Solution-processable 1,3,5-tri(9-anthracene)-benzene cored propeller-shaped materials with high Tg for blue organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2011.06.025

This study describes the synthesis and characterization of a series of new blue fluorescent materials, with propeller-like topology, consisting of 1,3,5-tri(9-anthracene)benzene core and various aromatic dendrons, such as naphthalene, 3,5-diphenylbenzene,

Full text of DOI:10.1016/j.orgel.2011.06.025

Organic Electronics 12 (2011) 1716–1723
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Solution-processable 1,3,5-tri(9-anthracene)-benzene cored propeller-
shaped materials with high Tg for blue organic light-emitting diodes
Hong Huang a, Qiang Fu b, Shaoqing Zhuang a, Guangyuan Mu a, Lei Wang a, , Jiangshan Chen b,
⇑
⇑
,
Dongge Ma b, Chuluo Yang c,
⇑
a Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology,
Wuhan 430074, PR China
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of Chinese Academy of Sciences,
Changchun 130022, PR China
c Department of Chemistry, Wuhan University, Wuhan 430072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 May 2011
Received in revised form 28 June 2011
Accepted 28 June 2011
Available online 18 July 2011
This study describes the synthesis and characterization of a series of new blue fluorescent
materials, with propeller-like topology, consisting of 1,3,5-tri(9-anthracene)benzene core
and various aromatic dendrons, such as naphthalene, 3,5-diphenylbenzene, carbazole,
and N,N-diphenylamine. These compounds show excellent thermal and morphological sta-
bility with high glass transition temperatures (Tg) (166–231 °C) and high thermal decom-
position temperatures (Td) (427–504 °C). Solution-processable double-layered OLEDs
fabricated with these materials as the light-emitting layer show stable blue emission
and good performance. The nondoped electronic device fabricated using compound 5c
exhibits a maximum brightness of 4754 cd/m2 and maximum current efficiency of
Keywords:
Blue material
1,3,5-Tri(9-anthracene) benzene core
Organic light-emitting device
2.0 cd/A (power efficiency, 1.71 lm/W) with Commission Internationale d’Eclairage (CIEx,y
)
color coordinates of (x = 0.16, y = 0.19) and the devices’ threshold voltage are only 3.4 eV.
Compound 5d shows an even higher efficiency of up to 4.90 cd/A with CIEx,y color coordi-
nates of (x = 0.17, y = 0.31) when doped with a blue fluorescent dopant, 4,40-bis[4-(di-p-tol-
ylamino)styryl]biphenyl (DPAVBi).
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
display applications and white solid-state lighting due to
their intrinsic wide band-gap [6–8]. Many blue dendritic
In the past decade, light-emitting dendritic materials
have attracted much interest, owing to them combining
the advantages of both the small molecules and polymer,
such as, precise chemical structures and high purity of
small molecular materials and good solution processability
of polymeric materials [1–4]. Recently, considerable efforts
in this field have been focused on developing stable den-
dritic materials with efficient blue electroluminescence
(EL) [5], which is of particular importance for full-color
fluorescent and phosphorescent light emitting materials
have been developed. For example, a kinked star-shaped
fluorene/triazatruxene co-oligomer has been reported [9]
with device efficiency (1.75 cd/A, CIE (0.16, 0.15)) and a
pyrene-centered starburst oligofluorene [10] with device
efficiency (1.48 cd/A, CIE (0.20, 0.32)). Lo’s research group
[11] have reported a host free phosphorescent dendrimer
material with an external quantum efficiency of 7.9%, and
CIE coordinates of (0.18, 0.35). Despite many exciting re-
ports, highly efficient devices exhibiting both high thermal
stability and robust emission still remains rare. Therefore,
blue fluorescent and phosphorescent OLEDs based on den-
drimers still require further improvements in terms of
thermal stability and EL efficiency.
⇑
Corresponding authors. Tel.: +86 27 87793032; fax: +86 27 87793032
(L. Wang), tel.: +86 0431 85262901; fax: +86 0431 85262901 (J.S. Chen).
E-mail addresses: wanglei@mail.hust.edu.cn (L. Wang), jschen@
ciac.jl.cn (J. Chen), clyang @whu.edu.cn (C. Yang).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.06.025

H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
1717
Among the blue-emitting materials, anthracene deriva-
tives are particularly attractive due to their excellent photo-
luminescence and electroluminescence properties [12–16].
