A pyrene-phenanthroimidazole derivative for non-doped blue organic light-emitting devices

Article abstract of DOI:10.1016/j.dyepig.2013.01.022

We report a newly synthesized blue fluorescent pyrene-phenanthroimidazole based emitter Py-BPI and its applications in non-doped organic light-emitting devices (OLEDs). Py-BPI has a high fluorescence quantum yield of 40% and good thermal stability with a high glass transition temperature of 137 °C. Based on Py-BPI, OLEDs with several configurations have been fabricated. Among these devices, an OLED with only two organic layers of Py-BPI and BPhen demonstrates blue emission with high efficiencies of 3.93 cd/A and 3.2 lm/W and maintains a high efficiency of 3.83 cd/A even at a brightness of 5000 cd/m². Combining with its hole-transporting ability, Py-BPI is shown to be a versatile blue emitter especially suitable for high brightness applications.

Full text of DOI:10.1016/j.dyepig.2013.01.022

Dyes and Pigments 98 (2013) 190e194
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A pyrene-phenanthroimidazole derivative for non-doped blue organic
light-emitting devices
,
,
c
c
Ying Zhang a b, Tsz-Wai Ng b, Feng Lu a, Qing-Xiao Tong a , Shiu-Lun Lai , Mei-Yee Chan ,
**
Hoi-Lun Kwong d, Chun-Sing Lee b
,
*
a Department of Chemistry, Shantou University, Guangdong 515063, China
b Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong,
China
c Department of Chemistry, University of Hong Kong, Hong Kong, China
d Department of Biology and Chemistry, City University of Hong Kong, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a newly synthesized blue fluorescent pyrene-phenanthroimidazole based emitter Py-BPI and
its applications in non-doped organic light-emitting devices (OLEDs). Py-BPI has a high fluorescence
quantum yield of 40% and good thermal stability with a high glass transition temperature of 137 ꢀC.
Based on Py-BPI, OLEDs with several configurations have been fabricated. Among these devices, an OLED
with only two organic layers of Py-BPI and BPhen demonstrates blue emission with high efficiencies of
3.93 cd/A and 3.2 lm/W and maintains a high efficiency of 3.83 cd/A even at a brightness of 5000 cd/m2.
Combining with its hole-transporting ability, Py-BPI is shown to be a versatile blue emitter especially
suitable for high brightness applications.
Received 1 August 2012
Received in revised form
29 January 2013
Accepted 30 January 2013
Available online 4 March 2013
Keywords:
Non-doped
Blue emitter
Pyrene-based
OLED
Ó 2013 Elsevier Ltd. All rights reserved.
Dual function
Hole transporter
1. Introduction
materials based on pyrene derivatives owing to their large
conjugation and strong tendency to form excimers, which causes
Blue organic light-emitting devices (OLEDs) have been exten-
sively researched for their applications in full-color displays and
solid-state lighting. However, performance of blue emitters is still
lagging behind those of red and green materials [1,2]. Develop-
ment of high performance blue emitters is thus an important task.
Common blue fluorescent molecules can be roughly classified into
several groups according to their emitting cores including
anthracene, fluorene, pyrene etc. Among these, pyrene derivatives
have rich electronic and photophysical properties and usually have
red-shifted emissions as well as decrease in the fluorescence ef-
ficiency in solid-state [10].
On the other hand, complicated multilayer device structures are
not preferred for large-scale industrial processing. Devices with
fewer layers are thus more favorable for simplifying the fabrication
process. Development of multifunctional materials, which can be
used both for carrier transport and light-emission, is thus highly
desirable [10]. By linking phenanthroimidazole with pyrene via a
thiophene bridge, we have obtained a yellowish green emitter Py-
TPI(Scheme 1) [11]. In this work, we explore the effects of changing
the thiophene bridge to a benzene bridge and obtained a new blue
emitter – Py-BPI. With this simple change, emission from the
corresponding OLED shows a sharp blue-shift, a significant change
in CIE coordinates from (0.32, 0.59) to (0.15, 0.18) as well as
enhanced device efficiency. Py-BPI also has a shallow highest
occupied molecular orbital (HOMO) energy level of 5.4 eV which
facilitates efficient hole-injection. In addition, it is also shown that
Py-BPI is capable to function as both a blue emitter and a hole-
transporter in OLED.
good carriers mobilities stem from their large
p-conjugated sys-
tems [3e5]. Many high efficiency organic materials conjugated
with pyrene unit have been reported in the past few years [6e9].
