349; Tel: +86 25 8349 2204
bInstitute of Advanced Materials (IAM), Nanjing University of Posts
and Telecommunications, 66 XinMofan Road, Nanjing 210003, People’s
Republic of China
Scheme 1 Reagents and conditions: (i), 2-ethylhexyl bromide,
K2CO3, DMF, 70 uC, 24 h; (ii), (1) Mg, I2, THF, (2) 2-bromofluor-
enone or 2,7-dibromofluorenone, THF, (3) saturated NH4Cl, H2O,
reflux; (iii), pyrene ( 5 equiv), CHCl3,CH3SO3H (1 equiv), 60 uC
20 min21; (iv), 4a/5 (1 : 1), Pd(PPh3)4, toluene, K2CO3 (2.0 M
aqueous), 90 uC, 48 h; v. 4b/5 (1 : 2), Pd(PPh3)4, toluene, K2CO3 (2.0 M
aqueous), 90 uC, 48 h.
cFaculty of Engineering, National University of Singapore, 9
Engineering Drive 1, Singapore 117576, Republic of Singapore
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pyrenes (EHOP1 and EHOP2) (Scheme 1). Our motivation of
designing the two materials is to combine the high thermal
stability of diarylfluorene with the high efficiency and hole-
injection ability of pyrene. We also expect that the long chain
alkyloxy group can give a better solubility and a low tendency
to crystallize in the devices. Due to the non-conjugated pyrene
groups at C9 of the fluorene moieties, the absorption spectra
of the two compounds showed some characteristic peaks and
they showed improved hole-injection ability than conjugated
fluorene derivatives. Preliminary results based on a simple
three-layer blue OLED, i.e. ITO/TCTA (8 nm)/EHOP1
(30 nm)/BCP (45 nm)/Mg:Ag, was obtained with low turn-on
2.2. Materials and synthesis
Commercial grade reagents were used without further
purification. All the solvents for characterization were used
after redistillation. THF was refluxed with sodium filament in
the presence of benzophenone until a persistent blue color
appears and then distilled.
1-Bromo-4-(29-ethylhexyloxy)benzene (2). To a solution of
K2CO3 (40.0 g, 289 mmol) and 4-bromophenol (15.0 g,
86.7 mmol) in DMF (300 mL) was added dropwise 2-ethyl-
hexyl bromide (20.1 g, 104 mmol). The mixture was heated to
70 uC for 24 h in the dark under the nitrogen. After the solids
were filtered off, the mother liquor was treated with 1 M HCl
and petroleum ether. The water phase was extracted three
more times with petroleum ether, and the combined extract
was washed three times with 1 M HCl, then with water for
three times. The mixture solution was dried with anhydrous
MgSO4. The solvents was removed by rotary evaporation, and
the crude product was purified by column chromatography
using petroleum ether as eluent to provide a colorless oil
(18.5 g, 75%). 1H NMR (400 MHz, CDCl3) d (ppm): 7.33–7.38
(m, 2H); 6.74–6.80 (m, 2H); 3.80 (dd, J = 5.6 Hz, 0.8 Hz, 2H);
1.68–1.75 (m, 1H); 1.24–1.55 (m, 8H); 0.92 (s, 6H).
voltage of 4 V, high maximum luminance of 11620 cd m22
,
and high current efficiency of 3.04 cd A21 in ambient air, and
the high performance of the OLED is originated from the
elaborate combination of 9-alkylphenyl-9-pyrenylfluorene and
pyrene groups.
2. Experimental
2.1 General experimental information
All reactions were monitored by TLC using pre-coated glass
sheets purchased by Yantai Huiyou Silica Gel Developing Co.,
Ltd (0.20 mm with fluorescent indicator UV254). Compounds
were visualized with UV light at 254 nm. Column chromato-
graphy was carried out using flash silica gel from Qingdao
Haiyang Chemical Co., Ltd (200–300 mesh). 1HNMR was
recorded using a Varian spectrometer at 400 MHz. Molecular
masses were determined by a SHIMADZU matrix-assisted
laser desorption–ionization time-of-flight mass spectrometer
(MALDI-TOF-MASS). Elemental analyses were performed
on a Vario EL III elemental analyzer. Thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC)
were performed using a purged nitrogen atmosphere at a
heating rate of 10 uC min21. Absorption and photolumi-
nescence (PL) emission spectra of the materials were measured
in dichloromethane using a SHIMADZU UV-3150 spectro-
photometer and a SHIMADZU RF-5301PC spectrophoto-
meter, respectively. Quantum yield was determined using
solutions in dichloromethane and was calculated by comparing
emission with that of a standard solution of 9,10-diphenyl-
anthracene in cyclohexane (Q = 0.9) at room temperature.
