Efficient nondoped blue organic light-emitting diodes based on phenanthroimidazole-substituted anthracene derivatives

Article abstract of DOI:10.1016/j.orgel.2012.08.032

A series of new blue materials based on highly fluorescent di(aryl)anthracene and electron-transporting phenanthroimidazole functional cores: 2-(4-(anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (ACPI), 2-(4-(10-(naphthalen-1-yl)anthracen-9-yl)phenyl)-1-p-henyl-1H- phenanthro[9,10-d]imidazole (1-NaCPI), 2-(4-(10-(naphthalen-2-yl)anthracen-9-yl) phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (2-NaCPI) were designed and synthesized. These materials exhibit good film-forming and thermal properties as well as strong blue emission in the solid state. To explore the electroluminescence properties of these materials, three layer, two layer and single layer organic light-emitting devices were fabricated. With respect to the three layer device 4 using ACPI as the emitting layer, its maximum current efficiency reaches 4.36 cd A^-1 with Commission Internationale del'Eclairage (CIE) coordinates of (0.156, 0.155). In the single layer device 10 based on ACPI, maximum current efficiency reaches 1.59 cd A^-1 with Commission Internationale del'Eclairage (CIE) coordinates of (0.169, 0.177). Interestingly, both device 4 and 10 has low turn on voltage and negligible efficiency roll off at high current densities.

Full text of DOI:10.1016/j.orgel.2012.08.032

Organic Electronics 13 (2012) 3050–3059
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Efficient nondoped blue organic light-emitting diodes based
on phenanthroimidazole-substituted anthracene derivatives
Shaoqing Zhuang a,c, Ronggang Shangguan a, Jiangjiang Jin a, Guoli Tu a, Lei Wang a,
,
⇑
b
c,
Jiangshan Chen b, , Dongge Ma , Xunjin Zhu
⇑
⇑
a Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,
People’s Republic of China
c Centre for Advanced Luminescence Materials, Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Hong Kong, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 May 2012
Received in revised form 12 July 2012
Accepted 20 August 2012
Available online 6 September 2012
A series of new blue materials based on highly fluorescent di(aryl)anthracene and electron-
transporting phenanthroimidazole functional cores: 2-(4-(anthracen-9-yl)phenyl)-1-phe-
nyl-1H-phenanthro[9,10-d]imidazole (ACPI), 2-(4-(10-(naphthalen-1-yl)anthracen-9-
yl)phenyl)-1-p-henyl-1H-phenanthro[9,10-d]imidazole (1-NaCPI), 2-(4-(10-(naphthalen-
2-yl)anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (2-NaCPI) were
designed and synthesized. These materials exhibit good film-forming and thermal proper-
ties as well as strong blue emission in the solid state. To explore the electroluminescence
properties of these materials, three layer, two layer and single layer organic light-emitting
devices were fabricated. With respect to the three layer device 4 using ACPI as the emitting
layer, its maximum current efficiency reaches 4.36 cd Aꢀ1 with Commission Internationale
del’Eclairage (CIE) coordinates of (0.156, 0.155). In the single layer device 10 based on ACPI,
maximum current efficiency reaches 1.59 cd Aꢀ1 with Commission Internationale del’Eclai-
rage (CIE) coordinates of (0.169, 0.177). Interestingly, both device 4 and 10 has low turn on
voltage and negligible efficiency roll off at high current densities.
Keywords:
Blue material
Anthracene
Phenanthroimidazole
Organic light-emitting devices
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
the hunt for blue fluorescent materials and devices are still
a subject of current interest [4].
Since the pioneering work by Tang and Van Slyke [1],
significant progresses in materials and device engineering
have led to the potential application of full-color OLEDs
and solid-state lighting [2]. For full-color displays, it is
essential to have the three primary colors, red, green and
blue. Red and green phosphorescent electroluminescent
devices with high efficiencies, long lifetimes, and proper
CIE coordinates have been well developed. However, blue
phosphorescent devices are still the bottleneck for the high
CIE coordinates (y-coordinate value >0.30) and short de-
vice lifetime [3]. Therefore, to achieve marketable OLEDs,
Based on anthracene, an well-known building block and
starting material in OLEDs, new diverse of blue material
and corresponding devices have been reported previously
[5]. For example, Park group [6] have synthesized carba-
zole-based anthracene derivatives 3,6-di(anthracen-9-yl)-
9-phenyl-9H-carbazole (P-DAC) and attained device effi-
ciency of 3.14 cd Aꢀ1, color coordinates (0.16, 0.14), and
threshold voltage 3.8 V. Li’s team [7] have shown fluorene
based naphthylanthracene derivatives with a maximum
luminance efficiency of 4.04 cd Aꢀ1, CIE coordinates of
(0.15, 0.13) and a turn on voltage of 4.1 V. Recently, phen-
anthroimidazole derivatives have attracted great attention as
electroluminescent materials because of their high thermal
stability and efficient electron transporting ability. Huang’s
group [8] reported bis(phenanthroimidazolyl)biphenyl
⇑
Corresponding authors. Tel./fax: +86 27 87793032 (L. Wang).
