Highly twisted bipolar emitter for efficient nondoped deep-blue electroluminescence

Article abstract of DOI:10.1016/j.dyepig.2017.01.028

Three novel bipolar deep-blue emitters, 1-(4-(tert-butyl)phenyl)-2-(4-(9,9-diphenylacridin-10(9H)-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (DPACTPI), 1-(4-(tert-butyl)phenyl)-2-(4'-(9,9-diphenylacridin-10(9H)-yl)-[1,1'-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazole (DPACPhTPI) and 2-(4-(9,9-diphenylacridin-10(9H)-yl)phenyl)-1-(4-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazole (DPACFPPI), employing the donor 9,9-diphenyl-9,10-dihydroacridine (DPAC) and the acceptor, phenanthroimidazole, were designed and synthesized. The highly twisted conformations between DAPC and phenanthroimidazole in the molecules efficiently interrupt molecular π-conjugation and inhibit π-π intermolecular interactions, resulting in good thermal stability and efficient deep-blue emission. The three phenanthroimidazole derivatives in non-doped OLEDs exhibited deep-blue emission. In particular, DPACPhTPI-based nondoped device showed an excellent performance with maximum external quantum efficiency (EQE) of 3.50%, maximum current efficiency (CE) of 1.38 cd/A, maximum power efficiency (PE) of 1.40 lm/W and CIE coordinate of (0.156, 0.047), which is among the best results for OLEDs with a similar color. The high radiative exciton utilization could be resulted from hybridized local and charge transfer (HLCT).

Full text of DOI:10.1016/j.dyepig.2017.01.028

Accepted Manuscript
Highly twisted bipolar emitter for efficient nondoped deep-blue electroluminescence
Zhi Huang, Bo Wang, Qing Zhang, Songpo Xiang, Xialei Lv, Lixiang Ma, Bing Yang,
Yu Gao, Lei Wang
PII:
S0143-7208(16)31175-5
10.1016/j.dyepig.2017.01.028
DYPI 5727
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 15 November 2016
Accepted Date: 12 January 2017
Please cite this article as: Huang Z, Wang B, Zhang Q, Xiang S, Lv X, Ma L, Yang B, Gao Y, Wang L,
Highly twisted bipolar emitter for efficient nondoped deep-blue electroluminescence, Dyes and Pigments
(2017), doi: 10.1016/j.dyepig.2017.01.028.
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Graphical Abstract

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Highly Twisted Bipolar Emitter for Efficient Nondoped
Deep-Blue Electroluminescence
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Zhi Huang , Bo Wang , Qing Zhang , Songpo Xiang , Xialei Lv , Lixiang Ma , Bing
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Yang , Yu Gao , Lei Wang *
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science
and Technology, Wuhan, 430074, P. R. China
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State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, P. R. China
Email: wanglei@mail.hust.edu.cn (Prof. L. Wang)
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Abstract:
1-(4-(tert-butyl)phenyl)-2-(4-(9,9-diphenylacridin-10(9H)-yl)phenyl)-1H-phenanthro[
9,10-d]imidazole (DPACTPI),
1-(4-(tert-butyl)phenyl)-2-(4'-(9,9-diphenylacridin-10(9H)-yl)-[1,1'-biphenyl]-4-yl)-1
H-phenanthro[9,10-d]imidazole (DPACPhTPI) and
2-(4-(9,9-diphenylacridin-10(9H)-yl)phenyl)-1-(4-(trifluoromethyl)phenyl)-1H-phena
nthro[9,10-d]imidazole (DPACFPPI), employing the donor
Three
novel
bipolar
deep-blue
emitters,
9,9-diphenyl-9,10-dihydroacridine (DPAC) and the acceptor, phenanthroimidazole,
were designed and synthesized. The highly twisted conformations between DAPC and
phenanthroimidazole in the molecules efficiently interrupt molecular π-conjugation
and inhibit π-π intermolecular interactions, resulting in good thermal stability and
efficient deep-blue emission. The three phenanthroimidazole derivatives in non-doped
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OLEDs exhibited deep-blue emission. In particular, DPACPhTPI-based nondoped
device showed an excellent performance with maximum external quantum efficiency
(EQE) of 3.50%, maximum current efficiency (CE) of 1.38 cd/A, maximum power
efficiency (PE) of 1.40 lm/W and CIE coordinate of (0.156, 0.047), which is among
the best results for OLEDs with a similar color. The high radiative exciton utilization
could be resulted from hybridized local and charge transfer (HLCT).
