High-performance non-doped pure-blue electroluminescent device based on bisphenanthroimidazole derivative with twisted donor-acceptor structure

Article abstract of DOI:10.1016/j.orgel.2021.106171

The development of efficient pure-blue emitters is of great significance for achieving high-quality display and lighting. Herein, a pure-blue emitter TPA-DPPI comprised of bisphenanthroimidazole and triphenylamine units with bipolar carrier transport property is designed and synthesized. For the two phenanthroimidazole units, one acts as the electron acceptor moiety, and the other is introduced to twist the molecule in order to suppress the intermolecular interactions in neat film. Highly twisted molecular configuration endows the compound with a high photoluminescence quantum efficiency yield of 0.56 in neat film. The non-doped device using TPA-DPPI as an emissive layer shows a pure-blue emission with a Commission International de l'Eclairage coordinates of (0.146, 0.097) and a high external quantum efficiency of 5.20%.

Full text of DOI:10.1016/j.orgel.2021.106171

Organic Electronics 94 (2021) 106171
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: http://www.elsevier.com/locate/orgel
High-performance non-doped pure-blue electroluminescent device based
on bisphenanthroimidazole derivative with twisted
donor-acceptor structure
,
,**
Zhibo Zhanga, Baoyan Liangb *, Yuanyuan Cuia, Kaiqi Yea, Chenglong Lia
a State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, 130012, People’s Republic of China
b Jihua Laboratory, 28 Huandao South Road, Foshan, 528200, Guangdong Province, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The development of efficient pure-blue emitters is of great significance for achieving high-quality display and
lighting. Herein, a pure-blue emitter TPA-DPPI comprised of bisphenanthroimidazole and triphenylamine units
with bipolar carrier transport property is designed and synthesized. For the two phenanthroimidazole units, one
acts as the electron acceptor moiety, and the other is introduced to twist the molecule in order to suppress the
intermolecular interactions in neat film. Highly twisted molecular configuration endows the compound with a
high photoluminescence quantum efficiency yield of 0.56 in neat film. The non-doped device using TPA-DPPI as
an emissive layer shows a pure-blue emission with a Commission International de l’Eclairage coordinates of
(0.146, 0.097) and a high external quantum efficiency of 5.20%.
Pure-blue emission
Non-doped devices
Bisphenanthroimidazole derivatives
Twisted configuration
Organic light-emitting diodes
1. Introduction
efficiency roll-offs and low highest brightness of pure-blue TADF devices
are also far away from the manufacture requirements. Hence, efficient
In the past few decades, organic light-emitting diodes (OLEDs) have
been paid considerable attention owing to their potential applications in
pure-blue materials used in OLED devices in the industry are still rare
and developing highly efficient and stable pure-blue emitters is
extremely urgent.
flat-panel
displays
and
solid-state
lighting
sources
[1].
High-performance pure-blue emitters meeting Commission Interna-
tional de l’Eclairage (CIE) coordinates (0.14, 0.08) defined by the Na-
tional Television System Committee (NTSC) are of particular
significance for full-color displays and white lighting [2,3]. Phospho-
rescent materials based on organic-metal complexes can utilize both the
singlet and triplet excitons via heavy atoms induced spin-orbit coupling
and achieve 100% internal quantum efficiency (IQE) theoretically
[4–6]. However, efficient pure-blue phosphors with CIE coordinates
around (0.14, 0.08) are extremely rare due to the inherent difficulty in
molecular design. Furthermore, high cost of phosphorecent materials is
also disadvantageous for the large-scale practical applications. Ther-
mally activated delayed fluorescence (TADF) materials proposed
recently can also achieve 100% exciton utilization efficiency (EUE) by
efficient reverse intersystem crossing (RISC), but the significant intra-
molecular charge transfer properties of TADF materials usually
contribute to red-shifted emission spectra, which increases the difficulty
of designing pure-blue TADF materials [7–10]. Moreover, the severe
The electroluminescence (EL) performance of blue emitting mate-
rials is often inferior to that of green and red emitters because of their
intrinsically wide band gaps which result in poor charge injection and
transportation in the emitting layer [11,12]. The choice of hosts of blue
materials is also a nerve-wracking issue, because the host materials must
possess even larger band gaps and well-matched frontier molecular
orbital energy levels. Construction of non-doped devices is an efficient
strategy to solve the above-mentioned issues. On the one hand, the
configuration of blue materials must be highly twisted for avoiding
aggregation induced quenching and red shifting of emission. On the
other hand, the blue materials must be bipolar for enlarging the exciton
recombination region and suppressing the efficiency roll-off at high
current density.
In this contribution, we designed and synthesized a twisted donor-
acceptor (D-A) type blue emitting molecule, N,N-diphenyl-4’-(1-(4-(2-
phenyl-1H- phenanthro[9,10-d]imidazole-1-yl)phenyl)-1H-phenanthro
[9,10-d]imidazole-2-yl)-[1,1′-biphenyl]-4-amine, namely TPA-DPPI. In
* Corresponding author.
** Corresponding author.
E-mail addresses: liangby@jihualab.com (B. Liang), chenglongli@jlu.edu.cn (C. Li).
https://doi.org/10.1016/j.orgel.2021.106171
Received 4 March 2021; Received in revised form 5 April 2021; Accepted 12 April 2021
Available online 20 April 2021
1566-1199/© 2021 Elsevier B.V. All rights reserved.