E-mail addresses: wanglei@mail.hust.edu.cn (L. Wang), jschen@-
ciac.jl.cn (J. Chen), xjzhu@hkbu.edu.cn (X. Zhu).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.08.032

S. Zhuang et al. / Organic Electronics 13 (2012) 3050–3059
3051
derivatives as excellent non-doped blue emitting materials
with optimized device efficiency of 7.3 lm Wꢀ1, CIE coordi-
nates of (0.15, 0.14). By using phenanthroimidazole as a
building block for luminescent materials, Ma’s group [9]
also reported pure blue devices with luminance efficiency
of 6.87 cd Aꢀ1, CIE coordinates of (0.15, 0.21) and a turn
on voltage of 2.8 V. Although these relatively successful
blue materials were reported, many of them, the efficiency
roll off at high current densities, so it is still a great need to
design and synthesize stable blue materials which exhibit
little efficiency roll off at high current densities
[4a,5a,5b,5e,8]. One of the basic designs for blue materials
is to increase their electron affinities to realize balanced
charge injection and transport [8,9]. It should be noted
that, in spite of increasing electron transporting properties,
the incorporation of electron-withdrawing moiety in those
saturated blue-emitted materials always results in lower
HOMO levels, which mismatch with side layer and gener-
ate larger hole-injection barriers at the hole-transporter/
emitter junctions. Hence, in designing pure blue light emit-
ting materials for OLEDs, a compromise is needed between
electron transporting ability and adjusting the HOMO/
LUMO levels.
(DIMENSION 3100). 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 between 0.05 and
0.1. The excitation was performed at 330 nm and 9,10-
diphenylanthracene (DPA) in cyclohexane
(U = 0.9 in
cyclohexane) was used as a standard [13]. Cyclic voltam-
metry measurements were carried out in a conventional
three electrode cell using a Pt button working electrode
of 2 mm in diameter, a platinum wire counter electrode,
and a Ag/AgNO3 (0.1 M) reference electrode on a com-
puter-controlled EG&G Potentiostat/Galvanostat model
283 at room temperature. Oxidations CV of all compounds
were performed in dichloromethane containing 0.1 M tet-
rabutylammoniumhexafluorophosphate (Bu4NPF6) as the
supporting electrolyte. The onset potential was deter-
mined from the intersection of two tangents drawn at
the rising and background current of the cyclic voltammo-
gram and calibrated to the ferrocene/ferrocenium (Fc|Fc+)
redox couple. DFT calculations have been performed to
characterize the 3D geometries and the frontier molecular
orbital energy levels of ACPI, 1-NaCPI and 2-NaCPI at the
B3LYP/6-31G⁄ level by using the ADF2009.01 program.
In previous work [10,11], we have introduced electron-
rich benzothiophene or flourene moieties into anthracene,
to adjust the emitter’s HOMO/LUMO level and carrier
transporting ability. Here, we designed and synthesized
2.2. Preparation of compounds
2.2.1. Synthesis of compound 1
three
asymmetric
phenanthroimidazole-substituted
A 250 ml round-bottomed flask was charged with 9,10-
phenanthrenequinone (2.08 g, 10 mmol), aniline (0.93 g,
10 mmol), 4-bromobenzaldehyde (1.85 g, 10 mmol) and
ammonium acetate (9.24 g, 120 mmol) under nitrogen
atmosphere. Subsequently, 120 mL acetic acid was added
to the solution and the reaction mixture was stirred under
reflux for 36 h under an argon atmosphere. After cooled to
room temperature, the mixture was poured into water. The
separated solid was filtered off, washed with methanol and
dried to give a white solid without futher purification.
anthracene derivatives. The introduction of phenanthroim-
idazole moieties effectively increases the electron injection
and transport ability and finely adjusts the ionization
potentials (Ip) of the compounds, and further reduces the
hole injection barrier at the interface of the hole-transport-
ing layer (HTL) and the emissive layer and balanced recom-
bination [12]. As expected, these materials show perfect
thermal stability and excellent EL performance with low
onset voltages, which indicate that these new materials
are a promising class of blue emitters for practical applica-
tions in panel display.
Yield 3.68 g (82%). 1H-NMR: (DMSO-d6, 400 MHz):
d
(ppm) 8.94 – 8.91 (d, J = 8.4 Hz, 1H), 8.89 – 8.86 (d,
J = 8.4 Hz, 1H), 8.71 – 8.68 (dd, J = 1.2, 8.0 Hz, 1H), 7.80 –
7.67 (m, 7H), 7.58 – 7.54 (m, 3H), 7.51 – 7.49 (dd, J = 2.0,
6.8 Hz, 2H), 7.51 – 7.49 (t, J = 7.2 Hz, 1H), 7.09 – 7.07 (d,
J = 8.0 Hz, 1H). MS (APCI) (m/z): [M+H+] calcd for C27H18-
BrN2, 450.3; found, 450.4.
2. Experimental section
2.1. Materials and methods
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. Mass spectra were carried out on an Agilent
(1100 LC/MSD Trap) using APCI ionization. Elemental anal-
yses of carbon, hydrogen, and nitrogen were performed on
an Elementar (Vario Micro cube) analyzer. Fluorescence
spectra were obtained on Edinburgh instruments (FLSP920
spectrometers) and UV–Vis spectra were measured using a
Shimadzu UV–VIS–NIR Spectrophotometer (UV-3600). The
differential scanning calorimetry (DSC) analysis was per-
formed under a nitrogen atmosphere at a heating rate of
10 °C/min using a PE Instruments DSC 2920. Thermogravi-
metric analysis (TGA) was undertaken using a PerkinElmer
Instruments (Pyris1 TGA) under nitrogen atmosphere at a
heating rate of 10 °C/min. AFM was measured using Veeco
2.2.2. Synthesis of compound ACPI
A 100 ml two-neck round-bottomed flask was added
compound 1 (0.90 g, 2 mmol), anthracen-9-ylboronic acid
(0.49 g, 2.2 mmol), toluene (30 ml), ethanol (15 ml), 2 M
K2CO3 (30 ml, 60 mmol) aqueous solution and tetrakis-
(triphenylphosphine) palladium (0) (0.12 g, 0.1 mmol) in
turn, then the reaction mixture was refluxed under nitro-
gen for 24 h in the absence of light. After the reaction
was completed, cooled to room temperature. The solution
was extracted with dichloromethane, dried by MgSO4.