Keywords: Deep-blue, Phenanthroimidazole, Twisted, Bipolar
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1. Introduction
Organic light emitting diodes (OLEDs) have attracted much attention owing to
their superior potential applications in full-color flat-panel displays and solid-state
lighting [1-2]. Three basic colors (blue, green and red) are essential to the full color
displays. Red and green phosphorescent OLEDs have already satisfied the
commercial requirement in efficiency and stability, while the blue OLEDs are still
one of the bottlenecks for the further application of OLEDs. In recent decades, great
effort has been made to develop deep blue phosphorescent emitters. Unfortunately,
blue phosphorescent OLEDs are still not stable enough for practical applications [3-5].
Recently, thermally activated delayed fluorescence (TADF) has drawn great attention
of researchers due to the possibility of solving the problems of blue OLEDs [6-8].
While most TADF molecules，which consist of donor and acceptor groups with
separated HOMO (highest occupied molecular orbital) and LUMO (lowest
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unoccupied molecular orbital) to decrease the singlet-triplet energy gap, exhibit broad
and structureless emission bands. It’s difficult to develop a deep-blue TADF emitters
due to the large stokes shift [9-11]. Highly efficient deep-blue TADF OLEDs with
CIE less than 0.1 and low efficiency roll-off at high brightness are rare [12]. Thus,
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efficient fluorescent materials are still considered to be promising for the realization
of efficient and stable deep-blue devices.
Constructing bipolar molecules has been proved to be an effective strategy to
develop highly efficient blue electroluminescence materials [13-18]. Bipolar emitting
materials can achieve better balanced charge carriers and a broad exciton formation
zone, consequently simplifying the device configurations and improving the device
performance. Nevertheless, the intramolecular charge transfer (ICT) interaction
between the donor and acceptor usually decreases the energy gap and leads to a
fluorescence maximum redshift. In order to construct a deep blue molecule in a D-A
system, the donor and acceptor should be very carefully selected to regulate the ICT
energy. Phenanthroimidazole is an ideal building block for deep blue/violet
light-emitting materials. It exhibits electron-donating properties when an
electron-withdrawing moiety is attached to the imidazole ring, but it shows electron
withdrawing properties when linked with an electron donor [14, 19-22]. However,
many phenanthroimidazole derivatives suffered from a large redshift in the solid state
due to π-π stacking [22-25], even shifted to blue-green region. Using bulky building
blocks is an effective method to inhibit molecular aggregation or π-π stacking in the
solid state [26-27]. Moreover, a twisted structure could not only avoid π-π stacking in
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the solid state [28-32] but also break the π conjugation to achieve a deep-blue
emission.
Inspired by this, we designed and synthesized three new blue-emissive materials
based on phenanthroimidazole and 9,9-diphenyl-9,10-dihydroacridine. DPAC was
chosen as an electron donor for its good hole-transporting mobility [33] and large
intramolecular torsion angle induced by the steric repulsion between the hydrogen
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atoms of the cross-linking phenylene [34]. Additionally, the sp C atom endows
DPAC unit with more rigidity and bulkiness. Furthermore, we introduce the tert-butyl
and trifluoromethyl group to the N1-substitution of phenanthroimidazole unit, which
can improve the solubility of the compounds in organic solvents, enhance the
morphological stability and suppress the formation of aggregation or π-π stacking in
the solid state. As expected, the three compounds exhibit excellent thermal stability,
bipolar carrier transport ability and deep-blue emission in the solid state. Nondoped
OLEDs based on the three novel emitters show deep-blue light emissions. The
DPACPhTPI-based device exhibited excellent EL performance with CIE coordinates
of (0.156, 0.047), low turn-on voltage of 3.0 V, high external quantum efficiency
(EQE) of 3.5%, current efficiency (CE) of 1.38 cd/A, and power efficiency (PE) of
1.40 lm/W, which is comparable to the reported high-efficiency OLEDs with similar
color.