However, there are few high efficiency blue light-emitting
* Corresponding author.
** Corresponding author.
E-mail addresses: qxtong@stu.edu.cn (Q.-X. Tong), apcslee@cityu.edu.hk
(C.-S. Lee).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.01.022

Y. Zhang et al. / Dyes and Pigments 98 (2013) 190e194
191
Scheme 1. A synthetic route for Py-BPI and the chemical structure of Py-TPI.
2. Experimental
min to melt and then quenched. Glass transition temperature (Tg) of
Py-BPI was determined by heating the quenched sample at a
heating rate of 10 ꢀC/min. Ionization potential (IP) of Py-BPI film on
ITO glass substrate was measured via ultraviolet photoelectron
spectroscopy (UPS) in a VG ESCALAB 220i-XL surface analysis sys-
tem. Electron affinity (EA) of Py-BPI was estimated by subtracting
from IP with its optical band gap (Egap) determined from the ab-
sorption spectra of its solid-state film.
2.1. Organic synthesis
All solvents and materials were used as received from com-
mercial suppliers without further purification. A synthetic route of
the product 1-(4-tert-butylphenyl)-2-(4-(pyren-1-yl)phenyl)-1H-
phenanthro [9, 10-d] imidazole (Py-BPI)is outlined in Scheme 1. In
particular,
2-(4-Bromophenyl)-1-(4-tert-butylphenyl)-1H-phe-
nanthro [9, 10-d] imidazole was prepared according to the pro-
cedures described in our previous work [12] and Py-BPI was
synthesized via a Suzuki coupling reaction [13].
2.3. OLED fabrication
Patterned indium-tin oxide (ITO) coated glass slides with a sheet
resistance of 30
U per square were sequentially cleaned with iso-
2.1.1. 1-(4-tert-butylphenyl)-2-(4-(pyren-1-yl)phenyl)-1H-
phenanthro [9,10-d] imidazole(Py-BPI)
propyl alcohol, Decon 90, rinsed in de-ionized water, then dried in
an oven, and finally treated in an ultraviolet-ozone chamber. The ITO
substrates were then transferred into a deposition chamber with a
base pressure of 10ꢁ6 mbar. Trilayer and bilayer devices with con-
figurations respectively of ITO/NPB (60 nm)/Py-BPI (40 nm)/elec-
tron transporting layer(20 nm)/LiF (0.5 nm)/Mg: Ag (100 nm) and
ITO/Py-BPI (100 nm)/ETL (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)were
fabricated by thermal vapor deposition.In these devices either
TPBI(1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene) or BPhen
(4,7-diphenyl-1,10-phenanthroline) was used as an electron-
transporting layer (ETL); Py-BPI was used as the emitting layer
(EML). NPB (4,40-bis[N-(1-naphthyl)-N-phenylamino] biphenyl)
and Py-BPI was respectively used as a hole-transporting layer (HTL)
in the trilayer and bilayer devices. Deposition rates for both organic
and metal layers were monitored with a quartz oscillation crystal
The synthesis procedure is similar to that reported in our pre-
vious work [12]. 1H NMR (400 MHz, CD2Cl2)
d1.50 (s, 9H), 7.28e7.37
(m, 2H), 7.52e7.66 (m, 5H), 7.67e7.87 (m, 6H), 8.01e8.10 (m, 3H),
8.12e8.30 (m, 6H), 8.75e8.91 (m, 3H).13C NMR(75 MHz, CDCl3)
d31.64 (s), 35.37 (s), 121.26 (s), 123.32 (s), 123.71 (s), 124.39 (s),
124.74e125.41 (m), 125.47 (s), 126.30 (s), 126.72 (s), 127.22e127.57
(m), 127.57e128.85 (m), 128.86 (s), 130.32 (d), 131.08 (d), 131.65 (s).
TOF MS (EIþ): 626.2777.Calc. for C47H34N2: 626.27.Anal. Found: C,
89.11; H, 6.34; N, 4.33. Calc. for C47H34N2: C, 90.06; H, 5.47; N, 4.47.
