Non-Doped Deep Blue and Doped White Electroluminescence Devices Based on Phenanthroimidazole Derivative

Article abstract of DOI:10.1007/s10895-016-1970-5

A novel deep-blue emitter PhImPOTD based on phenathroimidazole was synthesized, which is incorporated by an electron-donating dibenzothiophene unit and electron-withdrawing phenanthroimidazole and diphenylphosphine oxide moieties. Furthermore, the weak π–π stacking and intermolecular aggregation render the photoluminescence quantum yield is as high as 0.34 in the solid state. Non-doped organic light emitting diodes (OLEDs) based on PhImPOTD emitter exhibits a low turn-on voltage of 3.6?V, a favorable efficiency of 1.13?cd A^?1 and a deep blue emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.15, 0.08). The CIE is very close to the NTSC (National Television Standards Committe) blue standard (CIE: 0.14, 0.08). PhImPOTD is also utilized as blue emitter and the host for a yellow emitter (PO-01) to fabricate white organic light-emitting diodes (WOLEDs). This gives a forward-viewing maximum CE of 4.83?cd A^?1 and CIE coordinates of (0.32, 0.32) at the luminance of 1000?cd?m^?2. Moreover, the single-carrier devices unambiguously demonstrate that typical bipolar-dominant characteristics of PhImPOTD. This work demonstrates not only that the phenanthroimidazole unit is an excellent building block to construct deep blue emission materials, but also the introduction of a diphenylphosphine oxide deprotonation substituent is an efficient tactic for harvesting deep-blue emitting devices.

Full text of DOI:10.1007/s10895-016-1970-5

J Fluoresc
DOI 10.1007/s10895-016-1970-5
ORIGINAL ARTICLE
Non-Doped Deep Blue and Doped White Electroluminescence
Devices Based on Phenanthroimidazole Derivative
Shuo Chen1 & Yukun Wu2 & Shoucheng Hu2 & Yi Zhao2 & Daining Fang1,3
Received: 19 August 2016 /Accepted: 3 November 2016
#
Springer Science+Business Media New York 2016
Abstract A novel deep-blue emitter PhImPOTD based
on phenathroimidazole was synthesized, which is incor-
porated by an electron-donating dibenzothiophene unit
and electron-withdrawing phenanthroimidazole and
diphenylphosphine oxide moieties. Furthermore, the weak
π–π stacking and intermolecular aggregation render the
photoluminescence quantum yield is as high as 0.34 in
the solid state. Non-doped organic light emitting diodes
(OLEDs) based on PhImPOTD emitter exhibits a low
turn-on voltage of 3.6 V, a favorable efficiency of
1.13 cd A−1 and a deep blue emission with Commission
Internationale de l’Eclairage (CIE) coordinates of (0.15,
0.08). The CIE is very close to the NTSC (National
Television Standards Committe) blue standard (CIE:
0.14, 0.08). PhImPOTD is also utilized as blue emitter
and the host for a yellow emitter (PO-01) to fabricate
white organic light-emitting diodes (WOLEDs). This
gives a forward-viewing maximum CE of 4.83 cd A−1
and CIE coordinates of (0.32, 0.32) at the luminance of
1000 cd m−2. Moreover, the single-carrier devices unam-
biguously demonstrate that typical bipolar-dominant char-
acteristics of PhImPOTD. This work demonstrates not
only that the phenanthroimidazole unit is an excellent
building block to construct deep blue emission materials,
but also the introduction of a diphenylphosphine oxide
deprotonation substituent is an efficient tactic for harvest-
ing deep-blue emitting devices.
.
.
Keywords OLEDs Phenanthroimidazole derivative
.
.
Bipolar Non-doped deep blue Doped white
Introduction
In the field of consumer electronics, organic light-emitting
diodes (OLEDs) as solid-state lighting are progressively re-
placing the traditional display technology due to their supe-
rior lightweight, flexible and large viewing angle [1, 2].
Among diverse design strategies, the properties of three es-
sential emitting elements (red, green and blue) are directly
influence the performance of devices [3–5]. After decades of
development, several excellent green and red emitting mol-
ecules have been designed and applied successfully to im-
prove to the light emitting capability of OLEDs [6].
However, the overall performance and stability of blue (es-
pecially deep blue) fluorescence emitters and devices are
still inferior to those red and green counterparts [7, 8]. The
blue emitter can not only effectively reduce power consump-
tion of the devices but also be utilized to generate light of
other colors by energy cascade to lower energy fluorescent
or phosphorescent dopants. To date, the marketable OLEDs
mainly adopt the doped host-guest emitting layer.
Shuo Chen and Yukun Wu These authors contributed equally.
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-016-1970-5) contains supplementary material,
which is available to authorized users.