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2. Experimental section
2.1. Material and method
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All the reagents and solvents used for the syntheses and measurements were
purchased from commercial suppliers and were used without further purification
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unless otherwise noted. H NMR and C NMR spectra were conducted on a
Bruker-AF301 AT 400 MHz spectrometer. Mass spectra were recorded using an
Agilent (1100 LC/MSD Trap) using APCI ionization. High-resolution mass
spectrometry (HRMS) were carried out on a Bruker Daltonics SolariX 7.0T
spectrometer using APCI ionization. Elemental analyses of carbon, hydrogen, and
nitrogen were performed on an Elementar (Vario Micro cube) analyzer.
Thermogravimetric analyses (TGA) were undertaken using a PerkinElmer
Instruments (Pyris1 TGA) under nitrogen atmosphere at a heating rate of 10 � /min
from 30 to 600� . The differential scanning calorimetry (DSC) was performed on a
TA Q2000 DSC instrument under nitrogen at a heating rate of 10� /min from 30 to
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250� . The glass transition temperature (T ) was determined from the second heating
g
scan. UV-Vis spectra were measured using
a
Shimadzu UV-VIS-NIR
Spectrophotometer (UV-3600). Photoluminescence (PL) spectra were recorded on a
Hitachi F-7000 fluorescence spectrophotometer. The photoluminescence quantum
yields (PLQYs) were measured in DCM using 9,10-diphenylanthracene as a standard
(Φ=0.90 in cyclohexane). Cyclic voltammetry (CV) was performed on a
computer-controlled EG&G Potentiostat/Galvanostat model 283 at room temperature
with a conventional three electrode cell, which consisted of a platinum wire counter
electrode, an Ag/AgNO (0.1 M) reference electrode and a Pt carbon working
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electrode of 2 mm diameter. Oxidations of all compounds were performed in dried
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dichloromethane containing 0.1 M tetrabutylammonium hexafluorophosphate
(Bu NPF ) as supporting electrolyte. The onset potential was determined from the
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intersection of two tangents drawn at the rising and background current of the cyclic
voltammogram. The solutions were purged with a nitrogen stream for 10 min before
measurements.
2.2. Preparation of compounds
2.2.1. Synthesis
1-(4-(tert-butyl)phenyl)-2-(4-(9,9-diphenylacridin-10(9H)-yl)phenyl)-1H-phen
anthro[9,10-d]imidazole (DPACTPI)
mixture
of
A
of
2-(4-bromophenyl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (1.515
g, 3 mmol), 9,9-diphenyl-9,10-dihydroacridine (1 g, 3 mmol), t-BuONa (0.576 g, 6
mmol), tri-tert-butylphosphonium tetrafluoroborate (52 mg, 0.18 mmol) and
palladium acetate (20 mg, 0.09 mmol) was dissolved in dry toluene (50 mL), and
purged three times by nitrogen/vacuum cycle. And then, the reaction mixture was
refluxed for 12 hours. After cooling to room temperature, the solution was extracted
with dichloromethane. The organic layer was dried by anhydrous MgSO and
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concentrated. Then the crude product was purified by silica gel chromatography. The
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desired product was obtained as white powder (2.1 g, yield: 92%). H NMR (400
MHz, CDCl ) δ [ppm]: 8.93 (s, 1H), 8.79 (d, J = 8.4 Hz, 1H), 8.73 (d, J = 8.4 Hz, 1H),
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7.78-7.52 (m, 8H), 7.47 (d, J = 8.0 Hz, 2H), 7.31-7.20 (m, 6H), 7.04-6.97 (m, 9H),
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6.89-6.88 (m, 4H), 6.40 (d, J = 8.0 Hz, 2H), 1.45 (s, 9H); C NMR (100 MHz, CDCl )
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δ [ppm]: 146.35, 141.88, 131.65, 130.95, 130.36, 130.08, 128.44, 127.62, 127.21,
126.82, 126.46, 126.26, 124.16, 123.14, 120.99, 120.37, 114.16, 56.74, 35.10, 31.39;
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MS (APCI): calculated for C H N , 757.35; found, 758.4, [M+H] ; Elemental
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analysis calculated (%) C H N : C 88.74, H 5.72, N 5.54; found: C 88.53, H 5.87, N
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5.51.