Z. Zhang et al.
Organic Electronics 94 (2021) 106171
this molecule, triphenylamine (TPA) is used as the electron donor
moiety, and the adjacent PPI is chosen as the electron acceptor unit.
Additional PPI group is introduced to twist the molecule in order to
suppress the intermolecular interactions in neat film, which can not only
improve photoluminescence quantum efficiency yield (PLQY), but also
can lead to blue-shift of the fluorescent emission in neat film. As a result,
the compound exhibits a high PLQY of 0.56 in neat film and blue
emission with a peak wavelength at 460 nm. In addition, TPA-DPPI also
exhibits bipolar carrier transporting characteristic from the results of the
hole-only and electron-only devices. The non-doped OLED using TPA-
DPPI as the emitting layer shows pure-blue emission with a CIE co-
ordinates (0.146, 0.097) and high device performance with a maximum
external quantum efficiency (EQE) of 5.20%, which is among the highest
values reported for non-doped pure-blue OLEDs [13–18].
oxidation process with the onset voltage of ca. 0.47 V. Therefore, the
estimated highest occupied molecular orbital (HOMO) level is ꢀ 5.27 eV
for TPA-DPPI with regard to the energy level of ferrocene (4.8 eV below
vacuum). No clear reduction wave was observed within the potential
window of the CV, the lowest unoccupied molecular orbital (LUMO)
level was deduced from the HOMO energy level and the optical band gap
(3.05 eV) determined by the onset of the absorption spectrum (Fig. 2a).
The LUMO level was calculated to be ꢀ 2.22 eV for TPA-DPPI.
To gain insight into the structure-property relationship of TPA-DPPI
at molecular level, density functional theory (DFT) calculations with a
B3LYP/6-31G (d,p) basis set were carried out using Gaussian 09 pack-
age. The optimized structure and electron density distribution of the
compound are shown in Fig. 1. Large dihedral angles of 88.93◦ and
68.10◦ could be observed between each of the two PI moieties and the
C6H4 linker (P1) between them, respectively. In addition, the dihedral
angles were 29.39◦ and 33.63◦ between the C6H4 linker (P2) and adja-
cent imidazole/TPA units. The highly twisted molecular conformation
2. Results and discussion
The synthetic procedures and molecular structure of TPA-DPPI were
presented in Scheme 1. The intermediate NO2-PPI was prepared from
phenanthrene-9,10-dione, benzaldehyde, 4-nitroaniline and ammonium
acetate. The key precursor NH2-PPI was synthesized through reducing
NO2-PPI using Pd/C and hydrazine hydrate. The final product was
synthesized through a one-pot cyclizing reaction by refluxing a mixture
of phenanthrene-9,10-dione, NH2-PPI, TPAPh-CHO and ammonium
acetate in acetic acid for 5 h [19–21]. TPA-DPPI and the intermediates
were further characterized by 1H nuclear magnetic resonance (NMR)
(Fig. S1~Fig. S3), mass spectrometry (Fig. S4~Fig. S6) and elemental
analysis. 13C NMR spectrum of TPA-DPPI was not available owing to the
poor solubleness.
can effectively suppress intermolecular π-π interactions, leading to a
high PLQY in neat film. According to DFT calculations, the HOMO of
TPA-DPPI was predominantly located on the TPA moiety, P2 linker and
partly on the imidazole ring. The LUMO was mainly distributed on P2
and the adjacent PPI unit. It is worth noting that the LUMO and HOMO
orbits showed a slight overlap on the imidazole ring and P2, which could
contribute to highly efficient fluorescence radiative decay. In addition,
the additional PPI unit scarcely participated in the FMO distributions
but played a role in twisting the molecule. Theoretical calculation by
time-dependent DFT (TD-DFT) was also performed. The calculated en-
ergy level of S1 was 3.12 eV, which was consistent with the experimental
results. The natural transition orbitals (NTO) analysis results are shown
in Fig. S9, the transition from S0 to S1 was also charge transfer (CT)
dominated with a high oscillator strength of 0.3876, while the transition
from S0 to T1 was locally excited dominated.
The thermal properties of TPA-DPPI were investigated using thermal
gravimetric analysis (TGA) and differential scanning calorimetric (DSC)
measurements (Fig. S7), the corresponding thermal data are summa-
rized in Table S1. The compound exhibited excellent thermal stability
with a thermal decomposition temperature (Td, corresponding to 5%
weight loss) of 535 ◦C. A high melting point of 355 ◦C and a high glass-
The ultraviolet–visible (UV–vis) absorption and photoluminescence
(PL) spectra of TPA-DPPI in dichloromethane (DCM) solution and neat
thin film are shown in Fig. 2a and the corresponding data are summa-
rized in Table S1. The strong absorption band peaked at ca. 260 nm
◦
transition temperature (Tg) of 212 C were also obtained, which were
high enough for application in OLEDs and should be attributed to the
high molecular weight and rigid skeleton of DPPI. The excellent thermal
property of TPA-DPPI is advantageous to form stable thin films upon
thermal evaporation, and thus improves the operational performance of
the OLEDs.
could be attributed to the π–π* transition of the benzene rings. While the
longer wavelength absorption bands around 360 nm could be assigned
to the intramolecular charge transfer (ICT) transition [20,21]. Benefit-
ting from twisted molecular configuration induced weak intermolecular
interactions, the absorption spectrum of TPA-DPPI in the film state was
very similar to that measured in DCM solution. TPA-DPPI showed a blue
emission at 451 nm in DCM and 460 nm in the neat film. The PLQYs in
The electrochemical property of TPA-DPPI was measured by cyclic
voltammetry (CV). As depicted in Fig. S8, TPA-DPPI showed an
Scheme 1. The synthetic routes of TPA-DPPI. (i) phenanthrenequinone, benzaldehyde, ammonium acetate, glacial acetic acid, reflux; (ii) 10% Pd/C, 80% hydrazine
hydrate, ethanol, reflux; (iii) Pd(PPh3)4, K2CO3, THF, reflux; (iv) ammonium acetate, glacial acetic acid, reflux.
2