Cyclic voltammetry (CV) was performed on an Eco Chemie’s
Autolab. The quantum chemical calculations were performed
using AM1 in the Gaussian 03, B.04 program.
2-Bromo-9-(49-(20-ethylhexyloxyphenyl))-fluoren-9-ol (3a).
The Grignard reagents were prepared from magnesium turn-
ings (0.58 g, 24 mmol), a little iodine in THF (3 mL), and
1-bromo-4-(29-ethylhexyloxy)benzene (2) (8.56 g, 30 mmol) in
THF (30 mL), then the 2-bromofluorenone (4.0 g, 15.4 mmol)
in THF was dropped into the Grignard solution after the
Grignard solution was diluted with another THF (20 mL) and
cooled. The mixture was refluxed for 5 h and hydrolyzed
with saturated NH4Cl solution for 2 h after cooling at room
temperature, then was extracted with dichloromethane,
washed with water, dried with anhydrous MgSO4. The solvent
was removed by rotary evaporation, and the residue was
purified by column chromatography using ethyl acetate/
petroleum ether (1 : 10) as eluent to obtain the pale-green
solid (6.81 g, 95%). 1H NMR (400 MHz, CDCl3) d (ppm): 7.62
(d, J = 7.2 Hz, 1H); 7.45–7.54 (m, 3H); 7.24–7.42 (m, 5H);
6.78–6.82 (m, 2H); 3.80 (dd, J = 5.6 Hz, 0.4 Hz, 2H); 2.45 (s,
1H); 1.62–1.68 (m, 1H); 1.02–1.56 (m, 8H); 0.91 (s, 6H).
2,7-Dibromo-9-(49-(20-ethylhexyloxyphenyl))-fluoren-9-ol
(3b). 3b was synthesized according to the procedure described
for 3a using 2,7-dibromofluorenone (4.0 g, 11.7 mmol) to get a
Organic EL device were fabricated by using successive
vacuum-deposition of the materials on top of the ITO glass
substrate under a pressure of about 1 6 1024 Pa with the
deposition rate of about 0.1 nm s21. ITO glass was etched for
the anode electrode pattern and cleaned in ultrasonic baths of
chloroform, acetone and alcohol. The overlap area of Mg:Ag
alloy cathode and ITO electrodes is 6 mm2. A UV zone cleaner
was used for further cleaning before vacuum deposition of the
organic materials. The evaporation rate and the thickness of
the film were measured with a quartz oscillator. Current–
voltage–luminescence (I-V-L) characteristics and CIE color
coordinates were measured with a Keithley SMU238 and a
ST-86LA luminance-meter. EL spectra were measured with a
Photo Research’s PR 705.
1
pale-green solid (6.02 g, 11 mmol) in a high yield of 94%. H
NMR (400 MHz, CDCl3) d (ppm): 7.48 (s, 4H); 7.44 (d, J =
0.8 Hz, 2H); 7.24 (d, J = 8.4 Hz, 2H); 6.81 (d, J = 8.8 Hz, 2H);
3.80 (d, J = 5.6 Hz, 2H); 1.64–1.74 (m, 1H); 1.24–1.56 (m, 8H);
0.91 (s, 6H).
2-Bromo-9-(49-(20-ethylhexyloxyphenyl))-9-pyrenylfluorene
(4a). To a solution of pyrene (4.3 g, 21.5 mmol) and CH3SO3H
(0.41 g, 4.3 mmol) in CHCl3 (100 mL) was added dropwise 3a
(2.0 g, 4.3 mmol) in CHCl3. The mixture was heated to 60 uC,
and stirred for another 20 min. Then saturated sodium
bicarbonate solution was added into the mixture to quench
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the reaction. The organic phase was separated and washed
by saturated sodium bicarbonate, and the water phase was
extracted three times with dichloromethane. The combined
organic solution was dried with anhydrous MgSO4. The
solvent was removed by rotary evaporation, and the residue
was purified by column chromatography using petroleum
ether/dichloromethane (5 : 1) as eluent to get a white solid
(2.1 g, 75%). 1H NMR (400 MHz, CDCl3) d (ppm): 8.16 (d, J =
7.6 Hz, 1H); 8.10 (d, J = 7.6 Hz, 1H); 7.92–8.06 (m, 5H); 7.81
(d, J = 7.6 Hz, 2H); 7.70 (d, J = 8.0 Hz, 3H); 7.54–7.64 (broad,
1H); 7.38 (td, J = 7.6 Hz, 0.8 Hz, 1H); 7.23 (td, J = 7.6 Hz,
0.8 Hz, 1H); 3.80 (d, 5.6 Hz, 2H); 1.64–1.74 (m, 1H); 1.25–1.53
(m, 8H); 0.91 (s, 6H). LDI–TOF-MS (m/z): Anal. calcd. for
C43H3779BrO 648.2; found 648.2. Anal. calcd. for C43H3781BrO
650.2; found 650.2. Anal. calcd. C, 79.50; H, 5.74; found C,
79.45; H, 5.80.