2.2.2. Synthesis of 4'-bromo-[1,1'-biphenyl]-4-carbaldehyde (1)
mixture of (4-formylphenyl)boronic acid (0.6 g,
1-bromo-4-iodobenzene (1.3 g, 4.3 mmol), Pd(PPh ) (46 mg, 0.04 mmol), K CO
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mmol),
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(2.0 M aqueous solution, 60 mL), toluene (60 mL) and ethanol (30 mL) were stirred
under nitrogen at 90 °C for 12 h. After cooling to room temperature, the reaction
mixture was extracted with dichloromethane and further purified by column
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chromatography to obtain a white solid (1 g, yield: 95%). H NMR (400 MHz, CDCl )
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δ [ppm]: 10.06 (s, 1H), 7.96 (d, J = 8.0 Hz, 2H), 7.72 (d, J = 8.0 Hz, 2H), 7.61 (d, J =
8.0 Hz, 2H), 7.51 (d, J = 8.4 Hz, 2H).
2.2.3. Synthesis
of
2-(4'-bromo-[1,1'-biphenyl]-4-yl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,1
0-d]imidazole (2)
A
mixture of 9,10-phenanthrenequinone (0.8 g, 3.83 mmol),
4'-bromo-[1,1'-biphenyl]-4-carbaldehyde (1 g, 3.83 mmol), 4-(tert-butyl)aniline (0.57
g, 3.83 mmol) and ammonium acetate(1.8 g, 23 mmol) was dissolved in acetic acid
(50 mL), and was heated to reflux for 8 h under nitrogen atmosphere. After cooling to
room temperature, the mixture was poured into water. The solid product was filtered,
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washed with methanol and dried under vacuum to afford a pale-yellow solid (1.8 g,
yield: 80%). The crude product was used in the next step without further purification.
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H NMR (400 MHz, CDCl ) δ [ppm]: 8.92 (s, 1H), 8.77 (d, J = 8.4 Hz, 1H), 8.71 (d, J
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= 8.4 Hz, 1H), 7.76 (t, J = 7.6 Hz, 1H), 7.70-7.62 (m, 5H), 7.56-7.43 (m, 9H),
7.29-7.25 (m, 1H), 7.19 (d, J = 8.0 Hz, 1H), 1.46 (s, 9H); HRMS (APCI): calculated
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for C H BrN , 582.1494; found, 583.1540, [M+H] .
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2.2.4. Synthesis
of
1-(4-(tert-butyl)phenyl)-2-(4'-(9,9-diphenylacridin-10(9H)-yl)-[1,1'-biphenyl]-
4-yl)-1H-phenanthro[9,10-d]imidazole (DPACPhTPI)
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DPACPhTPI was prepared with the similar procedure as that of DPACTPI. H
NMR (400 MHz, CDCl ) δ [ppm]: 8.99 (s, 1H), 8.78 (d, J = 8.4 Hz, 1H), 8.72 (d, J =
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8.4 Hz, 1H), 7.79-7.48 (m, 13H), 7.30-7.19 (m, 8H), 7.13 (d, J = 8.0 Hz, 2H),
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7.08-7.00 (m, 6H), 6.91-6.86 (m, 4H), 6.48 (d, J = 8.0 Hz, 2H), 1.47 (s, 9H);
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NMR (100 MHz, CDCl ) δ [ppm]: 146.46, 142.13, 140.33, 140.00, 131.65, 130.41,
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130.05, 129.59, 129.45, 129.05, 128.52, 128.40, 127.62, 127.23, 126.86, 126.38,
126.26, 125.94, 125.10, 124.13, 123.12, 120.97, 120.20, 114.04, 56.76, 35.11, 31.45;
+
MS (APCI): calculated for C H N , 833.38; found, 834.8, [M+H] ; Elemental
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analysis calculated (%) C H N : C 89.28, H 5.68, N 5.04; found: C 89.03, H 5.81, N
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5.01.
2.2.5. Synthesis
of
2-(4-bromophenyl)-1-(4-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imid
azole (3)
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3 was prepared with the similar procedure as that of 2. Yield: 88% . H NMR
(400 MHz, CDCl ) δ [ppm]: 8.85 (d, J = 7.6 Hz, 1H), 8.77 (d, J = 8.4 Hz, 1H), 8.69 (d,
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J = 8.0 Hz, 1H), 7.88 (d, J = 8.0 Hz, 2H), 7.74 (t, J = 7.6 Hz, 1H), 7.69-7.63 (m, 3H),
7.54 (t, J = 7.6 Hz, 1H), 7.45-7.28 (m, 5H), 7.09 (d, J = 8.0 Hz, 1H); MS (APCI):
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calculated for C H BrF N , 516.04; found, 517.1, [M+H] .