* Daining Fang
fangdn@pku.edu.cn
1
State Key Laboratory for Turbulence and Complex System, College
of Engineering, Peking University, Chengfu Road, Haidian District,
Beijing 100871, People’s Republic of China
2
State Key Laboratory on Integrated Optoelectronics, College of
Electronics Science and Engineering, Jilin University, 2699 Qianjin
Street, Changchun 130012, People’s Republic of China
3
Institute of Advanced Structure Technology, Beijing Institute of
Technology, South Street No. 5, Zhongguancun, Haidian District,
Beijing 100081, People’s Republic of China

J Fluoresc
Nevertheless, the generally used heavy metals for phospho-
rescence are confined to iridium (Ir) and platinum (Pt)
[9–11], which are rather expensive and dependent on limited
global resources. Thus, organic molecule with free heavy-
metal containing meets the eco-friendly theme [12]. At the
same time, phosphorescent materials also suffer from the
lack of deep blue emitters and their inherent short lifetimes
when utilized to configure white organic light-emitting di-
odes (WOLEDs) [13]. Therefore, the development of high-
performance deep-blue emitting OLEDs (CIE criterion:
y < 0.15 and x + y < 0.30) and related materials is still a
challenging issue [14, 15]. In order to realize the deep blue
emission, the strategic molecular design should be focus on
relatively short conjugation and weak intermolecular aggre-
gation [16]. Also, the electron affinities of blue-emitting ma-
terials should be increased to realize balanced charge injec-
tion and transport, since the electron injection and transport
ability in organic semiconductors is relatively low compared
to that of hole [17, 18]. The n-type phenathroimidazole (PI)
derivatives always adopt less conjugative configuration such
as the twisted linkage or non-conjugative linkage [19–22].
The PI moiety as the core structure linked with a freely
rotatable benzene ring and end-capsulated with donor units
is the representative donor–π–acceptor dipolar molecule
[23–27]. In our molecule PhImPOTD, electron-deficiency
diphenylphosphine oxide is bonding with NH site at the
imidazole ring, which could effectively inhibit molecular
aggregation, π–π stacking and even fluorescence quenching
in the solid state. It is noted that diphenylphosphine oxide
unit, in contrast with the electron-rich group, also affords a
hypsochromic shift function of emission to ensure the deep
blue emitting OLEDs [28].
In this work, we report a triple twisted PI derivative,
which is designed based on donor–π–acceptor approach
and utilized phenyl bridge to concatenate two planar
electron-transporting PI and hole-transporting
dibenzothiophene (TD) units. In this way, such bipolar fea-
ture is beneficial to charge transport and balancing for
obtaining high-efficiency devices in comparison with con-
ventional unipolar materials. In addition, diphenylphosphine
oxide is employed to construct multi-dimension framework
by deprotonation on the site of imidazole. Therefore, the
yielding sterically hindered and highly twisted molecular
configuration is benefit to increase the photoluminescence
quantum yield. Thermal, photophysical, and electrolumines-
cent properties of the deep blue emitting PhImPOTD are
comprehensively investigated. Hybrid WOLEDs, combining
blue fluorophores and yellow phosphors, has been fabricated
and investigated. It is approved that PhImPOTD possesses
high luminescent efficiency, excellent luminous/thermo- sta-
bility and balanced charge carrier injection and transport,
which has opened us new avenues for designing deep blue
PI derivatives for high-performance OLEDs.
Experimental
Materials and Instruments
All the reagents and solvents used for the synthesis of the
PhImPOTD were purchased from Aldrich, J&K and TCI
companies and used as received. Dopant material PO-01
1
was purchased from Lumtec Corp. (taiwan). H magnetic
resonance (NMR) spectra were recorded using a Bruker
AVANCE III 500-MHz spectrometer, using CDCl3 as the
solvent and tetramethylsilane (TMS) as the internal standard.
High resolution mass spectra were recorded on a Bruker
APEX IV fourier transform ion cyclotron resonance mass
spectrometer. Elemental analysis for C, H, N and S were
performed on a Elementar Analysensysteme GmbH. All ma-
nipulations involving air-sensitive reagents were performed
in an atmosphere of dry Ar. Absorption and
photoluminescence (PL) emission spectra of the target com-
pound were measured using a Perkin Elmer Lambda-750
UV-Vis-NIR spectrophotometer and LS 55 fluorescence
spectrometer, respectively. The luminescence quantum yield
of compound was measured at room temperature and cited
relative to a reference solution of 9,10-dipenylanthracene
(Φ = 0.9 in cyclohexane) as a standard, and they were cal-
culated according to the well-known equation:
ꢀ
ꢁ
φoverall
φref
n
nref
2 Aref
A Iref
I
¼
ð1Þ
In Eq. (1), n, A, and I denote the refractive index of solvent,
the area of the emission spectrum, and the absorbance at the
excitation wavelength, respectively, and φref represents the
quantum yield of the standard 9,10-dipenylanthracene solu-
tion. The subscript ref. denotes the reference, and the absence
of a subscript implies an unknown sample. For the determina-
tion of the quantum yield, the excitation wavelength was cho-
sen so that A < 0.05. For the solid samples, the quantum yield
for the compound was determined at room temperature
through an absolute method using an Edinburgh
Instruments’ integrating sphere coupled to a modular
Edinburgh FLS 920 fluorescence spectrophotometer. The
values reported are the average of three independent determi-
nations for each sample. The absolute quantum yield was
calculated using the following expression:
Z
Lemission
Z
Z
Φ ¼
ð2Þ
Lreference
−
Lsample
In expression (2), Lemission is the emission spectrum of the
sample, collected using the sphere, Esample is the spectrum of

J Fluoresc
the incident light used to excite the sample, collected using the
sphere, and Ereference is the spectrum of the light used for
excitation with only the reference in the sphere. The method
is accurate to within 10%. Thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) were performed
on Perkin Elmer TGA 4000 and DSC 8000 thermal analyzers
under nitrogen atmosphere at a heating rate of 10 °C min−1.