2.2. Material characterization
Nuclear magnetic resonance (NMR) spectrum was recorded
using CD2Cl2(1HNMR) and CDCl3(13CNMR) as solvents with a Varian
Gemin-400 spectrometer. Mass spectrum was recorded on a PE
SCIEX API-MS system. Absorption and photoluminescence (PL)
spectra of Py-BPI were recorded on a PerkineElmer Lambda 2S
UVevisible spectrophotometer and a PerkineElmer LS50 fluores-
ꢀ
and controlled in the range from 1 to 2 A/s. A shadow mask was used
to define the cathode to make four devices each with an active area
of 0.1 cm2. Electroluminescence (EL) spectra, CIE coordinates, and
current densityevoltage-luminance (J-V-L) characteristics of OLEDs
were measured with a programmable Keithley model 237 power
source and Spectrascan PR 650photospectrometer under ambient
conditions without sample encapsulation.
cence spectrometer, respectively. Fluorescence quantum yields (Ff
)
in dichloromethane solution were determined according to previ-
ous literature [14] by a comparative method, using anthracene as a
standard reference with Ff ¼ 0.27 in ethanol. Thermogravimetric
analysis (TGA) was performed on a TA Instrument TGAQ50 at a
heating rate of 10 ꢀC/min under a nitrogen atmosphere. Differential
scanning calorimetric (DSC) measurement was performed on a TA
Instrument DSC2910. The sample was first heated at a rate of 10 ꢀC/
3. Results and discussion
The synthetic route for Py-BPI and its chemical structure are
shown in Scheme 1. Py-BPI was prepared via Suzuki coupling re-
actions between 2-(4-Bromophenyl)-1-(4-tert-butylphenyl)-1H-

192
Y. Zhang et al. / Dyes and Pigments 98 (2013) 190e194
Table 2
Performance data of the OLED based on Py-BPI.
a
Devices
VOnset
(V)
V at 20
mA/cm2 (V) (nm)
ELmax CEb
PEc
(lm/W)
CIE (x, y)
max
max
(cd/A)
Trilayer-BPhen 2.5
4.0
5.0
7.6
5.4
468
468
468
468
3.27
2.96
3.93
2.03
3.17
2.00
3.20
1.00
0.15,0.18
0.15, 0.19
0.16, 0.20
0.15, 0.15
Trilayer-TPBI
Bilayer-BPhen
Bilayer-TPBI
3.0
3.4
3.2
a
Obtained at 1 cd/m.2.
Current efficiency of the devices.
Power efficiency of the devices.
b
c
high Ff suggest that they have good potential to offer high effi-
ciency blue emission in OLEDs. The large blue-shift of the PL
spectrum of Py-BPI compared with Py-TPI might be due to the
weaker electron donating property of the benzene ring rendering
with a deeper HOMO energy level (ie. 5.4 eV) and thus a higher
energy gap in Py-BPI. Key thermal and photophysical data of the
two compounds are summarized in Table 1.
Fig. 1. UVeVis absorption and photo-luminescence (PL) for Py-BPI in dichloromethane
solution and thin solid film state.
It is well known that efficient carrier injection at interfaces
between different layers in OLEDs is essential for obtaining high
performance devices. It is thus important for the EML to possess a
shallow HOMO for facilitating hole-injection [10]. The HOMO value
for Py-BPI was measured via UPS to be 5.4 eV; while the lowest
unoccupied molecular orbital (LUMO) energy is estimated to be
2.7 eV by subtracting the HOMO value with the optical band gap as
determined from the absorption cutoff. Schematic energy level
diagrams for the Py-BPI based OLEDs are shown in Fig. 2.From the
diagrams, it can be seen that there are only a small energy step for
electron injection (0.2 eV) at the interface of Py-BPI/BPhen. In
addition, the similar HOMO energies of Py-BPI and NPB facilitate
efficient hole transport from NPB to Py-BPI.
Table 1
Photophysical and thermal data of Py-BPI and Py-TPI (For comparison purpose, data
for Py-TPI are extracted from Ref. [11]).
Compound Abs (nm)
PL (nm)
Ff
Td (ꢀC) Tg (ꢀC) Tm (ꢀC)
Py-BPI
242,262,352a/247, 439a/451b 40% 431
271,371b
137
300
Py-TPI
243,343,381a/243, 485a/504, 23% 422
128
249
406b
521b
a
Obtained from dichloromethane solution.