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2.2.6. Synthesis
of
2-(4-(9,9-diphenylacridin-10(9H)-yl)phenyl)-1-(4-(trifluoromethyl)phenyl)-1H
-phenanthro[9,10-d]imidazole (DPACFPPI)
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DPACFPPI was prepared with the similar procedure as that of DPACTPI. H
NMR (400 MHz, CDCl ) δ [ppm]: 9.00 (s, 1H), 8.81 (d, J = 8.0 Hz, 1H), 8.73 (d, J =
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8.0 Hz, 1H), 7.92 (d, J = 6.8 Hz, 2H), 7.81-7.71 (m, 6H), 7.58 (t, J = 7.6 Hz, 1H),
7.35-7.23 (m, 7H), 7.12-7.03 (m, 5H), 6.98-6.96 (m, 4H), 6.90-6.89 (m, 4H), 6.39 (d,
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J = 8.0 Hz, 2H); C NMR (100 MHz, CDCl ) δ [ppm]: 146.26, 141.79, 131.74,
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131.07, 130.35, 130.13, 129.72, 128.55, 127.65, 127.53, 126.86, 126.70, 126.31,
124.44, 123.19, 120.60, 120.56, 114.27, 56.77; HRMS (APCI): calculated for
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C H F N , 769.2705; found, 770.2785, [M+H] ; Elemental analysis calculated (%)
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C H F N : C 82.69, H 4.45, N 5.46; found: C 82.37, H 4.63, N 5.41.
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2.3. Computational Details
The geometrical and electronic properties were computed using the Gaussian 09
program package. The ground-state geometry was optimized using density functional
theory (DFT) with B3LYP hybrid functional at the basis set level of 6-31G(d). The
excited-state geometry was optimized by time-dependent density functional theory
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(TD-DFT) with the B3LYP functional at the same basis set level. To get a better
insight into the nature of the excited state, the natural transition orbitals (NTOs)
analyses were also carried out by TD-DFT calculations. Molecular orbitals were
visualized using Gauss view 5.0 program.
2.4. Device Fabrication and Measurement
The carrier-injection material MoO3, lithium fluoride (LiF), and
carrier-transporting material 4,4'-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline)
(TAPC), 3,3'-(5'-(3-(pyridin-3-yl)phenyl)-[1,1':3',1''-terphenyl]-3,3''-diyl)dipyridine
(TmPyPB) 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBi) were
commercially available. Commercial ITO coated glass with a sheet resistance of 20 ꢀ
per square was used as the substrate. Before device fabrication, the ITO glass
substrates were pre-cleaned carefully and treated with oxygen plasma for 5 min.
MoO was firstly deposited on the ITO substrate, followed by TAPC, the emission
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layer, and TmPyPB. Finally, a cathode composed of LiF and aluminum was
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sequentially deposited onto the substrate in the vacuum of 2×10 Torr. For all of the
OLEDs, the emitting areas were determined by the overlap of two electrodes as 0.09
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cm . The EL spectra, CIE coordinates and J-V-L curves of the devices were measured
using a PHOTO RESEARCH SpectraScan PR655 photometer and a KEITHLEY
2400 SourceMeter constant current source. The efficiency values were calculated
according to previously reported methods. All measurements were carried out at room
temperature under ambient conditions.
19 3. Results and discussion
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3.1. Synthesis
The synthetic routes and molecular structures are depicted in Scheme 1.
2-(4-bromophenyl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole and
9,9-diphenyl-9,10-dihydroacridine were synthesized according to the literature [25,
35]. The intermediate compound 1 was prepared through the palladium-catalyzed
Suzuki coupling reaction in high yield. The intermediate compound 2 and 3 were
synthesized in a one-pot reaction. The target products were prepared through the
palladium-catalyzed
Buchwald-Hartwig
reaction
between
9,9-diphenyl-9,10-dihydroacridine and the corresponding phenanthroimidazole
intermediates, then purified by column chromatography on silica gel. The chemical
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structures of the compounds were confirmed by H-NMR, C-NMR, elemental
analysis and mass spectrometry. Before fabrication of the OLED device, the three
new compounds were further purified by repeated temperature-gradient vacuum
sublimation.