Cyclic voltammetric (CV) 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 elec-
trode, and a saturated calomel electrode (SCE) reference elec-
trode on a computer-controlled CHI660d electrochemical
workstation at room temperature. Reduction CV of the com-
pou nd w a s pe r forme d i n C H2 C l2 containing
teterabutylammonium hexafluorophosphate (Bu4NPF6,
0.1 M) as the supporting electrolyte. Ferrocene was used as
an external standard. Electrochemistry was done at a scan rate
of 100 mV s−1.
(0.2 nm)/NPB (40 nm)/PhImPOTD (30 nm)/NPB (40 nm)/
MoO3 (0.2 nm)/Al (100 nm) (hole-only transporting Device)
and ITO (100 nm)/LiF (0.1 nm)/TPBi (40 nm)/ PhImPOTD
(30 nm)/TPBi (40 nm)/ LiF (0.1 nm)/Al (100 nm) (electron-
only transporting Device). The doped WOLEDs device B was
fabricated with the structure ITO/MoO3 (0.2 nm)/NPB
(40 nm)/PhImPOTD (15 nm)/PO-01 (0.2 nm)/PhImPOTD
(15 nm)/TPBi (40 nm)/LiF (0.1 nm)/Al.
Synthesis
Synthesis of Dibenzothiophene-4-Dioxaborolane (TD4B)
TD4B was synthesized according to the literature procedures
1
[28]. Yield: 80%. H NMR (TMS, CDCl3, 500 MHz): ppm
δ = 8.22 (d, J = 8.0 Hz, 1 H), 8.11 (t, J = 6.5 Hz, 2 H), 8.90 (d,
J = 7.5 Hz, 1 H), 7.84 (t, J = 4.0 Hz, 1 H), 7.45–7.40 (m, 3 H),
1.35 (s, 12 H); HR-ESI-MS: [M + H]+ m/z calcd for
C18H20BO2S: 311.12797, found: 311.12748; elemental anal-
ysis calcd (%) for C18H19BO2S: C 69.69, H 6.17, N 10.31, S
10.33; found: C 69.58, H 6.11, N 10.45, S 10.42.
Computational Details
The theoretical investigation of geometry optimization was
performed with the Gaussian 09 program package [29].
Density functional theory (DFT) was calculated at Beck’s
three-parameter hybrid exchange functional [30] and Lee,
and Yang and Parr correlation functional [31] B3LYP/6-31G
(d). The spin density distributions were visualized using
Gaussview 5.0.8.
Synthesis of 4-(Diphenylphosphinyl)Benzenamine
(POBA)
To a 100 mL round bottom flask fitted with magnetic bar, was
added NiCl2.6H2O (23.6 mg, 0.1 mmol), zinc (127.9 mg,
2.0 mmol), 2,2′-bipyridine (bpy) (31.2 mg, 0.2 mmol), 4-
bromoaniline (206.4 mg, 1.2 mmol), diphenylphosphine ox-
ide (202.1 mg, 1.0 mmol) and water (15 mL). The reaction
mixture was then stirred at 70 °C for 24 h. After completion of
the reaction, the mixture was allowed to cool to room temper-
ature and added with CH2Cl2 and water. The organic layer
was isolated and the remaining aqueous phase was further
extracted with CH2Cl2 (10 mL × 3). Then the organic phases
were combined and dried with anhydrous MgSO4, and puri-
fied by silica gel column chromatography using CH2Cl2/
MeOH (15:1) as the eluent to afford a white power. Yield:
75%. 1H NMR (TMS, CDCl3, 500 MHz): ppm δ = 7.59–7.5
(m, 6 H), 7.52–7.48 (m, 4 H), 7.21–7.17 (m, 2 H), 6.63 (dd,
J = 6.0, 2.5 Hz, 2 H), 5.79 (br s, 2 H); HR-ESI-MS: [M + H]+
m/z calcd for C18H17NOP: 294.10423, found: 294.10422; el-
emental analysis calcd (%) for C40H26N2O2: C 84.78, H 4.62,
N 4.94; found: C 84.68, H 4.71, N 4.86.