Such as, triarylamines [17,18], or the other moieties [19–21]
have been incorporated into anthracene unit to improve
hole injection and charge carrier transport properties.
Although a number of vacuum-deposited anthrance-based
molecules have been reported as efficient blue electrolumi-
nescent materials [22,23], solution-processed analogs re-
main rare and largely unexplored in OLEDs for its poor
solubility and aggregating properties. In our previous work
[24], we have demonstrated that 1,3,5-tri (9-anthracene)
benzene is a good core emitter, and its centered starburst
oligofluorenes show robust blue light emission in an OLED
device. Although these materials exhibit deep blue emis-
sion, and the current efficiency is up to 1.8 cd/A, but the val-
ues of Tg are only about 110 °C for the oligofluorenes
possessing long alkyl chains.
Here, we report the design and synthesis of four sym-
metric dendrimer molecules based on the following two
principles. Firstly, the propeller-like, and highly rigid
1,3,5-tri (9-anthracene) benzene group was used as the
core, which can reduce strong intermolecular interactions
and suppress fluorescence quenching. Secondly, using stil-
benzene as a conjugated bridge, the different electron do-
nor dendrons (naphthalene, diphenylbenzene, carbazole,
and diphenylamine) were introduced at the peripheries
of the core to improve solubility in common organic sol-
vents, influence the charge transporting properties, en-
hance the morphological stability and the thermal
stability and improve color purity. As we expected, all of
these materials possess high glass transition temperatures
(Tg) and good morphological properties, and exhibit good
EL performance with low driving voltages in solution-pro-
cessed double-layer OLED devices. These materials are a
promising class of solution-processable blue emitters for
practical applications.
performed at 360 nm and 9,10-diphenylanthracene(DPA)
in CH2Cl2 Uf = 0.9 in CH2Cl2) or solid film( DPA = 1) was
(
g
used as a standard [25,26]. Cyclic voltammetry measure-
ments were carried out in a conventionalthree electrodecell
using a Pt button working electrode of 2 mm in diameter, a
platinum wire counter electrode, and a Ag/AgCl (0.1 M) ref-
erence electrode on a computer-controlled EG&G Potentio-
stat/Galvanostat model 283 at room temperature.
Reductions CV of all compounds were performed in dichlo-
romethane containing 0.1 M tetrabutylammoniumhexaflu-
orophosphate (Bu4NPF6) as the supporting electrolyte.
2.2. Synthesis
The structures and synthetic routes of the four well-
defined compounds are shown in Scheme 1. 1,3,5-
tri(anthracen-9-yl)benzene (1) and 1,3,5-tris(10-bromo
anthracen-9-yl)benzene (2) were synthesized according
to the literature [24].
2.2.1. Synthesis of compound (3)
A 100 ml round-bottomed flask was charged with com-
pound 2 (422 mg, 0.5 mmol), toluene (20 ml), and ethanol
(10 ml) under nitrogen atmosphere. Subsequently, 4-form-
ylphenylboronic acid (375 mg, 2.5 mmol), tetrabutyl-
ammonium bromide (0.96 g, 2.98 mmol) and aqueous
potassium carbonate (15.0 ml, 30 mmol) were added
slowly to the solution. 10 min later, tetrakis (triphenyl-
phosphine) palladium (0) (173 mg, 0.15 mmol) was added
and the reaction mixture was stirred under reflux for 24 h
in the absence of light. After cooling to room temperature,
the precipitate was filtrated and washed with distilled
water and methanol, a yellow powder was obtained. Yield
0.34 g (74%). 1H NMR: (CDCl3, 400 MHz): d (ppm) 10.21 (s,
3H), 8.30–8.28 (d, J = 8.8 Hz, 6H), 8.15–8.13 (d, J = 8.0 Hz,
6H), 7.90 (s, 3H), 7.68–7.56 (m, 18H), 7.43–7.40 (t, J =
7.6 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 191.98, 145.92,
139.21, 136.94, 135.86, 135.77, 133.98, 132.18, 129.93,
129.87, 129.60, 126.85, 126.64, 125.72, 125.63. MS (FAB):
919.3 (m/z), calcd for C69H42O3: 919.3. Anal. Calcd. For
2. Experimental section
C
69H42 O3: C 90.17, H 4.61, O, 5.22; found: C 90.14, H
2.1. Material and methods
4.56, O 5.30.