The organic layer was concentrated under a reduced pres-
sure and purified by column chromatography and dried
under vacuum to yield a white solid. Yield: 90%. 1H-
NMR: (DMSO-d6, 400 MHz):
d (ppm) 8.95 – 8.93 (d,
J = 8.0 Hz, 1H), 8.90 – 8.88 (d, J = 8.0 Hz, 1H), 8.79 – 8.76
(d, J = 8.0 Hz, 1H), 8.66 (s, 1H), 8.15 – 8.12 (d, J = 8.0 Hz,

3052
S. Zhuang et al. / Organic Electronics 13 (2012) 3050–3059
2H), 7.84 – 7.77 (m, 5H), 7.76 – 7.68 (m, 4H), 7.59 – 7.46
(m, 5H), 7.44 – 7.33 (m, 5H), 7.15 – 7.13 (d, J = 8.0 Hz,
1H). MS (APCI) (m/z): [M+H+] calcd for C41H27N2, 547.7;
found, 547.2. 13C NMR (100 MHz, DMSO-d6) d (ppm)
150.68, 139.14, 138.75, 137.10, 135.92, 131.31, 131.25,
130.90, 130.81, 130.14, 129.79, 129.69, 129.60, 129.65,
128.94, 128.40, 128.23, 128.00, 127.25, 127.14, 126.46,
126.27, 126.23, 125.80, 125.75, 125.00, 124.17, 123.00,
122.53, 120.72. Anal. Calcd for C41H26N2: C, 90.08; H,
4.79; N, 5.12. Found: C, 89.90; H, 4.89; N, 5.21.
ent conditions. By the way, in counting the layers of the
device, we only consider the hole transport layer, emissive
layer and electron transport layer.
3. Results and discussion
3.1. Synthesis
The structures and synthetic routes of the three well-
defined compounds are shown in Scheme 1. Anthracen-
9-ylboronic acid, 9-(1-naphthyl)anthracene-10-boronic
acid, and 9-(2-naphthyl)anthracene-10-boronic acid were
synthesized according to the literature [14]. The interme-
diate 2-(4-bromophenyl)-1-phenyl-1H-phenanthro[9,10-
d]-imidazole was synthesized in high yield [12]. The title
products were prepared through the palladium-catalyzed
Suzuki coupling reaction of the intermediate and the corre-
sponding aryl boronic acid, then purified by column chro-
matography on silica gel using petroleum ether-
dichloromethane as the eluent. Repeated temperature-gra-
dient vacuum sublimation are required for further purifica-
tion of these materials used in OLEDs. All the new
compounds were fully characterized by 1H and 13C NMR,
mass spectrometry, and elemental analysis.
2.2.3. Synthesis of compound 1-NaCPI
The compound was synthesized using a similar proce-
dure as for compound ACPI, yield: 85%. 1H-NMR: (DMSO-
d6, 400 MHz): d (ppm) 8.99 – 8.97 (d, J = 8.0 Hz, 1H),
8.94 – 8.92 (d, J = 8.0 Hz, 1H), 8.79 – 8.77 (d, J = 8.0 Hz,
1H), 8.21 – 8.19 (d, J = 8.0 Hz, 1H), 8.15 – 8.13 (d,
J = 8.0 Hz, 1H), 7.96 – 7.71 (m, 10H), 7.62 – 7.51 (m, 7H),
7.46 – 7.28 (m, 8H), 7.17 – 7.15 (d, J = 8.0 Hz, 1H), 6.94 –
6.92 (d, J = 8.0 Hz, 1H). MS (APCI) (m/z): [M+H+] calcd for
C41H27N2, 547.7; found, 547.2. 13C NMR (100 MHz,
CDCl3) d (ppm) 136.68, 136.59, 135.31, 133.71, 133.58,
131.45, 131.40, 130.61, 130.29, 130.03, 129.76, 129.45,
129.25, 129.19, 128.40, 128.25, 128.16, 127.41, 127.11,
126.85, 126.61, 126.38, 126.29, 126.02, 125.77, 125.59,
125.22, 125.04, 124.20, 123.17, 123.06, 122.93, 120.96.
Anal. Calcd for C51H32N2: C, 91.04; H, 4.79; N, 4.16. Found:
C, 90.61; H, 4.87; N, 4.33.
3.2. Thermal properties
The thermal properties of the three compounds were
investigated by thermal gravimetric analysis (TGA) and dif-
ferential scanning calorimetry (DSC) under a nitrogen
atmosphere, and the related data are listed in Table 1.
Compounds ACPI, 1-NaCPI and 2-NaCPI exhibit good ther-
mal stability with decomposition temperatures (Td, 5%
weight loss) at 410, 441 and 473 °C, respectively. As shown
in Fig. 1, no obvious glass transition temperatures (Tg) are
observed for the three molecules, while endothermic melt-
ing transition temperatures (Tm) appear obviously at 290,
345 and 354 °C for ACPI, 1-NaCPI and 2-NaCPI, respec-
tively. Such high Tm and Td values indicate that these
compounds are stable and has the potential to be fabri-
cated into devices by vacuum thermal evaporation tech-
nology, which is highly desirable for high performance
OLED applications.