Z. Zhang et al.
Organic Electronics 94 (2021) 106171
Fig. 1. DFT-calculated spatial distributions of the HOMO and LUMO of TPA-DPPI.
Fig. 2. (a) UV–vis absorption and PL spectra of TPA-DPPI measured in DCM solution (1.0 × 10ꢀ 5 M) and neat thin film. (b) PL spectra of TPA-DPPI measured in
different solvents (10ꢀ 5 M) at room temperature.
DCM solution and film state were measured to be 0.95 and 0.56 by using
an integrating sphere, respectively. In addition, the PL spectra of
TPA-DPPI showed an obvious solvatochromic property. The emission
peaks red-shifted from 413 nm (in hexane) to 473 nm (in dimethyl
sulfoxide) with increasing solvent polarity (Fig. 2b), indicating the
excited state of TPA-DPPI is CT-dominated, which is in good agreement
with DFT calculation results. The phosphorescence spectrum of
TPA-DPPI in dilute toluene measured at 77 K is showed in Fig. S10, and
the energy level of the first triplet state was calculated to be 2.68 eV from
the onset of the phosphorescence spectrum.
To investigate the hole and electron transporting properties of the
compound, single-carrier devices with the configurations of [ITO/NPB
(10 nm)/TPA-DPPI (60 nm)/NPB (10 nm)/Al (100 nm)] for hole-only
device and [ITO/TPBi (10 nm)/TPA-DPPI (60 nm)/TPBi (10 nm)/LiF
(1 nm)/Al (100 nm)] for electron-only device were fabricated [22]. For
the hole-only device, only holes could be injected from the indium tin
Fig. 3. Current density versus voltage characteristics of the hole-only and
oxide (ITO) anode to the active layer due to a large energy barrier (2.0
electron-only devices of TPA-DPPI.
eV) from the Al cathode (work function: 4.3 eV) to the neighbouring
NPB (LUMO: ꢀ 2.3 eV) layer. Similarly, in the electron-only device, hole
(TPBi) was utilized as the electron-transporting and hole-blocking layer,
and LiF served as electron-injecting layer, respectively. The character-
istic curves and device data are shown in Fig. 5 and Table 1. The device
exhibited a pure-blue emission with a CIE coordinates of (0.146, 0.097),
which is very close to the NTSC blue standard (CIE: 0.14, 0.08) and
displayed little change over a wide range of driving voltages (Fig. S11).
The device showed an emission maximum at 460 nm, which was very
close to that of the thin-film PL spectrum, suggesting that the EL emis-
sion came from the intrinsic emission of TPA-DPPI. The device exhibited
a turn-on voltage (Von) of 3.4 V. High device performance with a
maximum luminance (Lmax) of 8561 cd mꢀ 2, a maximum current effi-
ciency (CEmax) of 4.45 cd Aꢀ 1, a maximum power efficiency (PEmax) of
4.11 lm Wꢀ 1, and a maximum external quantum efficiency (EQEmax) of
5.20% were achieved, which is comparable with the highest value re-
ported for non-doped pure blue fluorescent OLEDs (Table S2). In addi-
tion, the efficiency of the device was still maintained at a high level (CE:
3.16 cd Aꢀ 1 and EQE: 3.67%) at 1000 cd mꢀ 2, indicating a mild
injection from the ITO anode to the neighbouring TPBi layer was pre-
vented due to a large difference (1.4 eV) between the work function of
ITO (4.8 eV) and the HOMO level of TPBi (ꢀ 6.2 eV), and only electrons
could be injected. The current density-voltages (J–V) curves of
single-carrier devices indicated that TPA-DPPI was a bipolar trans-
porting material and was capable of transporting both electrons and
holes (Fig. 3). The balanced carrier transportation contributes to extend
exciton recombination region and suppress the efficiency roll off at high
current density [23,24].
To investigate the EL performance of TPA-DPPI as an emitter, non-
doped device with a configuration of [ITO/NPB (50 nm)/TCTA (5
nm)/TPA-DPPI (30 nm)/TPBi (25 nm)/LiF (1 nm)/Al (100 nm)] was
fabricated. The energy-level diagram and molecular structures of the
used materials are shown in Fig. 4. In this device, 1,4-bis[(1-naphthyl-
phenyl)amino]-biphenyl (NPB) was used as the hole-transporting
layer, 4,4′,4′′-tri(N-carbazolyl)triphenyl-amine (TCTA) was used as the
electron-blocking layer, 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene
3