All reagents and solvents were used as purchased from
Aldrich and were used without further purification. 1H
NMR spectra were recorded using a Bruker-AF301 AT
400 MHz. High resolution mass spectrometric measure-
ments were carried out using a Bruker autoflex MALDI-
TOF mass spectrometer. Fluorescence spectra were
obtained on a Perkin Elmer LS55 Luminescence spectrome-
ter and UV–Vis spectra were measured using a Shimadzu-
3150 UV–Vis–NIR spectrophotometer. The differential
scanning calorimetry (DSC) analysis was performed under
a nitrogen atmosphere at a heating rate of 10 °C/min using
a PE Instruments DSC 2920. Thermogravimetric analysis
(TGA) was undertaken using a SEIKO EXSTAR 6000 TG/DTA
6200 unit under nitrogen atmosphere at a heating rate of
10 °C/min. To measure the PL quantum yields (Uf), degassed
solutions of the compounds in CH2Cl2 were prepared. The
concentration was adjusted so that the absorbance of the
solution would be lower than 0.1. The excitation was
2.2.2. Synthesis of compound (4)
A 500 ml three-neck round-bottomed flask was charged
with 4-bromobenzylphosphonium bromide (5.21 g,
10.0 mmol), sodium hydride (0.75 g, 30.0 mmol) and THF
(200 ml) under nitrogen atmosphere. After stirring for 1 h,
at 40 °C, compound 3 (2.3 g, 2.5 mmol) was added. The solu-
tion was continuously stirred for 4 h at room temperature
and the product was precipitated by the addition of metha-
nol (200 ml). The precipitated solid was filtrated and
washed with methanol, a light yellow solid powder was ob-
tained. Yield 3.2 g (93%). 1H NMR: (CDCl3, 400 MHz): d
(ppm) 8.30–8.28 (d, J = 8.0 Hz, 6H), 7.89 (s, 3H), 7.78–7.75
(m, 12H), 7.56–7.39 (m, 33H), 7.26–7.21 (m, 3H). 13C NMR
(100 MHz, CDCl3) d 136.34, 132.20, 132.10, 131.98, 131.89,
131.82, 131.51, 131.41, 130.81, 130.59, 130.01, 128.58,
128.47, 128.07, 127.85, 127.13, 126.82, 126.62, 125.55,
125.16. MS (MALDI-TOF): 1378.1999 (m/z), calcd for

1718
H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
CHO
Br
Br
b)
a)
OHC
CHO
Br
1
2
3
R
Br
c)
d)
R
R
Br
Br
5a: R = naphthalene 5b: R = 3,5-diphenylbenzene
5c 5d
4
: R = carbazole
: R = diphenylamine
Scheme 1. Synthetic route of 5a, 5b, 5c and 5d. Reagents and conditions: (a) NBS, DMF, 30 °C; (b) toluene, K2CO3, ethanol, (4-formylphenyl)boric acid,
Pd(PPh3)4, reflux; (c) 4-bromobenzyl-phosphonium bromide, 40 °C, NaH, THF, 1 h; (d) (i) 1-naphthylboronic acid or 3,5-diphenylbenzeneboronic acid,
Pd(PPh3)4, toluene, EtOH; (ii) carbazole or diphenylamine, 18-crown-6, CuI, DMPU, K2CO3, 180 °C.
C90H57Br3: 1378.1994. Anal. Calcd. For C90H57Br3: C 78.44, H
4.17, Br, 17.39; found: C 78.50, H 4.15, Br, 17.35.
CDCl3) d 142.45, 141.79, 141.15, 140.36, 139.28, 138.47,
137.17, 136.71, 136.59, 136.41, 131.78, 130.01, 129.17,
128.87, 128.79, 128.60, 128.55, 127.64, 127.59, 127.38,
127.25, 127.18, 127.11, 126.81, 126.60, 126.12, 125.54,
125.32, 125.17, 124.94 MS (MALDI-TOF): 1825.7515 (m/
z), calcd for C144H96: 1825.7546. Anal. Calcd. For C144H96:
C 94.70, H 5.30; found: C 94.85, H 5.15.