Obtained from thin film state.
b
Other than the conventional trilayer devices (ITO/NPB (60 nm)/
Py-BPI (40 nm)/BPhen or TPBI (20 nm)/LiF (0.5 nm)/Mg:Ag
(100 nm)), we also explore the feasibility of using Py-BPI as the HTL
in bilayer devices (ITO/Py-BPI (100 nm)/BPhen or TPBI (20 nm)/LiF
(0.5 nm)/Mg:Ag (100 nm)). Key performance data of the four de-
vices were summarized in Table 2.
Fig. 3 shows that electroluminescence (EL) spectra of the four
devices are all similar to the PL spectrum of Py-BPI. This confirms
that the blue emissions of the devices are originated from Py-BPI.
The EL spectra peak at 468 nm for all four devices with full widths
at half maxima range from 82 to 93 nm. Commission Internationale
de L’Eclairage (CIE) coordinates of the devices are listed in Table 2.
All devices show blue emission with CIEy< 0.20. Comparing with
similarly structured trilayer device using Py-TPI as the emitter, the
CIE coordination moved from (0.32, 0.59) to (0.15, 0.18).
phenanthro [9, 10-d] imidazole and pyrenyl-1-boronic acid with a
yield of 58%. The molecular structure of Py-BPI was confirmed with
H nuclear magnetic resonance and mass spectrometry (details
shown in Fig. S1 and S2). Attached with a large and rigid pyrene
group, Py-BPI has good thermal stability with a high decomposi-
tion temperature (Td) of431 ꢀC in nitrogen, a high melting tem-
perature (Tm) up to 300 ꢀC and a glass-transition temperature (Tg) of
137 ꢀC (Fig. S3 and S4).
UVeVis absorption and PL spectra of Py-BPI in both dichloro-
methane solution and solid-state thin film formed on quartz sub-
strate are shown in Fig.1.Similarity of the solution and the solid film
spectra suggest that intermolecular interaction of Py-BPI in solid is
weak. Intensive blue emission peaked at wavelength of 451 nm and
absorption peaks at 247, 271, and 371 nm in thin-film state were
observed. In addition, Py-BPI shows a Ff of 40%, which is consid-
erably higher than that of Py-TPI (Ff ¼ 23%). Such substantial blue-
shift of 70 nm over that of the reported Py-TPI (i.e.521 nm) and the
Fig. 4 shows J-V-L characteristics of the four devices based on
Py-BPI. It can be seen that the driving voltages at the same current
Fig. 2. Schematic energy level diagrams for devices based on Py-BPI as an emitter or a dual functional material as both an emitter and a hole transporter.

Y. Zhang et al. / Dyes and Pigments 98 (2013) 190e194
193
Table 3
Summary of device performances for blue OLED with CIEy < 0.20 reported recently.
Emitter
CE (cd/A)
PE (lm/W)
CIE
Ref.
Py-BPI
TPA-BPI
BPyC
3.93
2.63
2.94
4.75
3.7
1.5
2.06
3.8
3.2
(0.15, 0.20)
(0.15, 0.09)
(0.15, 0.18)
(0.15, 0.18)
(0.17, 0.20)
(0.16, 0.07)
(0.15, 0.13)
(0.15, 0.09)
This work
[12]
[9]
[8]
[18]
[19]
[20]
[21]
2.53
2.72
2.55
2.5
2.19
e
DPEC
Ph3TPE
9TPAFSPO
BPTF
Silylene-bridged
2-(2-naphthyl)
indole
TPA-AN
MADN
TBADN
BDMA
NSA
3.6
4.54
1.43
2.00
2.20
7.85
3.08
6.00
4.69
2.06
3.2
4.02
0.7
e
(0.15, 0.19)
(0.15, 0.10)
(0.16, 0.14)
(0.16, 0.12)
(0.15, 0.18)
(0.17, 0.19)
(0.15, 0.19)
(0.15, 0.09)
(0.15, 0.09)
(0.15, 0.07)
(0.15, 0.08)
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
e
e
P2
DPF
TPIP
1.17
3.6
2.71
1.77
3.3
2.33
Fig. 3. Electroluminescence (EL) spectra for Py-BPI as an emitter in different devices.