9-(49-(20-Ethylhexyloxyphenyl))-2,7,9-tripyrenylfluorene
(EHOP2). EHOP2 was synthesized according to the procedure
described for EHOP1 using 5 (0.5 g, 1.5 mmol), 2,7-dibromo-
9-(49-(20-ethylhexyloxyphenyl))-9-pyrenylfluorene (4b) (0.51 g,
0.7 mmol), Pd(PPh3)4 (0.02 mmol), 2.0 M K2CO3 (1 mL) and
toluene (30 mL). The crude product was purified by column
chromatography using petroleum ether/dichloromethane (3 : 1)
as eluent to provide a pale green powder (0.47 g, 69%). 1H
NMR (400 MHz, CDCl3) d (ppm): 8.09–8.31 (m, 12H); 7.90–
8.08 (m, 15H); 7.74–7.89 (m, 4H); 7.58–7.73 (broad, 2H); 7.55
(d, J = 7.2 Hz, 2H); 6.96 (d, J = 8.0 Hz, 2H); 3.88 (d, J =
5.6 Hz, 2H); 1.75–1.86 (m, 1H); 1.32–1.60 (m, 8H); 1.04 (s,
6H). 13C NMR (400 MHz, CDCl3) d (ppm): 158.69, 153.53,
141.10, 139.69, 139.09, 138.85, 137.93, 131.76, 131.44, 121.20,
130.88, 130.80, 130.66, 130.18, 129.34, 128.75, 128.49, 127.92,
127.81, 127.71, 127.68, 126.91, 126.78, 126.22, 125.43, 125.37,
125.19, 125.05, 124.98, 121.04, 115.23, 70.81, 66.95, 39.79,
30.92, 29.50, 24.25, 23.43, 23.11, 14.50, 11.54. LDI–TOF-MS
(m/z): Anal. calcd. for C75H54O 970.4, found 970.7. Anal.
calcd. C, 92.75; H, 5.60; found C, 92.70; H, 5.63.
2,7-Dibromo-9-(49-(20-ethylhexyloxyphenyl))-9-pyrenylfluor-
ene (4b). 4b was synthesized according to the procedure
described for 4a using 2,7-dibromo-9-(49-(20-ethylhexyloxy-
phenyl))-fluorene-9-ol (3b) (4.0 g, 7.3 mmol), pyrene (7.38 g,
36.5 mmol) and CH3SO3H (0.70 g, 7.3 mmol) to get a white
solid (3.91 g, 5.4 mmol) in a yield of 73%. 1H NMR (400 MHz,
CDCl3) d (ppm): 8.17 (d, J = 7.6 Hz, 1H); 8.12 (d, J = 7.6 Hz,
1H); 7.94–8.08 (m, 5H); 7.78–7.92 (broad, 2H); 7.67 (d,
J = 8.0 Hz, 4H); 7.51 (dd, J = 8.0 Hz, 2.0 Hz, 2H); 3.80
(d, J = 6.0 Hz, 2H); 1.62–1.78 (m, 1H); 1.24–1.52 (m,
8H); 0.91 (s, 6H). LDI–TOF-MS (m/z): Anal. calcd.
for C43H3679Br79BrO 728.1; found 728.1. Anal. calcd. for
C43H3679Br81BrO 729.1; found 729.1. Anal. calcd. for
C43H3681Br81BrO 730.1; found 730.1. Anal. calcd. C, 70.89;
H, 4.98; found C, 70.86; H, 5.02.