Device Fabrication and Measurement
Prior to the device fabrication, the patterned ITO-coated glass
substrates were scrubbed and sonicated consecutively with
detergent water, deionized water, and acetone, dried in drying
cabinet, and then exposed to a UV-ozone environment for
30 min. After these processes, the substrates were transferred
into a vacuum chamber for sequential deposition of all the
organic layers by thermally evaporation with a base pressure
(~4.0 × 10−4 Pa) at a rate of 0.1–0.2 nm s−1 monitored in situ
with the quartz oscillator. LiF covered by Al is used as cathode
without breaking the vacuum. All the samples were measured
directly after fabrication without encapsulation at room tem-
perature under ambient atmosphere. The current-voltage-
luminance characteristics were carried out using a PR655
Spectrascan spectrometer and a Keithley 2400 programmable
voltage-current source. The external quantum efficiency
(EQE) and luminous efficiency (LE) were calculated assum-
ing Lambertian distribution, and then calibrated to the effi-
ciencies obtained at 1000 cd m−2 in the integrating sphere
(Jm-3200). The configurations of Device A was ITO/MoO3
(10 nm)/NPB (80 nm)/PhImPOTD (30 nm)/TPBi (40 nm)/LiF
(1 nm)/Al (100 nm). The nominal hole-only and electron-only
devices were fabricated with the configurations of ITO/MoO3
Synthesis of 2-(4-Bromophenyl)
-1-(4-Diphenylphosphinylphenyl)
-1H–Phenanthro[9,10-d]Imidazole (PhImPOBr)
A mixture of 4-bromobenzaldehyde (92.0 mg, 0.5 mmol),
phenanthrene-9,10-dione (104.0 mg, 0.5 mmol),
4-(diphenylphosphinyl)benzenamine (732.8 mg, 2.5 mmol),

J Fluoresc
Scheme 1 Synthetic pathways toward bipolar molecule PhImPOTD
ammonium acetate (130.1 mg, 2.0 mmol), and acetic acid
(30 mL) were refluxed under nitrogen in an oil bath. After
24 h, the mixture was cooled and concentrated under reduced
pressure, the crude product was extracted by CH2Cl2/H2O. It
was then purified by chromatography using CH2Cl2/MeOH
(15:1) as an eluent to obtain the product as yellow powder.
Yield: 62%. 1H NMR (TMS, CDCl3, 500 MHz): ppm δ = 7.21
(d, J = 8.0 Hz, 1H), 7.30 (t, J = 7.5 Hz, 1H), 7.39 (d,
J = 8.5 Hz, 2H), 7.44 (d, J = 8.5 Hz, 2H), 7.53–7.55 (m,
5H), 7.60–7.68 (m, 5H), 7.70–7.75 (m, 5H), 7.91 (dd,
J = 8.0 Hz, 2H), 8.70 (d, J = 8.5 Hz,1H), 8.78 (d,
J = 8.0 Hz, 1H), 8.8 (d, J = 9.0 Hz, 1H); HR-ESI-MS: [M +
H]+ m/z calcd for C39H27BrN2OP: 649.10389, found:
649. 10200; elemental analysis calcd (%) for
C39H26BrN2OP: C 72.12 H 4.03, N 4.31; found: C 72.18, H
4.13, N 4.43.
8.75 (d, J = 8.5 Hz,1H), 8.83 (d, J = 8.5 Hz, 1H), 8.92 (d,
J = 8.0 Hz, 1H); HR-ESI-MS: [M + H]+ m/z calcd for
C51H34N2OPS: 753.21240, found: 753.21298; elemental
analysis calcd (%) for C51H34N2OPS: C 81.36, H 4.42, N
3.72, S 4.26; found: C 81.38, H 4.37, N 3.71, S 5.31.
Results and Discussion
Synthesis
The synthetic route for compound PhImPOTD is outlined
in Scheme 1. The detailed procedures for the syntheses of
the reaction intermediates and final products are depicted in
synthesis part. PhImPOTD is composed of three main
100
Synthesis of 2-(4-Dibenzothiophene Sulfone)
-1-(4-Diphenylphosphinylphenyl)
-1H–Phenanthro[9,10-d]Imidazole (PhImPOTD)
Lost 5%
90
80
70
60
50
40
30
20
A mixture of PhImPOBr (974.3 mg, 1.5 mmol), TD4B
(465.3 mg, 1.5 mmol), tetrakis(triphenylphosphine)palladium
(173.3 mg, 0.15 mmol), tetrabutylammonium bromide
(48.5 mg, 0.15 mmol), and aqueous solution of sodium hy-
droxide (2 mol L−1, 9 mmol) in THF (20 mL) was stirred
under argon at 80 °C for 48 h. After quenched with aqueous
NH4Cl solution, the mixture was extracted with CH2Cl2. The
combined organic extracts were washed with brine and dried
over anhydrous MgSO4. After removing the solvent, the res-
idue was purified by column chromatography on silica gel
using ethyl acetate as the eluent to give a white power.