2.2.3. Synthesis of compound (5a)
A 500 ml three-neck round-bottomed flask was added
compound 4 (137.8 mg, 0.1 mmol), 1-naphthaleneboronic
acid (111 mg, 0.5 mmol), toluene (20 ml), ethanol (10 ml),
2 M K2CO3 (15 ml, 30 mmol) aqueous solution and tetra-
kis-(triphenylphosphine) palladium (0) (173 mg, 0.15
mmol) in turn, then the reaction mixture was refluxed un-
der nitrogen for 24 h in the absence of light. After cooling
to the room temperature, the precipitate was collected by
filtration and washed with distilled water and methanol.
The crude product was purified by column chromatography
and dried under vacuum to yield a yellowish solid. Yield:
92%. 1H NMR: (CDCl3, 400 MHz): d (ppm) compound 5a:
8.30–8.28 (d, J = 8.0 Hz, 6H), 8.00–7.81 (m, 26H), 7.74–7.72
(m, 6H), 7.58–7.26 (m, 40H). 13C NMR (100 MHz, CDCl3) d
140.24, 139.87, 139.32, 138.46, 137.20, 136.67, 136.41,
134.05, 133.88, 131.79, 131.58, 130.50, 130.05, 128.75,
128.60, 128.24, 128.13, 127.87, 127.74, 127.21, 126.88,
126.61, 126.51, 126.09, 125.99, 125.96, 125.82, 125.56,
125.41, 125.17. MS (MALDI-TOF): 1519.6144 (m/z), calcd
for C120H78: 1519.6132. Anal. Calcd. For C120H78: C 94.83, H
5.17; found: C 94.58, H 5.42.
2.2.5. Synthesis of compound (5c)
A mixture of compound 4 (689 mg, 0.5 mmol), carbazole
(376 mg, 2.25 mmol), CuI (14.25 mg, 0.075 mmol), 18-
crown-6 (19 mg, 0.25 mmol), K2CO3 (310.5 mg, 2.25 mmol)
and 1,3-di-methyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone
(DMPU, 2.0 ml) was heated to 190 °C for 24 h under a nitro-
gen atmosphere. After cooling down to room temperature,
the mixture was quenched with 1 N hydrochloric acid, then
extracted with dichloromethane, washed with NH3ꢀH2O and
water. The combined organic phases were dried (MgSO4)
and concentrated under reduced pressure. The residue was
purified by column chromatography on silica gel using hex-
ane as the eluent to give compound 5c as a yellow solid.
Yield: 80%. 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.33–8.31
(d, J = 9.2 Hz, 6H), 8.17–8.15 (d, J = 7.6 Hz, 6H), 7.92 (s,
3H), 7.83–7.80 (m, 18H), 7.63–7.38 (m, 42H), 7.33–7.29
(m, 6H). 13C NMR (100 MHz, CDCl3) d 140.80, 140.70,
139.30, 138.69, 137.12, 137.00, 136.51, 136.44, 136.42,
134.04, 131.84, 130.42, 130.03, 129.25, 128.08, 127.90,
127.27, 127.18, 126.84, 126.68, 125.98, 125.57, 125.20,
123.46, 120.35, 120.02. MS (MALDI-TOF): 1636.6458 (m/
2.2.4. Synthesis of compound (5b)
The compound was synthesized using a similar proce-
dure as for compound 5a, yield: 85%. 1H NMR: (CDCl3,
400 MHz): d (ppm) 8.32–8.30 (d, J = 8.8 Hz, 6H), 7.91–
7.72 (m, 50H), 7.57–7.35 (m, 40H). 13C NMR (100 MHz,
z), calcd for
126H81N3: C 92.45, H 4.99, N 2.57; found: C 92.31, H 5.17,
N 2.52.