3. Results and discussion
The two 9-(49-(20-ethylhexyloxyphenyl))fluorene substituted
pyrenes, EHOP1 and EHOP2, were synthesized according
to the procedure sketched in Scheme 1. Compound 2 was
synthesized by Williamson reaction (75% yield).8 Grignard
reagent was prepared from Mg and compound 2 in THF.
Then it was reacted respectively with 2-bromofluorenone
and 2,7-dibromofluorenone to get the fluorene-9-ol 3a and
3b (.94% yield), which were followed by Friedel–Crafts
reaction with an excess amount of pyrene in chloroform
promoted by methanesulfonic acid to get the compounds 4a
and 4b, respectively (.70% yield). The Suzuki coupling
reaction was employed between the pyrene boric acid and
monobromide 4a or dibromide 4b to achieve the target
compounds EHOP1 and EHOP2 with 80 and 69% yield,
respectively.9 Both the two compounds in this report were
purified by flash column chromatography. They were fully
characterized by 1H and 13C NMR, MALDI-TOF-MASS and
elemental analysis. The results were consistent with the
proposed structures.
9-(49-(20-Ethylhexyloxyphenyl))-2,9-dipyrenylfluorene
(EHOP1).
1-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)
pyrene (5) (0.5 g, 1.5 mmol), 2-bromo-9-(49-(20-ethylhexyloxy-
phenyl))-9-pyrenylfluorene (4a) (0.97 g, 1.5 mmol), and
Pd(PPh3)4 (0.05 mmol) and aqueous 2.0 M K2CO3 (2 mL)
were mixed in a flask containing with nitrogen saturated
toluene (50 mL). The reaction mixture was stirred at 90 uC for
48 h. After it was cooled to room temperature, the reaction
mixture was quenched with saturated sodium bicarbonate
solution and extracted twice with dichloromethane. The com-
bined organic extracts were dried with anhydrous MgSO4. The
crude product was purified by column chromatography using
petroleum ether/dichloromethane (4 : 1) as eluent to provide a
pale green powder (0.92 g, 80%). 1H NMR (400 MHz, CDCl3)
d (ppm): 8.08–8.22 (m, 4H); 7.83–8.07 (m, 15H); 7.75 (d, J =
9.6 Hz, 1H); 7.68 (d, J = 7.6 Hz, 2H); 7.46 (t, J = 8.8 Hz, 1H);
7.16–7.35 (m, 4H); 6.74–6.88 (m, 2H); 3.80 (d, J = 5.6 Hz, 2H);
1.65–1.78 (m, 1H); 1.23–1.53 (m, 8H); 0.92 (s, 6H). 13CNMR
(400 MHz, CDCl3) d (ppm): 158.41, 152.89, 140.76, 139.93,
139.55, 137.87, 131.65, 131.17, 131.07, 130.72, 130.62, 130.24,
129.89, 129.02, 128.58, 128.18, 127.97, 127.78, 127.68, 127.58,
127.52, 126.72, 126.13, 125.34, 125.30, 125.21, 125.12, 125.05,
124.92, 124.81, 120.82, 120.70, 115.02, 70.67, 66.54, 39.66,
30.77, 29.96, 29.35, 24.10, 23.29, 14.34, 11.38. LDI – TOF-MS
(m/z): Anal. calcd. for C59H46O 770.4; found 770.6. Anal.
calcd. C, 91.91; H, 6.01; found C, 91.90; H, 6.03.
The thermal stability of the two materials in nitrogen was
evaluated by thermogravimetric analysis (TGA). The two
materials have the similar thermal decomposition temperature
over 430 uC. The TGA results indicate that the materials are
thermally stable enough to be used as EL materials. But
the phase-transition properties the two materials were more
outstanding, which is determined by DSC in nitrogen atmo-
sphere at a heating rate of 10 uC min21 (Fig. 1). The DSC
examination revealed that EHOP1 and EHOP2 are stable
amorphous materials, with glass transition temperature (Tg) of
103 and 191 uC for EHOP1 and EHOP2, respectively. One
more rigid pyrene at the C7 of the fluorene moiety in EHOP2
results in higher Tg than that of EHOP1. In contrast, although
pyrene has the high efficiency and high carrier mobility, it is
ease to crystallize, which is the reason why the direct using of
pyrene as emitters in doped or non-doped OLEDs is neglected.
So this result indicates that the incorporation of the more rigid
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Fig. 1 The DSC traces of EHOP1 and EHOP2.