First Heat
Second Heat
50
100
150
200
250
Temperature/oC
100
200
300
400
500
600
700
800
Temperature (oC)
Fig. 1 TGA thermogram of PhImPOTD (at 10 °C min−1 under notrogen
atmosphere). Inset: Differential scanning calorimetry (DSC) spectrum of
the first and second heating cyclings for PhImPOTD at a heating rate of
10 °C min−1 under nitrogen flushing
1
Yield: 64%. H NMR (TMS, CDCl3, 500 MHz): ppm
δ = 7.35 (t, J = 8.5 Hz, 1H), 7.47–7.63 (m, 12H), 7.68–7.84
(m, 13H), 7.98 (t, J = 8.5 Hz, 2H), 8.23 (t, J = 7.0 Hz, 2H),

J Fluoresc
Fig. 2 The optimized molecular
geometries of PhImPOTD
calculated with DFTon a B3LYP/
6-31G(d) level
components: PI as the acceptor moiety, dibenzothiophene as
the donor moiety and diphenylphosphine oxide as the
branch of phenathroimidazole. Firstly, TD4B is successively
achieved by bromination and borate acidification of
dibenzothiophene at C-4 positions. Secondly, the
diphenylphosphine oxide (PO) group is not directly treated
with deprotonation for the NH group on imidazole. We
chose the moderate synthetic route to construct
4-(diphenylphosphinyl)benzenamine by zinc and nickel co-
catalysis. The key backbone PhImPOBr is prepared by one-
pot cyclizing reaction with high yields. This synthesis meth-
od could conveniently construct PI derivatives with various
structures by tuning aromatic aldehyde and primary amine
[32]. The target molecule is obtained through the typical
Suzuki-Miyaura cross-coupling reactions between the bro-
mide intermediate PhImPOBr and the boronic ester TD4B
catalyzed by Pd(PPh3)4–NaOH in 64% yields. In order to
pursue the maximized yields, the selective aprotic solvents
THF, toluene and 1,4-dioxane were utilized for this reaction.
The results prove that the utilization of THF can be apt to
the maximized yields. The identity of PhImPOTD is fully
characterized by standard spectroscopic techniques, which
gives satisfactory analysis data corresponding to chemical
structure and demonstrates its high purity. PhImPOTD has
good solubility in common organic solvents such as THF,
dichloromethane, chloroform and toluene.
Thermal Properties
The thermal properties of PhImPOTD are examined by ther-
mogravimetric analysis (TGA) and differential scanning cal-
orimetry (DSC) measurements, as shown in Fig. 1.
PhImPOTD shows good thermal stability, which is indicated
by high decomposition temperature (Td, corresponding to 5%
weight loss) of 321 °C. Such value is ca. 60 °C higher than PI
unit (Td = 262 °C). In addition, no phase transition, like the
glass transition and the melting temperature, is observed in the
tested temperature range, indicating the noncrystalline or
amorphous characteristics of PhImPOTD [33]. The nonpla-
nar PO and TD units in molecule hinder close packing, which
is beneficial for the formation of amorphous thin film by vac-
uum deposition.
Theoretical Calculations and Electrochemical Properties
To gain further insight into the structure-property relationship,
we performed the geometry optimization of PhImPOTD
using the DFT calculations with the Gaussian 09 series of
programs using the B3LYP hybrid functional and 6–31G(d)
0.15
0.10
0.05
Abs. PhImPOTD in CH2Cl2
1.00
0.75
0.50
0.25
0.00
1.00
0.75
0.50
0.25
0.00
Abs. PhImPOTD in film
FL PhImPOTD in CH2Cl2
FL PhImPOTD in film
Ex = 365 nm
powder
CH2Cl2
0.00
1.20 V
-0.05
Crystal
300
400
500
600
700
800
0.8
1.0
1.2
1.4
1.6
Wavelength (nm)
Voltage (V vs. SCE)
Fig. 4 Normalized absorption spectra and fluorescence spectra of
PhImPOTD in CH2Cl2 at 10−6 M and in spin-coating film on a quartz
plate at 298 K, respectively. Insert: photographs of PhImPOTD in
CH2Cl2 and in the solid state under 365 nm hand-lamp irradiation;
photographs of single crystal captured through optical microscope
Fig. 3 Cyclic voltammogram of PhImPOTD. In each case, the anodic
scan was performed in CH2Cl2 at a scan rate of 100 mV s−1. The working
electrode: platinum wire; the auxiliary electrode: platinum wire with a
porous ceramic wick; the reference electrode: calomel electrode

J Fluoresc
Table 1 The photophysical and thermal properties of PhImPOTD
max,abs/nm a max,PL/nm a max,film/nm b PLQYsolution
a
b
λ
λ
λ
PLQYfilm
0.34
Tg/oC c Td/oC d HOMO/eV e LUMO/eV f Eg/eV g
PhImPOTD 230, 260, 339 414
438
0.62
–
321
5.60
2.41
3.19
a Measured in CH2Cl2 solution at room temperature
b Measured in spin-coating film at room temperature
c Tg: glass transition temperature, obtained from DSC measurements
d Td: decomposition temperature at weight loss of 5 %, obtained from TGA measurements
e HOMO was calculated from the onset value of the oxidation potential
f LUMO was calculated from the HOMO and the optical band gap Eg
g Eg: the optical band gap was calculated from the absorption spectra
basis set. Figure 2 presents the distributions of the frontier
molecular orbitals (FMO), i.e., the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO). The HOMOs are situated on the dibenzothiophene
unit, while the LUMOs are mainly localized on the imidazole,
diphenylphosphine oxide group and the linker phenyl seg-
ments due to the inductive effect of P = O. It is noteworthy
that with a most twisted conformation due to TD at the 4-
position of PI, it brings about the separated HOMO and
LUMO level distribution localized on the D and A moieties,
which indicates that HOMO–LUMO excitation would shift
the electron density distribution from one side of the TD
groups as the donor to the other side PI as the acceptor.