3.2. Thermal properties
The thermal properties of the three compounds were investigated by thermal
gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a
nitrogen atmosphere. As presented in Fig. 1 and Table 1, compounds DPACTPI,
DPACPhTPI and DPACFPPI exhibit high decomposition temperatures (Td,
corresponding to 5% weight loss) of 439 � , 476 � and 419 � , respectively. The three
new compounds did not show any glass transition temperatures up to 250� . It is most
likely due to their highly rigid and twisted structures which might mitigate the
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intermolecular interactions of these molecules in the solid state [28]. These results
reveal that the three materials are suitable for vacuum thermal evaporation
technology.
3.3. Theoretical calculations
To gain a better insight into the electronic structures of the three compounds, the
molecular configuration and frontier molecular orbitals were optimized using DFT at
the B3LYP/6-31G(d) level in the Gaussian 09 software. Fig. 2 shows the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) distribution of the three compounds. The HOMO is mainly distributed on
the donor DPAC moiety. The LUMO of DPACTPI and DPACPhTPI are localised
on the phenanthroimidazole and the adjacent phenyl or biphenyl rings, while the
LUMO of DPACFPPI is mostly spread over the acceptor benzotrifluoride and
extended to phenanthroimidazole. It is obviously found that the HOMO and the
LUMO are almost completely spatially separated for the three compounds, which can
provide hole- and electron-transporting channels for intermolecular hole and electron
hopping along their respective conducting pathways, improving the device
performance. In the optimized configurations of the three compounds as shown in Fig.
2, very large dihedral angles of 84.5°, 83.9° and 83.3° were found between DPAC and
the phenyl ring connected to the phenanthroimidazole, which might effectively
prevent intramolecular extending of π-electron and suppress molecular packing in the
solid state. In addition, these molecules also show the large dihedral angles between
the N-substituted benzene and phenanthroimidazole planes.
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3.4. Photophysical properties
The UV-vis absorption and photoluminescence (PL) spectra of the three
compounds are measured in dilute CH Cl , as shown in Fig. 3a. The fluorescence
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spectra of compounds in solid film at room temperature is showed in Fig. 3b. Key
optical parameters are summarized in Table 1. All compounds show a strong
absorption band at around 260 nm, which may originate from the π-π* transition of
the benzene ring. The weaker absorption band at around 300-360 nm were assigned to
the π-π* transition of the phenanthroimidazole moiety. The absorption spectrum of
DPACTPI and DPACFPPI is very similar. While DPACPhTPI shows higher
absorption intensity between 300 and 360 nm, which could be assigned to the
extended π-conjugation for the phenanthroimidazole moiety.
The maximum emission
wavelengths of DPACTPI, DPACPhTPI and DPACFPPI in CH Cl were observed
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at 435, 451 and 447 nm, respectively. Compared with DPACTPI, the redshift of 12
and 16 nm for DPACFPPI and DPACPhTPI could be attributed to the introduction
of the trifluoromethyl group and the increased conjugation by the insert of the
benzene ring. It is noteworthy that the emission peaks of the three compounds in the
film state show a large blue-shift of more than 20 nm, revealing little intermolecular
interactions in the film state. The blue-shift could be explained by the large dihedral
angle between DPAC and the phenyl ring connected to the phenanthroimidazole.
Using 9,10-diphenylanthracene as a standard, the PL quantum yields (Ф) in CH Cl
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were determined as high as 0.359, 0.352 and 0.287, respectively. Additionally, optical
band-gap (E ) of DPACTPI, DPACPhTPI and DPACFPPI were calculated to be
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3.26 eV, 3.18 eV and 3.22 eV from its absorption edge in solution, respectively.
3.5. Electrochemical properties
The electrochemical properties and energy levels of the three compounds were
investigated by cyclic voltammetry. As shown in Fig. 4, the onset oxidation peaks
(E ) for DPACTPI, DPACPhTPI and DPACFPPI are 0.54, 0.53, and 0.53 eV
ox
+
versus the Ag/Ag redox couple, respectively. With the same DPAC donor unit, all
show a similar oxidation potential and HOMO level, which is in accordance with the
computed result. The HOMO energy levels of DPACTPI, DPACPhTPI and
DPACFPPI are estimated to be 5.44 eV, 5.43 eV and 5.43 eV with regard to
ferrocene. The LUMO energy levels were calculated from E and HOMO values,
g
which were listed in Table 1.
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Table 1
2
Physical properties of DPACTPI, DPACPhTPI and DPACFPPI.