9-alkyloxyphenyl-9-pyrenylfluorene moiety onto the pyrene
can significantly enhance the amorphous stability of the
materials. Amorphous stability is a basic requirement for
materials used in OLEDs, and the microcrystallization of the
materials has detrimental effects on the device stability. So the
high thermal stability may in turn improve the operating
lifetime of the EL device.10
The UV–Vis absorption and photoluminescence spectra of
the two materials were measured both in CH2Cl2 and in film
and the corresponding data were summarized in Table 1. From
EHOP1 to EHOP2, the onset absorption shows a red shift due
to the increasing of conjugation length, resulting from a more
conjugated pyrene group at the C7 of fluorene moiety in
EHOP2 (Fig. 2a). The two absorption peaks at y334 nm and
y322 nm, all resulted from the non-conjugated pyrene group
at the C9 of fluorene moiety, which is consistent with the
absorption of pyrene in CH2Cl2 (336 nm, 322 nm). Although
the conjugation chains for EHOP1 and EHOP2 are different,
they all show the absorption lmax at y351 nm. 2,7-Dipyrenyl-
9,9-diphenylfluorene (DPhDPF)11 has the similar conjugated
chain to EHOP2, but the absorption lmax is at 362 nm, thus
the p–p* conjugation absorption of the EHOP1 should be
covered up by the peaks at y351 nm, and the EHOP2’s should
be the shoulder peak at the y365 nm, which was also partly
covered up. Except for the absorption lmax at 360 nm, there is
no other absorption peaks for DPhDPF. From the above
comparison, we think the sharp peaks at y351 nm is the
characteristic absorption peak of 9-pyrenylfluorene substituted
pyrenes, which might originated from the interaction of the
non-conjugated pyrene groups at the C9 of the fluorene and
the conjugated pyrene groups at the C2 of the fluroene moiety,
and the further research is on the way.
Fig. 2 (a) UV–Vis absorption. (b) Emission spectra of pyrene,
EHOP1, and EHOP2. (c) Film PL of pyrene, EHOP1, and EHOP2,
also included are the PL spectra of EHOP1 and EHOP2 film after
annealed at 150 uC for 24 h.
Table 1 Physical properties of EHOP1 and EHOP2.
lem,max/nm
Substituted pyrene
W (%)
labs,max/nm
Solution
Film
DE/eV (abs. edge/nm)
HOMO/eV
LUMOa/eV
EHOP1
EHOP2
a
75
74
351
352
405
425
467
459
2.98 (416)
2.83 (439)
25.39
25.32
22.41
22.49
LUMO = HOMO + DE (band gap).
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The emission spectra display maxima at 405 and 425 nm, for
EHOP1 and EHOP2, respectively. From the unsubstituted
pyrene to substituted EHOP1 to EHOP2, the PL spectra also
show a gradual red shift, concomitant with the increasing
conjugation length (Fig. 2b). The PL quantum yields of the
materials in dichloromethane solution were estimated to be
0.75 and 0.74 for EHOP1 and EHOP2, respectively, by using
9,10-diphenylanthrancene as the reference standard ((cyclo-
hexane solution, W = 0.9).12 Although the solution emission
spectra of pyrene, EHOP1 and EHOP2 were obviously
different to each other, both the film PL maxima of EHOP1
and EHOP2, prepared by vacuum vapor deposition (100 nm),
seem to be similar to that of pyrene (Fig. 2c), which could be
attributed to the aggregation of the pyrene groups. It is also
the reason that why the names of the two compounds are
abbreviated to be pyrene derivatives. In addition, no obvious
change in the emission film was observed even after annealing
at 150 uC for 24 h under nitrogen, which indicates that we have
successfully improve the luminescent stability of pyrene by
incorporating the bulky 9-alkyloxyphenyl-9-pyrenylfluorene
group onto the pyrene ring.
calculated results showed that the non-conjugated pyrene
groups at the C9 of the fluroene moieties make significant
contribution to the HOMO energy levels. In comparison with
this, the non-conjugated benzene groups and long-chain
alkyloxy groups have no contribution to the HOMO, including
the diphenyl groups at the C9 of fluorene moiety in DPhDPF.