These observations are in accord with the fact that TD is a
hole transport unit and PI is an electron transport unit for
PhImPOTD. It is more important, such separation of
HOMO and LUMO can provide hole- and electron-
transporting channel respectively. Moreover, the dihedral an-
gels between the adjacent phenyl linker and PI are 31.7°. The
dihedral angels between the imidazole and deprotonation
group are 75.2°. Closer inspection reveals the conjugated phe-
nyl linker and the adjacent TD plane intersect with the
approximately perpendicular dihedral angels of 86.2°. The
twisted conformation brings out the less overlapped spatial
distributions of the HOMO and LUMO, which is beneficial
to charge balance and exciton recombination in devices.
The experimental LUMO and HOMO energy levels of
PhImPOTD was estimated with cyclic voltammetry (CV),
as shown in Fig. 3. PhImPOTD shows a single irreversible
oxidation peak with the similar onset voltages around 1.20 eV,
corresponding to the HOMO energy levels of −5.60 eV (as-
suming that the absolute energy level of the Fc/Fc+ redox
couple was 4.40 eV below vacuum). The energy level of
LUMO was calculated by using Eqs. (3) and (4):
.
Eg ¼ 1240 λonset
ð3Þ
ð4Þ
ELUMO ¼ EHOMO þ Eg
The corresponding LUMO energy level of PhImPOTD
was calculated to be −2.41 eV. The molecular orbital data
indicate that the HOMO/LUMO level of diphenylphosphine
oxide substituted PhImPOTD are obviously lower than
in hexane
1.0
0.8
0.6
0.4
0.2
0.0
in toluene
in ether
in chloroform
in dichloromethane
in acetomitrile
350
400
450
500
550
600
Wavelength/nm
Scheme 2 Energy level of device A and chemical structures of the carrier
Fig. 5 Solvatochromic shifts of PL spectra of PhImPOTD
transporting materials

J Fluoresc
Table 2 Key performance parameters of non-doped deep blue devices
Material
Device Von (V)a
λ
max (nm) FWHM (nm) CEmaxb (cd A−1
)
PEmaxb (lm W−1
)
EQEmaxb (%) CIE (x, y)c
PhImPOTD
A
B
3.6
3.6
444
62
1.13
4.83
0.50
2.15
1.00
2.2
(0.151, 0.084)
(0.317, 0.315)
PhImPOTD + PO01
444 + 556
−
a Voltage required for 1 cd m−2
b current efficiency (CEmax), power efficiency (PEmax), external quantum yield (EQEmax
c The CIE are measured at 1000 cd m−2
)
those of the electron-rich substituted one [28], which proved
the electrophilic substituent group pulls the HOMO/LUMO
levels down.
with that in solution, which is likely to be caused by π–π
stacking and intermolecular aggregation in the solid state.
The photoluminescence quantum yield (PLQY) is determined
by 9,10-dipenylanthracene (Φ = 0.9 in cyclohexane) as a stan-
dard. PhImPOTD shows the PLQY is as high as 0.62 in
CH2Cl2. However, PLQY is decreased to 0.34 in the solid
state, which should be attributed by the aggregation
quenching. Then, the solvatochromic shifts are investigated
in different polarity solvents. These solvents are n-hexane,
toluene, chloroform, ether, dichloromethane and acetonitrile,
respectively. The orientation polarization of these solvents are
0.0012, 0.014, 0.15, 0.17, 0.22 and 0.31, respectively. The
emission colors obtained from various solutions, and
solvatochromic shifts of PL spectra for PhImPOTD are
shown in Fig. 5. The wavelength of maximum PL intensity
for PhImPOTD red-shifted from 398 (n-hexane) to 420 nm
(acetonitrile), with increasing solvent polarity. Such
solvatochromic behavior demonstrates the existence of CT
moiety in the excited state [37].