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3.6. Carrier transporting characteristics
Before characterizing the performance of these blue emitters in OLEDs,
hole-only and electron-only devices were fabricated to investigate the charge
transporting properties. The device configurations are shown as follows: ITO/MoO3
(10 nm)/TAPC (20 nm)/DPACTPI, DPACPhTPI or DPACFPPI (30 nm)/TAPC (20
nm)/MoO (10 nm)/Al (100 nm) and ITO/LiF (1 nm)/TPBi (20 nm)/DPACTPI,
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DPACPhTPI or DPACFPPI (30 nm)/TPBi (20 nm)/LiF (1 nm)/Al (100 nm). MoO3
and LiF performed as the hole injection layer (HIL) and the electron injection layer
(EIL), respectively. TAPC was used as the electron-blocking material, whereas the
TPBi layer was used to prevent hole injection from the anode. Fig. 5 shows the
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current density versus voltage (J-V) curves of these single-carrier devices. The J-V
characteristics of single-carrier devices demonstrated the good bipolar transport
property of the three compounds. All of the compounds show higher and similar hole
current density due to the good hole-transporting mobility of DPAC donor unit [36].
Besides, the barrier of carrier injection from TAPC layer (~0.13 eV) is smaller than
TPBi layer (>0.45 eV), which also resulted in higher hole current density in a
hole-only device. The introduction of strong electron-withdrawing trifluoromethyl
group endow DPACFPPI-based device with higher electron current density at low
voltage (<9 V), while the poor electron-transporting mobility at high voltage (>10 V)
could be ascribed to the poor stability of the trifluoromethyl unit [30, 37]. The
superior bipolar transporting characteristic of DPACPhTPI indicate a better
electroluminescence performance.
3.7. Electroluminescence
To evaluate the EL performances of DPACTPI, DPACPhTPI and DPACFPPI
as blue emitters, we initially fabricated nondoped devices I-III with a multilayer
structure: ITO/MoO (10 nm)/TAPC (40 nm)/EML (30 nm)/TmPyPB (20 nm)/LiF (1
3
nm)/Al (100 nm). In these devices, indium tin oxide (ITO) was used as the anode,
MoO was used as hole injection layer, TAPC was used as hole transporting layer
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(HTL), TmPyPB was used as electron transporting layer (ETL), LiF was used as
electron injecting layer, and Al was used as cathode. The relative HOMO/LUMO
energy levels of the materials are illustrated in Scheme 2, and detailed structures and
key performance of these devices are summarized in Table 2.
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The current density-voltage-luminance (I-V-L) and current density-current
efficiency-power efficiency characteristics are shown in Fig. 6. As can be seen in Fig.
6 (a), the turn-on voltages of three devices were 3.6 V, 3.3 V and 3.6 V, respectively.
The lower turn-on voltage of DPACPhTPI-based device II should result from a
relatively smaller injection barrier from TmPyPB. All the three non-doped devices
show deep-blue emission with the maximum emission wavelengths of 428 nm, 436
nm and 424 nm, respectively. Compared with the fluorescence spectra in films, the
EL emission peaks showed a small redshift, which may be caused by the
intermolecular interactions and the electrical field polarization in the excited states.
The DPACTPI-based nondoped device I achieves a maximum current efficiency
(CEmax) of 1.85cd/A, a maximum power efficiency (PEmax) of 1.76 lm/W and a
maximum external quantum efficiency (EQEmax) of 2.31%. DPACPhTPI exhibits the
best EL performance with a CEmax of 3.84 cd/A, a PEmax of 4.02 lm/W and a EQEmax
of 3.51%. DPACFPPI displays a CEmax of 1.51 cd/A, a PEmax of 1.40 lm/W and a
EQEmax of 3.03%. However, because of the poor electron mobility at high voltage,
DPACFPPI shows the lowest luminance and a large efficiency roll-off.
Inspired by the relatively better device performance of DPACPhTPI, the blue
electroluminescent device based on DPACPhTPI was further optimized with the
device configuration IV-VIII as follows: ITO/MoO3 (10 nm)/TAPC (x nm)
/DPACPhTPI (20 nm)/TmPyPB (y nm)/LiF (1 nm)/Al (100 nm). The detailed device
structures, current density-voltage-luminance (I-V-L) and current density-current
efficiency-power efficiency characteristics are shown in Fig. 7. Devices IV-VII show
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the same turn-on voltage of 3.0 V. While, device VIII exhibits a higher turn-on
voltage of 3.3V. By changing the thickness of HTL and ETL, the EL spectra of the
devices vary from 420 nm to 444 nm. With the increasing of the thickness of HTL or
ETL, the emission peak is shifted gradually bathochromically, which could be
induced by the movement of exciton formation zone. The microcavity effect arising
from the change of the thickness may also contribute to this bathochromic
shift [38-39]. Device VII showed outstanding performance with the CIE coordinates
of (0.156, 0.047) and a maximum EQE of 3.50%, which was among the best results
for OLEDs with similar color (Table 2).