2.2.4. Synthesis of compound 2-NaCPI
The compound was synthesized using a similar proce-
dure as for compound APCI. Yield: 90%. 1H-NMR: (DMSO-
d6, 400 MHz): d (ppm) 8.97 – 8.95 (d, J = 8.0 Hz, 1H),
8.92 – 8.90 (d, J = 8.0 Hz, 1H), 8.80 – 8.77 (d, J = 8.0 Hz, 1H),
8.20 – 8.17 (d, J = 8.0 Hz, 1), 8.13 – 8.10 (d, J = 8.0 Hz, 1H),
8.04 – 8.01 (d, J = 10.0 Hz, 2H), 7.90 – 7.70 (m, 9H), 7.67 –
7.54 (m, 8H), 7.79 – 7.35 (m, 7H), 7.17 – 7.14 (d, J = 8.0 Hz,
1H). MS (APCI) (m/z): [M+H+] calcd for C51H33N2, 673.8;
found, 673.5. 13C NMR (100 MHz, DMSO-d6) d (ppm)
150.70, 139.38, 138.78, 137.18, 137.11, 136.35, 136.08,
133.50, 132.86, 131.37, 130.95, 130.88, 130.28, 130.25,
129.84, 129.74, 129.67, 129.55, 129.07, 128.60, 128.46,
128.29, 128.24, 128.03, 127.25, 127.18, 127.12, 127.04,
126.67, 126.31, 126.20, 126.14, 125.78, 125.04, 124.21,
123.02, 122.52, 120.73. Anal. Calcd for C51H32N2: C, 91.04;
H, 4.79; N, 4.16. Found: C, 90.61; H, 4.87; N, 4.33.
3.3. Morphology properties
2.3. Device fabrication and measurement
Since efficient film-forming properties of light emitting
materials are crucial for the performance of the devices,
the surface morphologies of vacuum-deposited thin films
of the three new compounds, ACPI, 1-NaCPI and 2-NaCPI,
were studied by atomic force microscopy (AFM). For a di-
rect comparison, we prepared both unannealed films and
thermally annealed samples films at 120 °C for 2 h under
an N2 atmosphere. As shown in Fig. 2, the unannealed film
samples exhibit a root-mean-square (RMS) roughness of
0.361, 0.384 and 0.280 nm for ACPI, 1-NaCPI and 2-NaCPI,
respectively. The annealed film exhibits a fairly smooth
The EL devices were fabricated by vacuum deposition of
the materials at a base pressure of 5 ꢁ 10ꢀ6 Torr onto glass
precoated with a layer of indium tin oxide (ITO) with a
sheet resistance of 25
X/square. Before deposition of an or-
ganic layer, the clear ITO substrates were treated with ox-
gen plasma for 5 min. The deposition rate of organic
compounds was 0.9–1.1 Å sꢀ1. Finally, a cathode composed
of cesium pivalate (2 nm) and aluminum (100 nm) was
sequentially deposited onto the substrate in the vacuum
of 10ꢀ5 Torr. The L–V–J of the devices was measured with
a Keithley 2400 Source meter and PR655. All measure-
ments were carried out at room temperature under ambi-
surface morphology with
a root-mean-square (RMS)
roughness of 0.283, 0.321 and 0.274 nm for ACPI, 1-NaCPI
and 2-NaCPI, respectively. The small RMS difference

S. Zhuang et al. / Organic Electronics 13 (2012) 3050–3059
3053
NH2
CHO
Br
O
O
N
N
N
+
i) NH4Ac
HAc
+
ii) Ar(OH)2
Pd(PPh3)4
Br
N
1
ACPI
ii
ii
N
N
N
N
1-NaCPI
2-NaCPI
Scheme 1. Synthetic route of APCI, 1-NaCPI and 2-NaCPI. Reagents and conditions: (i) NH4Ac, HAc, reflux; (ii) toluene, K2CO3, ethanol, aryl boric acid,
Pd(PPh3)4, reflux.
Table 1
The optical, photophysical and thermal properties of ACPI, 1-NaCPI, and 2-NaCPI.
Compound
HOMO/LUMO (eV)a
HOMO/LUMO (eV)b
UF
Eg (eV)
Eox (V)
PL (nm)
Solution
Abs (nm)
Tg/Tm/Td (°C)
Film
ACPI
1-NaCPI
2-NaCPI
ꢀ5.62/ꢀ2.54
ꢀ5.58/ꢀ2.56
ꢀ5.55/ꢀ2.55
ꢀ5.30/ꢀ2.29
ꢀ5.25/ꢀ2.35
ꢀ5.24/ꢀ2.29
0.87
0.71
0.64
3.08
3.02
3.00
0.82
0.78
0.75
427
431
437
456
455
457
349, 367, 387
358, 376, 396
359, 376, 396
ND/290/410
ND/345/441
ND/354/473
a
Determined from the onset of oxidation potentials and the Eg = HOMO–LUMO.
Values from DFT calculation.
b
100
(a) ACPI
(b)
ACPI
80
1-NaCPI
1-NaCPI
2-NaCPI
2-NaCPI
60
40
20
50
100 150 200 250 300 350
TemperatureoC
100 200 300 400 500 600 700
TemperatureoC
Fig. 1. (a) DSC curves of ACPI, 1-NaCPI and 2-NaCPI; (b) TGA curves of ACPI, 1-NaCPI and 2-NaCPI.
between unannealed and annealed film samples suggests
their excellent thermal and amorphous stability, which
indicates that attaching phenanthroimidazole moiety to
anthracence’s 9-position may influence the arrangement
of the molecules in the thin films and makes them desir-
able for good performance OLEDs.
HOMO and LUMO levels are depicted in Table 1. As shown
in Fig. 3, the spatial distributions of electron densities of
HOMO and LUMO for all the three compounds are mostly
localized on the anthracene units, it implies that the
absorption and emission process may mainly be attributed
to the
p–
p⁄ transition centered at the anthracene moiety.
The HOMO level of ACPI, 1-NaCPI and 2-NaCPI locate at
ꢀ5.30, ꢀ5.25 and ꢀ5.24 eV, respectively. The HOMO levels
for ACPI, 1-NaCPI and 2-NaCPI increase about 0.04–0.05 eV
for the elongation of conjugation when intruducing naph-
thyl group on the 9-position of anthracene of ACPI. In addi-
tion, the calculated geometries of ACPI, 1-NaCPI and 2-
3.4. Theoretical calculations
The three-dimensional geometries and the frontier
molecular orbital energy levels of these compounds were
calculated using density functional theory. The resulted

3054
S. Zhuang et al. / Organic Electronics 13 (2012) 3050–3059
Fig. 2. AFM topographic images of the three compounds in unannealed and annealed films.