Z. Zhang et al.
Organic Electronics 94 (2021) 106171
Fig. 4. Energy level diagrams and the chemical structures of the relevant compounds used in the devices based on TPA-DPPI.
Fig. 5. (a) Current density–voltage–brightness (J–V–L) characteristics; (b) Current efficiencies and power efficiencies versus current density curves; (c) External
quantum efficiency versus current density curve; (d) EL spectrum of device based on TPA-DPPI at 100 cd mꢀ 2
.
efficiency roll-off. The small efficiency roll-off was attributed to the bi-
polar transport nature of TPA-DPPI.
Table 1
Electroluminescent performance of the OLED based on TPA-DPPIa.
Emitter
λem
nm
/
Von
/
Lmax
/
CEb/cd
PEb/lm
EQEb/
%
CIE (x,
3. Conclusions
V
cd mꢀ 2
8561
Aꢀ 1
Wꢀ 1
y)c
TPA-
DPPI
a
460
3.4
4.45,
3.16
4.11,
1.24
5.20,
3.67
0.146,
0.097
In summary, we have designed and synthesized a D-A type pure-blue
emitting molecule TPA-DPPI. Between TPA and the adjacent PPI charge-
transfer process exists, the additional PPI moiety makes the whole mo-
lecular with a highly twisted configuration, which contributes to sup-
press aggregation-induced redshift and quenching of TPA-DPPI in neat
film. Eventually, TPA-DPPI exhibits a pure-blue emission at 460 nm and
a high PLQY of 0.56 in neat film. The non-doped OLED using TPA-DPPI
as the emitting layer shows pure blue emission with a CIE coordinates of
(0.146, 0.097) and high device performance with a maximum EQE of
5.20%, which is among the best results reported for non-doped pure blue
OLEDs. This study provides an efficient strategy to develop highly effi-
cient blue emitting materials.
Abbreviation: λem: emission peak of EL spectrum; Von: turn-on voltage
recorded at the luminance of 1 cd mꢀ 2; Lmax: maximum luminance; CE: current
efficiency; PE: power efficiency; EQE: external quantum efficiency; CIE: Com-
mission Internationale de l’Eclairage coordinates.
b
c
Values given in the order of maximum and value at 1000 cd mꢀ 2
Recorded at a driving voltage of 5 V.
4

Z. Zhang et al.
Organic Electronics 94 (2021) 106171
4. Experimental section
org/10.1016/j.orgel.2021.106171.
Preparation of Materials: All commercially available reagents were
used as received without further purification. All reactions were carried
out using Schlenk techniques under a nitrogen atmosphere. 4’-(diphe-
nylamino)biphenyl-4-carbaldehyde (TPAPh-CHO) were synthesized
according to literature methods [25,26].
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Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation
of China (51803069), Jihua Laboratory (X201231XF200) and the Sci-
ence and Technology Research Project of the Education Department of
Jilin Province, China (Grant No. JJKH20211043KJ).
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