Photophysical Properties
The electronic absorption and steady-state photoluminescence
spectra of PhImPOTD were measured at room temperature in
dichloromethane and in thin film, as illustrated in Fig. 4 and
Table 1. The compound features the intense absorption bands
in the region of 230–390 nm in dichloromethane, which con-
sist of three bands around 230, 260 and 339 nm. The strong
absorption peak located at 230 nm originated from the π → π*
transition of diphenylphosphinyl moiety [34]. The absorption
bands at around 260 nm can be assigned to the isolated ben-
zene ring connected with imidazole [35], the π → π* transi-
tion of dibenzothiophene (280–305 nm) partially merges with
this absorption band, the broad absorption bands at 339 nm
might be corresponds to the π–π* transition of the substituent
on the 2-imidazole position to the PI unit [36]. Moreover, the
optical gap in solution was 3.19 eV for PhImPOTD, estimat-
ed according to the onset of the absorption spectrum in
CH2Cl2. In the photoluminescence spectra, PhImPOTD
shows a structureless emission peak at 414 nm. As expected,
the emission maxima of PhImPOTD in a thin film (at
438 nm) is bathochromic shifted by ca. 24 nm in comparison
Electroluminescence Properties
To explore the EL properties of PhImPOTD, we construct the
device Awith a frequently used multilayered structure: indium
tin oxide (ITO)/MoO3 (10 nm)/ N,N′-Bis-(1-naphthalenyl)-
N,N′-bis-phenyl-(1,1′-biphenyl)-4,4′-diamine (NPB)
(80 nm)/ PhImPOTD (30 nm)/ 1,3,5-Tri(1-phenyl-1H-
benzo[d]imidazol-2-yl)phenyl (TPBi) (40 nm)/LiF (1 nm)/Al
1.0
1.00
0.95
500
0.8
Device A current destiny
0.90
Device A luminance
1000
0.85
400
0.6
0.80
420
430
440
450
460
470
300
100
6V
7V
8V
9V
0.4
0.2
0.0
200
10
100
10V
0
400
500
600
700
800
1
16
0
2
4
6
8
10
12
14
Wavelength (nm)
Voltage (V)
Fig. 7 Current density–voltage–luminance characteristics for devices A
Fig. 6 Electroluminescence spectra for device A at different voltages.
The inset shows a magnified view of the emission peaks

J Fluoresc
1.2
0.9
6V
7V
8V
9V
10V
1.0
0.8
0.6
0.4
0.2
0.0
0.6
0.3
0.0
0.50
0.25
400
500
600
700
Wavelength (nm)
Fig. 10 Electroluminescence spectra for device B at different voltages
0.00
1.0
no vibronic feature under the applied voltages ranging from 6
to 10 V. Evidently, such the EL spectrum is very similar to the
PL spectra observed from film. In detail, the full width at half
maximum (FWHM) of EL and PL is 62 nm and 68 nm,
replectively, Noticeably, no other emission from any other
layers was observed for device A, suggesting that all excitons
generated in the device are effectively confined in the emitting
layer (EML). At the same time, no exmicer or exciplex emis-
sion was observed. Due to the substituent on the imidazole
position and the polyaromatic hydrocarbon moieties,
PhImPOTD exists the locally twisted structure, which results
in large steric hindrance. Figure 7 and Fig. 8 show the current
density-voltage-luminance (J–V–L) characteristics and plots
of efficiency versus current density of the fabricated deep blue
device, respectively. The device yields a maximum luminance
of 1341 cd m−2, an external quantum yield of 1.00%, and a
maximum luminous efficiency of 1.13 cd A−1. Despite the
efficiencies of device with PhImPOTD are slightly lower
than those of the reference devices [28, 36], it is definitely a
worthwhile work that Commission Internationale de
l’Eclairage (CIE) color coordinates of (0.15, 0.08) are
0.5
0.0
0
200 400 600 800 1000 1200 1400
Luminance (cd m-2)
Fig. 8 Efficiency versus luminance curves of non-doped blue device
based on PhImPOTD
(100 nm), in which MoO3 and LiF are utilized as a hole and an
electron-injecting layer, respectively. NPB and TPBi serve as
a hole and an electron-transporting layer, respectively
(Scheme 2). The key parameters of device are summarized
in Table 2. From the EL spectra, shown in Fig. 6, we could
see an obvious deep-blue emission with λmax at 444 nm and
Hole only
Electron only
100
10000
500
Device B current destiny
Device B luminance
400
300
200
100
0
1000
100
10
1
0.01
0
2
4
6
8
10
12
14
1
16
0
2
4
6
8
10
12
14
Voltage (V)
Voltage (V)
Fig. 11 Current density–voltage–luminance characteristics for device B
Fig. 9 Current density versus voltage characteristics of hole-only and
electron-only devices for PhImPOTD

J Fluoresc
2.5
2.0
1.5
1.0
0.5
0.0
the phosphor PO-01. The low driving voltage of 3.6 V for
onset is realized owing to the well matching HOMO/LUMO
level between EML and transport layers. (Fig. 11).