Fig. 3. Three-dimensional structures and calculated HOMO and LUMO density maps of ACPI, 1-NaCPI and 2-NaCPI.
NaCPI indicate the non-planarity of their molecular struc-
tures. These geometrical characteristics can effectively pre-
3.5. Photophysical properties
vent intermolecular interactions between
thus, suppress molecular recrystallization.
p
-systems and,
Fig. 4(a) shows the UV–Vis spectra of compounds ACPI,
1-NaCPI and 2-NaCPI at room temperature in a CH2Cl2

S. Zhuang et al. / Organic Electronics 13 (2012) 3050–3059
3055
solution. And Fig. 4(b) shows the fluorescence spectra of
compounds ACPI, 1-NaCPI and 2-NaCPI at room tempera-
ture in both CH2Cl2 solution and solid state. The related
photophysical properties are summarized in Table 1. All
of these compounds have similarly structured absorption
spectra in the range of 380–425 nm in solution, which
phosphate (Bu4NPF6) dissolved in dichloromethane
(Fig. 5). The onset oxidation peaks (Eox) for the three com-
pounds are 0.82, 0.78, and 0.75 eV versus the ferrocenium/
ferrocene redox couple, respectively. Thus, the HOMO lev-
els were determined to be ꢀ5.62, ꢀ5.58 and ꢀ5.55 eV
respectively (Table 1) by the method – (Eox + 4.80) eV re-
ported by Chen [16]. The HOMO levels for both 1-NaCPI
and 2-NaCPI with a peripheral naphthalene group are low-
er than that of ACPI. It could be rationalized that the elec-
tron-donating substituent might reduce the oxidation
potentials of the molecule and increase the HOMO energy
level. This trend is in good agreement with the calculated
values. The lower energy barrier between emitting layer
of 1-NaCPI or 2-NaCPI and the hole transporting layer will
facilitate hole injection into the emission layer.
are assigned to the
p–
p⁄ transition of the characteristic
vibrational structures of the isolated anthracene groups.
The 10 nm blue shift of ACPI compared to 1-NaCPI and 2-
NaCPI can be attributed to its short conjugation. And upon
excitation at 330 nm, the maximum emission wavelengths
of ACPI, 1-NaCPI and 2-NaCPI in CH2Cl2 solution were ob-
served at 427, 431 and 437 nm, respectively. Compared
with ACPI, 1-NaCPI and 2-NaCPI show bathochromic shifts
of 4 and 10 nm respectively, due to the lengthened conju-
gation by the naphthalene group. On the other side, the
greater steric hindrance of 1-naphthyl than 2-naphthyl
leads to an increasing non-planarity property of naphtha-
lene ring to anthracene group, which also annotate that
2-NaCPI shows larger bathochromic shift than 1-NaCPI
[14]. The maximum emission peaks of ACPI, 1-NaCPI and
2-NaCPI in thin films are red-shifted by 29, 24 and 20 nm
respectively, compared to the corresponding ones in
CH2Cl2 solution. These features are reasonable for organic
and polymer materials due to intermolecular interactions
or exciton hopping in the solid state [15]. Moreover, a
gradual decreasing red-shift effects was observed with an
increasing elongation of conjugation, ACPI (29 nm) > 1-
NaCPI (24 nm) > 2-NaCPI (20 nm). The quantitative
enhancement of emission was evaluated by the PL quan-
tum yields (UF), using 9,10-diphenylanthracene as a stan-
dard. In toluene, blue emitters ACPI, 1-NaCPI and 2-NaCPI
exhibit quantum yields of 0.87, 0.71 and 0.62, respectively.
Therefore, these compounds are excellent candidates for
using as efficient emitting materials in OLEDs.
3.7. Electroluminescence
To explore the electron injection and transport ability of
ACPI, 1-NaCPI and 2-NaCPI, an electron-only devices with
configuration: Al (100 nm)/TPBI(40 nm)/ACPI, 1-NaCPI or
2-NaCPI or m-ADN (40 nm)/LiF (1 nm)/Al (100 nm) were
fabricated, respectively. For the electron-only devices, the
TPBI layer, which has a low-lying HOMO level of ꢀ6.2 eV,
was used to prevent hole-injection from the Al anode
(ꢀ4.3 eV) to the organic layers. Here, a typical anthracene
derivative 2-methyl-9,10-di(naphthalen-2-yl)anthracene
(m-ADN) was used as a stand. All of the three new com-
pounds exhibit higher current density than m-ADN at the
same voltage (see Supplementary Data Fig. S1), which
means the introduction of phenanthroimidazole group to
anthracene increases the electron injection and transport
ability of anthracene.
To evaluate the EL performances of ACPI, 1-NaCPI and
2-NaCPI as blue emitters, three type (three layer: anode/
HTL/emitter/ETL/cathode, two layer: anode/HTL/emitter/
cathode, Single layer: anode/emitter/cathode), a total of
12 devices were fabricated. In these devices, indium tin
oxide (ITO) is used as the anode, 4,40,400-tris[N-(1-naph-
thyl)-N-phenylamino]triphenylamine (2-TNATA) or MoO3
are used as the hole-injection layer, 4,4-bis[N-(1-naph-
thyl)-N-phenyl-l-amino]biphenyl (NPB) is used as the hole
transporting layer (HTL), tris(8-hydroxy-quinolinato)alu-
minum (Alq3) or 1,3,5-tris(1-phenyl-1H-benzimidazol-2-
yl)benzene (TPBI) are used as the electron transporting
3.6. Electrochemical properties
In addition to the photophysical properties, the electro-
chemical properties are also very important in evaluating a
material’s usefulness for optoelectronic applications. The
electrochemical behaviors of the compounds ACPI, 1-NaCPI
and 2-NaCPI were examined by cyclic voltammetry using a
standard three-electrode electrochemical cell in an electro-
lyte solution of 0.1 M tetrabutylammoniumhexafluoro-
1.0
1.0
(a)
ACPI solution PL
1-NaCPI solution PL
(b)
ACPI
1-NaCPI
2-NaCPI
2-NaCPI solution PL
0.8
0.8
ACPI film PL
1-NaCPI film PL
2-NaCPI film PL
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
300
350
400
450
350
400
450 500
550
600 650
Wavelength (nm)
Wavelength (nm)
Fig. 4. (a) Normalized absorption spectra of compound ACPI, 1-NaCPI and 2-NaCPI in dichloromethane; (b) normalized photoluminescence spectra in
dichloromethane and film state.