C126H81N3: 1636.6461. Anal. Calcd. For
C

H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
1719
2.2.6. Synthesis of compound (5d)
Thermal properties of these compounds were studied
using DSC and TGA. The thermal properties were measured
under N2 gas condition at heating rates of 10 °C/min, and
the related data are listed in Table 1. As shown in Fig. 1a,
compounds 5a–5d exhibit a high glass transition tempera-
ture (Tg) at 166, 189, 260, and 231 °C, respectively. The
high Tg are attributed to the rigid core and the star shaped
dendrons can effectively suppress the crystallization (or
arm aggregation). High decomposition temperatures (Td,
5% weight loss) of these compounds were also observed
at 484, 504, 497, and 427 °C (Fig. 1b), respectively, which
may be due to the highly stability of the 1,3,5-tri (9-
anthracene) benzene core and the side groups. The stable
aromatic side group can supply electron to the styryl group
and enhance the thermal stability of material.[28] The rel-
atively high Tg and Td of these star shaped compounds is
very desirable for high performance OLED applications.
Efficient film-forming properties of light emitting mate-
rials used in OLEDs are crucial for the performance of the
devices, so the surface morphologies of thin films of the
four compounds (5a, 5b, 5c and 5d) were examined. As
showed in Fig. 2, the AFM images show smooth and amor-
phous thin films formed by spin-coating the compounds
(5a, 5b, 5c and 5d) from toluene solution. The deposited
films were annealed under N2 gas condition at 150 °C for
0.5 h. The annealed film exhibits a fairly smooth surface
morphology with a root-mean-square (rms) roughness of
0.5539, 0.6211, 0.5202 and 0.6565 nm for 5a, 5b, 5c and
5d, respectively. This suggests that all of the compounds
possess excellent thermal and amorphous stability, which
indicates that the rigid propeller core influences the
arrangement of the molecules in the thin films, and
1,3,5-tri(9-anthracene)benzene is a good core emitter for
star-shaped fluorophore.
Fig. 3 shows the normalized UV–Vis and fluorescence
spectra of compounds 5a, 5b, 5c and 5d in a CH2Cl2 solu-
tion and in the thin film. The related photophysical proper-
ties were listed in Table 1. All of these compounds have
similarly structured absorption spectra (range: 330–
440 nm) and emission spectra (range: 400–550 nm) in
solution. This indicates that the photophysical properties
are mainly determined by the core 1, 3, 5-tri(9-anthra-
cene)benzene. The absorption in the range 330–440 nm
is assigned to the S1 S0 transition of the anthracene moi-
ety. In the solid state, all of them show strong pure-blue
emission with a main peak at 448, 453, 451 and 460 nm,
respectively, which shows that the introduction of dipenyl-
amine results in a notable red-shift in PL spectra and
peripheral dendrons can affect the molecular interaction.
The quantum yields of compound 4a-4d in CH2Cl2 were
0.68, 0.66, 0.67 and 0.64, respectively, which is even higher
than the typical blue material m-ADN [25]. The quantum
yield in solid state was also measured, the relative high
values indicated that no obviously aggregating or quench-
ing phenomena appeared. In order to further investigate
the morphological stability of these materials, the Abs
and PL spectra before/after annealing state (see Figure
S4.) was also researched, the absorbance and PL emission
have no clearly change after annealing (150 °C for 1 h),
which showed that the morphological stability of material
is excellent. The optical energy bandgaps of the compound
The compound was synthesized using a similar proce-
dure as for compound 5c. Yield: 74%. 8.30–8.28 (d, J =
8.8 Hz, 6H), 8.17–8.15 (d, J = 7.6 Hz, 6H), 7.89–7.88 (s, 3H),
7.78–7.72 (m, 18H), 7.60–7.37 (m, 48H), 7.27–7.21 (m,
6H). 13C NMR (100 MHz, CDCl3) d 139.25, 138.68, 137.19,
136.62, 136.36, 136.29, 134.01, 131.86, 131.79, 131.72,
131.49, 130.57, 129.99, 129.30, 129.10, 128.87, 128.75,
127.81, 127.44, 126.80, 126.54, 126.31, 125.53, 125.17,
124.55, 121.45. MS (MALDI-TOF): 1641.6924 (m/z), calcd
for C126H87N3: 1641.6900. Anal. Calcd. For C126H87N3:
92.11, H 5.34, N 2.56; found: C 92.10, H 5.39, N 2.51.