PhQ-CVz
POAn
5a
2.8
a
9-(4-(10-(Naphthalen-1-yl)anthracen-9-yl)phenyl)-9H-carbazole.
(VOnset is defined as voltage required to give a luminance of 1 cd/
m2):trilayer-BPhen (2.5 V) < trilayer-TPBI (3.0 V); bilayer-BPhen
(2.8) < bilayer-TPBI (3.2 V). The reason for the lower driving and
onset voltages for the BPhen devices is attributed to the higher
electron mobility (me) and deeper LUMO for BPhen as compare to
those of TPBI. me under the electric field of 4.9 ꢂ 105 V/cm from
time-of-flight measurement are
5
ꢂ
10ꢁ4 cm2/V [15] and
3.6 ꢂ 10ꢁ5 cm2/V [16] for BPhen and TPBI; while LUMO values are
2.9 and 2.7 eV for BPhen and TPBI as we determined. In addition,
the incorporation of high hole mobility material NPB
(
mh ¼ 7 ꢂ 10ꢁ3 cm2/V [17] under the electric field of 4.9 ꢂ 105 V/
cm2) in trilayer device further reduces the operation voltages.
Fig. 5 shows the power and current efficiencies of the Py-BPI
based devices under different current densities. The maximum
power and current efficiencies for trilayer devices with TPBI are
2.00 lm/W and 2.96 cd/A, respectively whereas those for the tri-
layer device with BPhen are 3.17 lm/W and 3.27 cd/A, respectively.
Interestingly, efficiencies in the bilayer device with BPhen can be
further improved to 3.20 lm/W and 3.93 cd/A. Importantly, the
current efficiency undergoes negligible change at very high current
density of 300 mA/cm2. It indicates the holes and electrons are well
balanced in the bilayer BPhen device. In addition, it is worth noting
that performance of Py-BPI based devices are comparable to some
of the best blue emitters reported recently (Table 3) [8,9,12,18e32].
Fig. 4. Current densityevoltage and luminanceevoltage characteristics of OLEDs based
on Py-BPI. (To avoid overdriving the devices and/or protection of the measurement
system, the maximum brightness was limited to 10000 cd/m2.)
density or luminance for Py-BPI based devices using a BPhen ETL is
lower than those of the devices with a TPBI ETL. For instance, at the
current density of 20 mA/cm2 (or at a typical display brightness of
100 cd/m2), the operating voltage for devices trilayer and bilayer
devices with a BPhen ETL are 4.0 and7.6 V (3.2 and 6.2 V) while
those using a TPBI ETL are 5.0 and 5.4 V (4.0 and 4.8 V), respectively.
In addition, we observed the same trend in the turn-on voltage
5.5
3.5
Trilayer-TPBI
5.0
Trilayer-TPBI
Trilayer-BPhen
3.0
Trilayer-BPhen
4.5
Bilayer-TPBI
Bilayer-TPBI
Bilayer-BPhen
4.0
2.5
Bilayer-BPhen
3.5
3.0
2.5
2.0
1.5
1.0
0.5
2.0
1.5
1.0
0.5
0.0
0
50
100
150
200
250
300
0
100
200
300
2
Current Density (mA/cm )
2
Current Density (mA/cm )
Fig. 5. Power and current efficiencies of the Py-BPI based devices under different current densities.

194
Y. Zhang et al. / Dyes and Pigments 98 (2013) 190e194
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A newly synthesized blue fluorescence emitter based on pyrene-
phenanthroimidazole, namely Py-BPI, has been characterized and
applied in non-doped OLEDs. Py-BPI shows good thermal stability
with a Tg of 137 ꢀC and a high Ff of 40%. It is found that the PL
spectrum of Py-BPI was successfully blue-shifted as much as 70 nm
from the previously reported green emitter Py-TPI by replacing the
thiophene bridge with a benzene bridge. A simple blue OLED with
only two organic layers of Py-BPI and BPhen demonstrated high
efficiencies of 3.93 cd/A and 3.20 lm/W and little current efficiency
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This work was supported by National Natural Science Founda-
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Research Program of China (No. 2013CB834803) and the Research
Grants Council of the Hong Kong Special Administrative Region,
China (Project No. T23-713/11).
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
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