3056
S. Zhuang et al. / Organic Electronics 13 (2012) 3050–3059
(2 nm)/Al (100 nm), were fabricated for further evaluation
ACPI
1-NaCPI
2-NaCPI
of EL performances. Since TPBI has lower HOMO of 6.2 V,
using TPBI as electron transport material to instead Alq3,
can prevent the injection of hole into Alq3 and avoid the
emission of Alq3.
As predicted, the CIE of devices 4, 5 shifted to blue obvi-
ously (see Fig 7(b)) when using TPBI to instead of Alq3,
from (0.1560, 0.2168) to (0.1562, 0.1551), from (0.1655,
0.1996) to (0.1503, 0.1669), respectively. However, for
the compound 2-NaCPI, the CIE exhibits negligible change,
from (0.1505, 0.1605) to (0.1508, 0.1573). These phenom-
ena can be explained from the energy diagrams, the HOMO
level gap in Alq3/EML interface (0.08, and 0.12 eV for ACPI,
1-NaCPI, respectively) is too small to block the diffusion of
exciton, especially for ACPI. Meanwhile, narrower EL peaks
in the devices 4, 5 and 6 with FWHMs of 65, 71 and 59 nm
respectively, (see Fig. 7), which is almost correlated with
the PL in solid state. With respect to devices 4, 5 and 6,
TPBI acts both as an electron-transporter and as a hole-
blocking layer, and the exciton can be confined to the
emissive layer efficiently. Noteworthily, whether the elec-
tron transporting layer is Alq3 or TPBI, 2-NaCPI exhibits
high efficiency, and pure blue light emission in the device
due to its suitable HOMO level. Fig. 8 reveals the current
density–current efficiency characteristics of devices 4, 5
and 6. Devices 4, 5 and 6 exhibit a maximum brightness
of 11,470, 14,680, and 11,840 cd mꢀ2, and luminous effi-
ciency of 4.36, 4.22, 4.68 cd Aꢀ1, respectively (see Table 2).
For APCI and 2-NaCPI, the maximum power efficiency was
0.2
0.4
0.6
0.8
1.0
1.2
+
Potential(V vs Fc/Fc )
Fig. 5. The CV measurement of ACPI, 1-NaCPI and 2-NaCPI.
layer (ETL), and (CH3)3CCOOCs is used as the electron
injecting layer [17]. The relative HOMO/LUMO energy lev-
els of the materials are illustrated in Scheme 2, and de-
tailed structures and key performance of the 12 devices
are summarized in Table 2.
Non-doped devices 1, 2 and 3 were fabricated with typ-
ical configuration as follows: ITO/2-TNATA(60 nm)/NPB
(10 nm)/emitting layer (40 nm)/Alq3 (15 nm)/ (CH3)3-
CCOOCs (2 nm)/Al (100 nm). The current density–volt-
age–luminance (I–V–L) and current density–current
efficiency characteristics are shown in Fig. 6. Simple de-
vices 1, 2, 3 utilizing compounds ACPI, 1-NaCPI and 2-NaC-
PI as host exhibit a maximum brightness of 18,460, 16,000,
and 12,250 cd mꢀ2
, luminous efficiency of 4.62, 4.36,
4.65 cd Aꢀ1, low threshold voltage of 2.83, 2.73, 2.71 V,
respectively (see Table 2). The low turn on voltage can be
attributed to the fact that reasonable HOMO level of the
compounds reduce the hole injection barrier at the inter-
face between the hole-transporting layer and the emissive
layer. The EL spectra of 1, 2 and 3 show blue light emission
with peaks at 472, 468 and 460 nm, respectively (Fig. 7(a)).
Narrow EL peaks in the ACPI, 1-NaCPI and 2-NaCPI devices
with FWHMs of 77, 72 and 61 nm, respectively, are ob-
served. At a current density of 20 mA cmꢀ2, the CIE coordi-
nates of devices 1, 2 and 3 are (0.1560, 0.2168), (0.1655,
0.1996) and (0.1505, 0.1605), respectively.(see Table 2)
The EL spectra of the devices fabricated using ACPI and
1-NaCPI as the emitting layers have a little red shift com-
pared to 2-NaCPI. It may be ascribed to a slight contribu-
tion from Alq3 emission [18].
enhanced to 3.03 and 3.6 lm Wꢀ1
. We attribute this
enhancement to the better electron transporting ability
of TPBI.
To explore the possible electron transporting properties
of these materials, a bilayer device 7, 8, 9 with a structure
of ITO/2-TNATA (60 nm)/NPB (10 nm)/ACPI, 1-NaCPI or 2-
NaCPI (60 nm)/(CH3)3CCOOCs (2 nm)/Al (100 nm) were
also fabricated. Here, ACPI, 1-NaCPI or 2-NaCPI was used
as both the ETL and the emitter. Devices 7, 8 and 9 exhibit
a
maximum brightness of 10,570, 11,650, and
15,030 cd mA mꢀ2, and luminous efficiency of 3.32, 2.52,
3.64 cd Aꢀ1
the CIE coordinates of (0.1573, 0.1529),
,
(0.1531, 0.1711), (0.1603, 0.1879), respectively (see Ta-
ble 2). Compared with Alq3 based devices 1, 2 and 3, the
contiguous numerical value of working voltage suggests
that these materials are an electron-transporting type blue
host material.