PhImPOTD and PO-01 endow device B the high efficiencies
with 4.83 cd A−1 for maximum current efficiency (C.E.),
2.15 lm W−1 for power efficiency (P.E.) and a maximum lu-
minance of 8472 cd m−2 (Fig. 12). The white light emitting
device is with the CIE coordinates of (0.32, 0.32) at the lumi-
nance of 1000 cd m2, which is very close to the standard white
light point of (0.33, 0.33). In addition, a small offset of CIE
coordinates of emitted light is observed under the various
biases (at the luminance of 300–7500 cd A−1), which reflects
good color stability for doped WOLEDs. The inset of Fig. 11
is a snapshot of the WOLED at 14.0 V; a suitable white light
emission with a uniform emitting area is seen.
5
4
3
2
1
0
0
2000
4000
6000
8000
10000
Luminance (cd m-2)
Fig. 12 Efficiency versus luminance curves of doped white device B
based on PhImPOTD
attained. This value is very close to the NTSC (National
Television Standards Committe) blue standard (CIE: 0.14,
0.08). Meanwhile, the device A shows low turn-on voltage
of 3.6 V, which should be attributed to the well matched
HOMO level of diphenylphosphine substituent PI emitters
(5.6 eV) and the adjacent NPB (5.4 eV). To further understand
the charge injection/transportation characteristics of
PhImPOTD, we fabricated the single-carrier devices with
the configurations of ITO/MoO3 (0.2 nm)/NPB (40 nm)/
PhImPOTD (30 nm)/NPB (40 nm)/MoO3 (0.2 nm)/Al
(100 nm) (hole-only transporting device) and ITO/LiF
(0.1 nm)/ TPBi (40 nm)/PhImPOTD (30 nm)/TPBi (40 nm)/
LiF (0.1 nm)/Al (100 nm) (electron-only transporting device).
MoO3 and TPBi layers are used to prevent electron and hole
injection from the cathode and anode, respectively. As
depicted in Fig. 9, the current density–voltage (J–V) charac-
teristics illustrate that the hole current density values of
PhImPOTD approximate the same as the electron current
density, which undoubtedly evidences the balanced bipolar
charge transport capacity of PhImPOTD. All these results
indicate that PhImPOTD is an excellent bipolar material,
which can be served as non-doped deep blue OLEDs.
WOLEDs have drawn tremendous intentions and been
studied extensively because of their great promise for univer-
sal application in future solid-state lighting sources and back-
lights for full-color displays [38]. White-light emission can be
acquired by complementary color (blue plus yellow or blue,
green plus red) or broadband emission. To further explore the
potential of deep blue emitter PhImPOTD [39, 40], we tried
to fabricate device B through precisely controlling the ratio of
PhImPOTD and the complementary yellow emitter PO-01
(acetylacetonatobis(4-phenylthieno[3,2-c]pyridinato-N,C2’)
Iridium), aiming for achieving WOLEDs. The typical EL
spectra of the WOLEDs at different voltages are shown in
Fig. 10. The dual emissions EL spectra could be divided into
their blue emission (444 nm) corresponding to the fluorophore
PhImPOTD and yellow emission (556 nm) corresponding to
Conclusion
In summary, a new phenanthroimidazole derivative
PhImPOTD with ambipolar transport behavior has been syn-
thesized and characterized. By combing the advantages of
phenanthroimidazole backbone and the D–π–A structure,
high efficient deep blue emitter was achieved. Further, the
deprotonation of n-type imidazole moiety by electron-
withdrawing diphenylphosphine oxide increased the unsym-
metrical configuration, which could efficiently prevent the
strong π–π stacking and molecular interactions. It is reason-
able that PhImPOTD possesses high quantum efficiency and
thermal stability. PhImPOTD has not only been used as emit-
ter to fabricate deep-blue OLEDs, but also as host materials to
construct highly efficient WOLEDs. Non-doped PhImPOTD
multilayer device exhibits a deep-blue emission (CIE: 0.15,
0.08), together with a low driving voltage of 3.6 V, efficiencies
of 1.13 cd A−1 for C.E., and 1.00% for E.Q.E. Doped
WOLED with CIE coordinates of (0.32, 0.32) gave the per-
formance of 4.83 cd A−1 for C.E., 2.15 lm W−1 for P.E. and
3.6 V for onset voltage. It also possesses good color stability
under broad luminance. We believe that this work gives a
novel clue for high performance deep blue and white emitting
fluorescent OLEDs.
Acknowledgements The authors are grateful for the support by
National Natural Science Foundation of China (Grant No. 111572002)
and the Foundation for Innovative Research Groups of the National
Natural Science Foundation of China (Grant No. 11521202).
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