C
2.3. Device fabrication
To investigate the electroluminescent (EL) properties of
the symmetric starburst compounds containing different
surface groups, double layer OLEDs were fabricated via
solution processing according to the previous litera-
ture.[27] The device structures of the OLEDs were as fol-
lows: ITO/PEDOT:PSS/5a–5d or (doping DPAVBi) /TPBI/
LiF/Al. Polyethylene dioxythiophene-polystyrene sulfate
(PEDOT-PSS) and 2,20,200-(1,3,5-benzenetriyl)tris[1-phe-
nyl-1H-benzimidazole] (TPBI) were used as hole injection
and hole-blocking/electron-transporting layer, respec-
tively. The active layer was spin-coated from toluene solu-
tion, and TPBI layer was deposited by means of
conventional vacuum deposition onto the ITO-coated glass
substrates. The J–V–L characteristics of the EL devices were
measured using a Keithley 2400 source meter and a Keith-
ley 2000 source multimeter equipped with a calibrated sil-
icon photodiode. The EL spectra were measured using a JY
SPEX CCD3000 spectrometer. The thicknesses of the films
were measured using the instrument of Profiler DEK-
TAK150. All of the measurements were carried out at room
temperature under ambient conditions.
3. Results and discussion
3.1. Synthesis, Thermal, Film-forming, Optical and
Electrochemical properties
Four target compounds, 5a, 5b, 5c and 5d, were pre-
pared as shown in Scheme 1. Detailed synthesis of the
1,3,5-tris(10-bromoanthracen-9-yl)benzene (2) has been
described in previous work [24]. The intermediate 3
was synthesized by the Pd-catalyzed Suzuki cross-cou-
pling of formylated benzene boric acid with compound
2 with excellent yields, and were easily purified by a
simple filtration and washed with distilled water and
methanol. Then Wittig reaction of 4-bromobenzyl-phos-
phonium bromide and compound 3 gave the compound
4 in 93% yield at 40 °C. The target compounds 5a and
5b were obtained in 92% and 85% of isolated yield, by
an easy Suzuki coupling reaction, respectively. The target
compound 5c and 5d were obtained in 80% and 74% of
isolated yield by the CuI-catalyzed Ullman reaction,
respectively. All the four target compounds were soluble
in common organic solvents (CH2Cl2, toluene), and char-
acterized fully by 1H and 13C NMR, MALDI-TOF, and ele-
mental analysis.

1720
H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
Table 1
The optical, photophysical and thermal properties of 5a–5d.
HOMO
LUMO
U
/
a gb
Eg (eV)
PL (nm)
Solution
Abs (nm) in solution
Tg/Td (°C)
Film
5a
5b
5c
5d
ꢁ5.57
ꢁ5.54
ꢁ5.52
ꢁ5.50
ꢁ2.65
ꢁ2.59
ꢁ2.58
ꢁ2.57
0.68/0.69
0.66/0.68
0.67/0.64
0.64/0.61
2.92
2.95
2.94
2.93
436
436
436
437
448
453
451
460
331,358,380,401
340,358,380,401
344,358,380,401
320,358,380,401
166/484
189/504
260/497
231/427
a
Using DPA as a standard; kex = 360 nm (
Using DPA as a standard; kex = 360 nm (
U
= 0.9 in CH2Cl2).
b
g
= 1 in thin film).
10
100
a)
5a
5b
5c
5d
9
b)
c
90
80
70
60
8
7
b
6
5
4
3
5a
5b
5c
5d
a
d
100
150
200
250o 300
350
200
300
400
500
600
700
800
Temperature (οC)
Temperature ( C)
Fig. 1. (a) DSC curves of compounds 5a, 5b, 5c and 5d measured under nitrogen at a heating rate of 10 °C/min; (b) TGA curves of star-shaped compounds
measured at a heating rate of 10 °C/min under nitrogen.
Fig. 2. AFM topographic images of the four compounds (5a, 5b, 5c and 5d) in thin solid films (ca. 40 nm thick).
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
a)
b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
5a
5b
5c
5d
5a
5b
5c
5d
300 350 400
500 550 600 650
Wa4v5e0length (nm)
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. (a) Absorption and photoluminescence spectra of compound 5a, 5b, 5c and 5d in CH2Cl2; (b) Absorption and PL spectrum of the compounds 5a, 5b,
5c and 5d in thin film (ca. 40 nm).
5a, 5b, 5c, and 5d are 2.92, 2.95, 2.94, and 2.93 eV, respec-
tively, which is calculated from the threshold of the optical
absorption.