The special influences of the non-conjugated pyrene group at
C9 of fluorene moiety are consistent with the difference
between our experimental HOMO of EHOP2 (25.32 eV) and
the reported HOMO of DPhDPF (25.7 eV).11
The two materials have good hole-injection and hole-
transporting ability, because the HOMOs of the EHOP1 and
EHOP2 are close to the common hole-injection materials
such as CuPc and PEDOT:PSS (y25.2 eV).15 Thus in order
to simplify the device architecture, a light-emitting device was
obtained without the hole-injection layer, ITO/TCTA (8 nm)/
EHOP1 (30 nm)/BCP (45 nm)/Mg:Ag, where the 4,49,40-
tri(N-carbazolyl)triphenylamine (TCTA) was used as a buffer
and hole-transporting layer, the 2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline (BCP) as a buffer and electron-transport-
ing layer, considering that there is always fluorescence
quenching at the interface between the metal electrode and
the emitter layer. A blue emission from the EHOP1 was
observed (Fig. 4), with 1931 CIE (Commission Internationale
DE 19Eclairage) coordinates of (y0.18, y 0.23). The device
has a low turn-on voltage of 4.0 V, and the maximum
luminance of 11620 cd m22 was obtained at the voltage of
14.6 V, with the current density of 668 mA cm22 (Figs 5 and 6).
The maximum efficiency is 3.04 cd/A (8.8 V, 2891 cd m22 and
95 mA cm22). At the more practical current densities of y50
The electrochemical behavior of the materials were investi-
gated by cyclic voltammetry (CV) with a standard three
electrodes electrochemical cell in a 0.1 M tetra-n-butylammo-
nium hexafluorophosphate (Bu4NPF6) in CH2Cl2 at room tem-
perature under nitrogen with a scanning rate of 200 mV s21
.
A platinum working electrode, a platinum counter electrode
and an Ag/AgNO3 (0.1 M) reference electrode were used. The
oxidation onset potentials were measured to be 0.69 and 0.66 V
for EHOP1 and EHOP2, respectively. The corresponding
HOMO energy levels were thus estimated to be 25.39 and
25.36 eV for EHOP1 and EHOP2,13 respectively, which
indicates that the electron-rich pyrene derivatives do have
obviously improved hole-injection ability than conjugated
fluorene derivatives (25.6 to 25.8 eV).4 The LUMOs can be
estimated from the DE, calculated from the absorption edges.
The corresponding data were also summarized in Table 1.
A typical semiempirical quantum chemical calculation
methodology, Austin Model1 (AM1),14 was employed to
simulate the HOMOs and LUMOs of the EHOP1, EHOP2
and DPhDPF (Fig. 3). For EHOP1 and EHOP2, the
and 25 mA cm22, the device has efficiency of 2.8 cd A21
,
1.12 lm W21 and 2.4 cd A21, 1.12 lm W21, respectively. It is
noteworthy that the device still keeps a considerable efficiency
of 1.74 cd A21, when the luminescence reaches its maximum.
This implies that the device is stable enough to endure extreme
operating conditions. Considering the simple three-layer
device fabrication, low turn-on voltage, high luminance, high
efficiency and efficiency stability when current increases, the
performance of this non-doped blue OLED can competitive
with the best of the doped and nondoped blue OLEDs,11,16
which is originated from the elaborate combination of the
Fig. 3 The calculated frontier orbitals for EHOP1, EHOP2, and DPhDPF.
4078 | J. Mater. Chem., 2006, 16, 4074–4080
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that the materials are promising materials for application to
optoelectronics.
3. Conclusion
In summary, two new 9-(49-(20-ethylhexyloxyphenyl))-9-pyr-
enylfluorene substituted pyrenes, i.e. EHOP1 and EHOP2,
have been synthesized and characterized. The two materials
are stable amorphous materials, with Tg of 103 and 191 uC for
EHOP1 and EHOP2, respectively. Due to the non-conjugated
pyrene groups at C9 of the fluorene moieties, the absorption
spectra of the two compounds showed some characteristic
peaks and they showed improved hole-injection ability
than conjugated fluorene derivatives. The preliminary simple
three-layer blue EL device, ITO/TCTA (8 nm)/EHOP1
(30 nm)/BCP (45 nm)/Mg:Ag, was obtained with high bright-
ness (11620 cd/m2), low turn-on voltage (4.0 V) and high
efficiency (3.04 cd/A), without the need for hole-injection
layers. Results of this work have also proof that we can
simplify the device technology though the elaborate design of
material structures.
Fig. 4 The EL spectra of the light-emitting device.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China under Grants 60325412,
90406021, and 50428303 as well as the Shanghai Commission
of Science and Technology under Grants 03DZ11016 and
04XD14002 and the Shanghai Commission of Education
under Grant 2003SG03. We wish to thank Mr. Hui Xu and
Ms Ping Zhao for helpful discussions.
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