In order to confirm the above hypothesis, devices 4, 5
and 6 for ACPI, 1-NaCPI and 2-NaCPI with the same config-
uration as follows: ITO/2-TNATA (60 nm)/NPB (10 nm)/
emitting layer (40 nm)/TPBI (15 nm)/(CH3)3CCOOCs
For organic semiconductors, conductivities of emissive
materials with electron-transport properties are generally
2
4
6
2.3
2.5
2.56
2.54
2.55
2.7
2.9
LUMO
(CH3)3CCOOCs/Al
I
I
P
A
I
P
I
3
C
P
T
B
C
B
a
q
a
P
C
l
A
P
N
N
-
N
A
A
N
ITO
HOMO
T
-
1
T
2
2
5.2
5.3
5.55
5.58
5.62
5.7
6.2
Scheme 2. Energy-level diagram of the materials used in the devices.

S. Zhuang et al. / Organic Electronics 13 (2012) 3050–3059
3057
Table 2
EL performance of devices 1–12.
Device Von
Lmax (Voltage) (cd mꢀ2
)
gP (lm
gC
gext (%)g,h L (Voltage) (cd mꢀ2
)
kem (FWHM)
CIE (x,y)k
(V)a
(V)b
Wꢀ1)c,d
(cd Aꢀ1)e,f
(V)i
(nm)j
1
2
2.83
2.73
2.71
2.76
3.07
2.66
3.03
2.86
2.76
3.05
2.96
2.90
18,460 (10.80)
2.29 (2.96)
2.46 (3.22)
2.48 (3.18)
2.55 (3.03)
2.12 (2.46)
2.95 (3.60)
1.89 (2.13)
1.35 (1.42)
2.06 (2.59)
0.94 (1.36)
0.48 (0.78)
0.85 (1.40)
4.30 (4.62)
4.32 (4.36)
4.63 (4.65)
4.17 (4.36)
4.13 (4.22)
4.67 (4.68)
3.10 (3.32)
2.45 (2.52)
3.61 (3.64)
1.50 (1.59)
0.86 (0.93)
1.64 (1.72)
2.72
(3.02)
2.82
(2.88)
3.59
(3.61)
3.27
(3.52)
3.13
(3.23)
3.65
(3.68)
2.46
(2.73)
1.83
(1.89)
2.47
(2.52)
1.04
(1.16)
0.64
(0.69)
0.99
860 (5.91)
864 (5.52)
926 (5.87)
834 (5.13)
826 (6.11)
934 (5.01)
620 (5.15)
490 (5.69)
722 (5.52)
300 (4.99)
172 (5.50)
328 (6.08)
472 (77)
468 (72)
460 (61)
456 (65)
468 (71)
460 (59)
456 (66)
468 (65)
468 (63)
456 (70)
468 (65)
468 (67)
(0.1560,
0.2168)
(0.1655,
0.1996)
(0.1505,
0.1605)
(0.1562,
0.1551)
(0.1503,
0.1669)
(0.1508,
0.1573)
(0.1573,
0.1529)
(0.1531,
0.1711)
(0.1603,
0.1897)
(0.1699,
0.1774)
(0.1578,
0.1658)
(0.1916,
0.2267)
16,000 (10.50)
12,250 (10.30)
11,470 (8.25)
14,680 (10.50)
11,840 (9.25)
10,570 (8.50)
11,650 (10.50)
15,030 (9.75)
7357 (7.50)
3
4
5
6
7
8
9
10
11
12
3153 (11.00)
7109 (11.00)
(1.03)
a
b
c
d
e
f
Von: turn-on voltage.
Lmax: maximum luminance. Voltage: voltage at the maximum luminance.
P: power efficiency measured at 20 mA cmꢀ2
Maximum power efficiency.
C: current efficiency measured at 20 mA cmꢀ2
Maximum current efficiency.
g
.
g
.
g
h
i
g
ext: external quantum efficiency measured at 20 mA cmꢀ2
Maximum external quantum efficiency.
L: luminance measured at 20 mA cmꢀ2. Voltage: voltage at the luminance.
V: FWHM: full width at half maximum at 20 mA cmꢀ2
.
j
.
k
kem: CIE at 20 mA cmꢀ2
.
500
400
300
200
100
0
4
3
2
(b)
5
4
3
2
1
(a)
104
103
102
101
100
device 1
device 2
device 3
1
0
device 1
device 2
device 3
0
0
50
100
150
200
250
2
4
6
8
10
Current Density (mA/cm2)
Voltage (V)
Fig. 6. (a) Voltage–current density–luminance (V–J–L) characteristics and (b) efficiencies curves for blue devices 1, 2 and 3. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)
lagging behind those materials with hole-transport proper-
ties. So it is possible to construct a single layer device using
the electron-type material. In order to verify the hypothe-
sis, a single layer device 10, 11, 12 with a structure of ITO/
MoO3 (10 nm)/ACPI, 1-NaCPI or 2-NaCPI (100 nm)/
(CH3)3CCOOCs (2 nm)/Al (100 nm) were also fabricated.
Devices 10, 11 and 12 exhibit a maximum brightness of
7357, 3153, and 7109 cd mꢀ2, and luminous efficiency of
1.59, .0.93, 1.72 cd Aꢀ1, the CIE coordinates of (0.1699,
0.1774), (0.1578, 0.1658), (0.1916, 0.2267), respectively
(see Table 2). Compared to the multilayer devices, part of
CIE coordinates of single device has changed obviously, it
may be caused by the intermolecular interactions and the
thickness of emitting layer, and the same phenomena were
also observed in the reported single layer device [5g].