In addition to the photophysical properties, the electro-
chemical properties are also very important in evaluating a
material usefulness for optoelectronic applications. The
electrochemical behaviors of the compounds 5a, 5b, 5c
and 5d were examined by cyclic voltammetry using a stan-
dard three-electrode electrochemical cell in an electrolyte
solution of 0.1 M tetrabutylammoniumhexafluorophos-
phate (TBPAPF6) dissolved in dichloromethane [29]. The
onset oxidation peaks (Eox) for 5a–5d were 1.26, 1.23,

H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
1721
Table 2
Electroluminescence Characteristics of the Devices.a
Lmax (cd/m2) voltages (V)
gc.max (cd/A)
gp.max (lm/W)
CIE (x, y)e
Von (1 cd/m2) (V)
EL(peak) (nm)
b
c
d
Device
Emitter
1
2
3
4
5
6
7
8
5a
5b
5c
5d
3366, 11.3
2740, 11.1
4754, 8.5
1562, 11.5
3635, 8.6
6807, 12.4
7470, 11.6
11204, 11.4
1.7
1.6
2.0
2.4
3.6
4.2
3.6
4.9
1.10
1.14
1.71
1.54
2.40
2.81
2.25
3.22
(0.15,0.17)
(0.15, 0.14)
(0.16, 0.19)
(0.18, 0.28)
(0.16, 0.31)
(0.16, 0.30)
(0.17, 0.33)
(0.17, 0.31)
4.8
4.5
3.4
4.9
3.2
3.6
3.6
3.4
468
456
468
476
472
472
476
472
5a:DPAVBi
5b:DPAVBi
5c:DPAVBi
5d:DPAVBi
a
b
c
Devices configuration: ITO/PEDOT:PSS (40 nm)/ 5a–5d (40 nm) or 5a–5d: DPAVBi (5%)(40 nm)/TPBI (40 nm) /LiF (1 nm) /Al (100 nm).
Maximum luminance.
Maximum current efficiency.
Maximum powder efficiency.
Commission International de I0Eclairage coordinates.
d
e
104
1000
a)
103
800
600
400
200
0
Device 1
Device 2
Device 3
Device 4
1
0.1
102
101
100
10-1
10-2
10-3
b)
Device 1
Device 2
Device 3
Device 4
0.01
10-2
10-1
100
101
102
103
3
4
5
6
7
8
9
10 11 12 13
Current Density (mA/cm2)
Voltage (V)
1.0
0.8
0.6
0.4
0.2
0.0
c)
d)
1
0.1
Device 1
Device 2
Device 3
Device 4
Device 1
Device 2
Device 3
Device 4
0.01
10-2
10-1
100
101
102
103
400
500
600
700
Current Density (mA/cm2)
Wavelength (nm)
Fig. 4. (a) J–V–L curve of the four devices; (b) current efficiency-current density characteristics; (c) powder efficiency-current density characteristics; (d) EL
spectra of the four devices.
1.21 and 1.19 eV, respectively (see Figure S3). Thus, the
HOMO levels were determined to be ꢁ5.57, ꢁ5.54, ꢁ5.52
and ꢁ5.50 eV by the method ꢁ(Eox + 4.31) eV reported by
Li et al. [25] (Table 1). The values were very close to that
of 9,10-diphenylanthracene (5.54 eV), which indicates the
HOMO level of these compounds were mainly decided by
1,3,5-tri(9-anthracene)-benzene center and the side
groups were only finely tune the energy gap and the
HOMO level. Generally speaking, the electrochemical prop-
erties and photophysical properties of compound 5a–5d in
dilute solution are very similar, mirroring the anthracene
unit’s characteristics. It shows every individual arm’s elec-
trochemical properties are remained unperturbed for the
propeller topology serving as a spacer to effectively block
conjugation.
3.2. Device performance
To study the EL properties of these compounds, double-
layered devices with the configuration ITO/PEDOT:PSS
(40 nm)/5a–5d (40 nm)/TPBI (40 nm)/LiF (1 nm) /Al
(100 nm) (emitter: 5a, Device 1; 5b, Device 2; 5c, Device
3; 5d, Device 4) were fabricated by spin-coating from a tol-
uene solution (ca. 7 mg/ml). The EL performances of these
compounds are listed in Table 2. Double-layer devices uti-
lizing compounds 5a and 5b exhibit a maximum bright-
ness of 3366, 2740 cd/m2, luminous efficiency of 1.7,
1.6 cd/A, with CIE coordinates 5a (0.15, 0.17), 5b (0.15,
0.14). For compounds 5c and 5d, the double-layer devices
showed a maximum brightness of 4754, 1562 cd/m2, lumi-
nous efficiency of 2.0, 2.4 cd/A, with CIE coordinates 5c

1722
H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
600
500
400
300
200
100
0
10
a)
b)
104
103
1
102
Device 5
Device 6
Device 7
Device 8
101
0.1
Device 5
Device 6
Device 7
Device 8
100
10-1
0.01
100
101
Current Density (mA/cm2)
102
4
6
8
10
12
Voltage (V)
10
1
1.0
0.8
0.6
0.4
0.2
0.0
d
)
c)
Device 5
Device 6
Device 7
Device 8
Device 5
Device 6
Device 7
Device 8
0.1
0.01
100
101
102
400 450 500 550 600 650 700 750
Wavelength (nm)
Current density (mA/cm2)
Fig. 5. (a) J–V–L curve of the four doped devices; (b) current efficiency-current density characteristics; (c) powder efficiency-current density
characteristics; (d) EL spectra of the four doped devices.
(0.16, 0.19), 5d (0.18, 0.28). Fig. 4 shows the EL spectra, the
current density–voltage-brightness and the current den-
sity-efficiency characteristics of the devices (1, 2, 3, 4). EL
spectra of the devices based on the emitters 5c and 5d have
obviously red-shift compared to the compounds 5a and 5b,
it is induced by the carbazole and diphenylamine group’s
electronic-donating properties. Especialy, the EL spectrum
of the device 4 has a red shift about 20 nm compared to
that of device four. Additionally, the electroluminescence
(EL) of all devices remained quite stable under different
driving voltages (see Figure S1).
In order to further improve the device efficiency, we
adopted a host-dopant system using these materials as
the host and a highly fluorescent blue dye, DPAVBi, as dop-
ant at a concentration of 5%. Table 2 summarizes the key EL
performance of doped devices (5, 6, 7, 8), all of them show a
strong blue emission with current efficiency of 3.6–4.9 cd/
A. Fig. 5. shows the EL spectra, the current density–volt-
age-brightness and the current density-efficiency charac-
teristics of devices (5, 6, 7, 8). The device 8 exhibits a
maximum brightness of 11204 cd/m2 at 11.4 V, and a max-
imum current efficiency of 4.9 cd/A with CIE coordinates of
(0.17, 0.31). We ascribe the higher efficiency to the more
efficient Förster energy transfer because there is more over-
lap between the emission of compound 5d and the absorp-
tion of DPAVBi. As shown in Fig. 5d, the EL spectra of all
DPAVBi doped devices (5, 6, 7, 8) exhibit bright blue emis-
sions at 472 nm with a shoulder at around 496 nm, which is
the intrinsic characteristic of the emissions from the dopant
DPAVBi. This result indicates completed energy transfer
from the host to dopant. From the Fig. 6, it is obviously that
the quantum efficiency of the doped device has been im-
proved, which indicated that the better device performance
in the doped device is not due to the change of emission col-
ors. More interestingly, the driving voltage of all the devices
is low (ca. 3.6 eV), in which device 5 exhibits the lowest
driving voltage of 3.2 V. This indicates that the energy lev-
els of the host materials and the dopant DPAVBi are well
matched. Additionally, the dopant DPAVBi can also act as
the hole trapper in all of doped devices, so the turn on volt-
age of the most of doped devices are lower than the un-
doped devices(see Table 2) [30]. Excellent EL stability (see
Figure S2) and the efficient color purity render these mate-
rials promising candidates for display applications. We be-
lieve that optimization could further improve the device
2.5
Device 1
Device 2
2.0
Device 3
Device 4
Device 5
1.5
Device 6
Device 7
1.0
Device 8
0.5
0.0
100
101
102
103
Current density (mA/cm2)
Fig. 6. Quantum efficiency-current density characteristics of devices 1–8.

H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
1723
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