Clearly, the maximum efficiencies of these single devices

3058
S. Zhuang et al. / Organic Electronics 13 (2012) 3050–3059
1.0
(b)
1.0
0.8
0.6
0.4
0.2
0.0
(a)
device 4 CIE(0.1562, 0.1551)
device 1 CIE(0.1560, 0.2168)
device 2 CIE(0.1655, 0.1996)
device 3 CIE(0.1505, 0.1605)
device 5 CIE(0.1503, 0.1669)
device 6 CIE(0.1508, 0.1573)
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 7. (a) EL spectrum of devices 1, 2 and 3 and (b) EL spectrum of devices 4, 5 and 6.
500
400
300
200
100
0
6
5
4
3
2
1
0
device 4
(a)
(b)
104
103
102
101
100
device 5
device 6
device 4
device 5
device 6
5
4
3
2
1
0
2
4
6
8
10
0
50
100
150
200
250
Current Density (mA/cm2)
Voltage (V)
Fig. 8. (a) Voltage–current density–luminance (V–J–L) characteristics and (b) efficiencies curves for blue devices 4, 5 and 6. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)
(a)500
(b) 4
Device 7
Device 9
Device 11
Device 8
Device 10
Device 12
Device 7
Device 8
Device 9
Device 10
Device 11
Device 12
104
103
102
101
100
400
300
200
100
0
3
2
1
0
0
100
200
300
400
2
4
6
8
10
Current Density (mA/cm2)
Voltage (V)
Fig. 9. (a) Voltage–current density–luminance (V–J–L) characteristics and (b) efficiencies curves for devices 7–12.
are below those of the bilayer and trilayer devices (devices
the performance of the ACPI based devices are comparable
in terms of turn-on voltage, luminance and efficiency and
efficiency roll off to anthracene-based non-doped devices
with CIE x + y ꢂ 0.30 [6,7,19–24].
1–9). Devices 10 and 12 showing better perfromance than
device 11 could be attributed more large steric hindrance
of 1-NaCPI which will reduce its electron-transporting
ability [14]. On the other hand, it is worth noting that all
the single layer devices show little efficiency roll-off as
current density increases (Fig. 9).
4. Conclusions
To further access the performance of ACPI as a blue
emitter, performance of device 4 was compared with those
of similarly structured blue emitting devices reported
recently (Table 3) [6,7,19–24]. Device 4 has negligible effi-
ciency roll off at high current densities. Such as, compared
to the maximum current efficiency, the efficiency roll off of
device 4 is only 8.2% when the current density increased to
200 cd Aꢀ1. By the way, it can be seen from the Table 3 that
In summary, a series of new robust blue fluorescence
materials were developed based on anthracene and phen-
anthro[9,10-d]imidazole functional cores. These com-
pounds with fitly energy levels and increasing electron
transport ability, exhibit strong blue emission in both solid
state and EL device. Non-doped device 4 using ACPI as the
emitting layer, its maximum electroluminescence effi-

S. Zhuang et al. / Organic Electronics 13 (2012) 3050–3059
3059
Table 3
Key characteristics of devices 4 and other recently reported anthracene-based non-doped blue emitting devices with CIE x + y ꢂ 0.3.
a
c
c
g2c00
grcoll off
(%)
Emitter
Von
(V)
gmc ax
gmP ax
CIE (x, y)
Moiety
Lmax
(cd mꢀ2
Refs.
(cd Aꢀ1
)
(lm Wꢀ1
)
(cd Aꢀ1
)
)
ACPI
2.76
4.36
4.0
8.2
3.03
(0.156,
Phenanthroimidazole 11,470
Device
4
0.155)d
TPVAn
P-DAC
NAF1
4.9
3.8
4.1
na
5.3
3.2
3.16
2.6
3.7
1.2
1.4
na
39.6
+2
na
(0.14, 0.12)
(0.16, 0.14)
(0.15, 0.13)
(0.15, 0.19)
(0.17, 0.16)
(0.15, 0.16)
(0.15, 0.14)
(0.15, 0.16)
(0.18, 0.21)
Tetraphenylethylene
Carbazole
Fluorene
Arylamine
Carbazole
Fluorene, arylamine
Truxene
Fluorene, arylamine
Fluorene
No
na
[4a]
[6]
[7]
[19]
[20]
[21]
[22]
[23]
[24]
3.14
4.04
4.54
2.10
1.65
2.98
1.65
5.1
0.92e
2.20
4.02f
na
35.6
18.5
42.8
15.2
na
6528
24,910
1576
4586
1318
4586
12,508
TPA-AN
tCz9Ph2Ant 5.5
4
4.6
5.9
4.6b
6.5
na
1.52
na
NPAT
2
DPAPFB
na
3.9
na
23.5
1.10
Part of data was obtained from the figure of the reference.
a
Von: onset voltage obtained at 1 cd mꢀ2
Obtained at 10 cd mꢀ2
.
B
c
d
e
f
.
gmc ax gc200 gPmax and Lmax are maximum current efficiency, current efficiency at 200 mA cmꢀ2, power efficiency and luminance respectively.
, ,
Obtained at 20 mA cmꢀ2
.
Obtained at 100 mA cmꢀ2
PEDOT:PSS inserted.
.
ciency reaches 4.36 cd Aꢀ1 with Commission Internatio-
nale del’Eclairage (CIE) coordinates of (0.156, 0.155). In
the single layer device 10, its maximum efficiency reaches
1.59 cd Aꢀ1 with Commission Internationale del’Eclairage
(CIE) coordinates of (0.169, 0.177). Interestingly, both de-
vice 4 and 10 has negligible efficiency roll off at high cur-
rent densities, which is among the best reported results
for anthracene-based nondoped simple devices.
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