The EQE for OLEDs can be calculated by the following equation:
EQE = γ × Ф × η × η ,
PL
r
out
where EQE is the maximum external quantum efficiency; γ is the carrier
recombination efficiency, which is assumed to be 100%; ФPL is the PLQY of the
emission layer; η is the radiative exciton yield; and ηout is light out-coupling
r
efficiency, which is 20~25% for glass substrates if there are not any out-coupling
enhancing structures in the device. Considering ФPL of DPACPhTPI is 35.2% and
the maximum EQE is 3.51%, the calculated radiative exciton yield is near 50%. This
result breaks the 25% theoretical limit of spin statistics for conventional fluorescence
OLEDs. The possibility of the triplet-triplet annihilation (TTA) seem to be excluded
because the brightness increases linearly with the increase of the current density at
low current injection and less than linearly at higher current injection [40], as shown
in Fig. S1. TADF is unlikely to take place due to the large ꢁEST (0.72 eV, Fig.
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S2) [12]. Therefore, the high radiative exciton yield may have possibly resulted from
hybridized local and charge transfer (HLCT) [41]. TD-DFT calculations and NTOs
analyses were performed for DPACPhTPI, as shown in Fig. 8. According to the
NTOs analyses of S excited state, DPACPhTPI exhibits HLCT characteristic and
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CT state is dominant in the emission, which also explains the moderate PLQY of
DPACPhTPI. The solvatochromism behavior of DPACPhTPI agrees well with the
NTOs calculations as shown in Fig. S4. Fig. 8 also shows that the T excited state of
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DPACPhTPI is similar to the S excited state and the energy difference between T
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and S is estimated to be of only 0.011 eV, which suggests the possibility of the
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reverse intersystem crossing (RISC) from T to S . These results further show that
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DPACPhTPI should be HLCT emitter instead of others.
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Table 2
Key characteristics of devices based on DPACTPI, DPACPhTPI and DPACFPPI and other
recently reported non-doped blue emitting devices with CIE < 0.08
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In conclusion, we have designed and synthesized three bipolar, fluorescent
emitters, DPACTPI, DPACPhTPI and DPACFPPI, incorporating
9,9-diphenyl-9,10-dihydroacridine as electron-donor and phenanthroimidazole as
electron-acceptor. The three compounds exhibit strong blue PL emissions, good
thermal stability, good film-forming property, together with appropriate energy levels.
These molecules showed large intramolecular torsion angles, which avoided π-π
stacking in the solid state. The experimental results were well supported by DFT
theoretical calculations. The nondoped OLED device based on DPACPhTPI showed
a good performance with maximum EL peak at 436 nm, EQE of 3.51%, CE of 3.84
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cd/A, PE of 4.02 lm/W and CIE coordinate of (0.160, 0.078). What's more, with
optimized device structure, DPACPhTPI-based nondoped device also exhibited a
near ultraviolet emission, a high maximum EQE of 3.50% with a CIE coordinate of
(0.156, 0.047).
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04 Acknowledgements
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This research work was supported by the NSFC/China (51573065), the science
and technology support program of Hubei Province (2015BAA075) , the National
Basic Research Program of China (973 Program 2013CB922104) and the Analytical
and Testing Center at Huazhong University of Science and Technology is
acknowledged for characterization of new compounds.
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Table 1
Physical properties of DPACTPI, DPACPhTPI and DPACFPPI.
HOMO/
HOMO/
a
a/b
c
d
g
Compound
Abs (nm)
PL (nm)
Φ
Eg (eV)
Eox (V)
T (℃ )
d
e
f
LUMO(eV)
LUMO(eV)
2
61, 287,
06, 361
62, 326,
60
59, 286,
06, 358
a Measured in dilute CH Cl solution at room temperature.
DPACTPI
DPACPhTPI
DPACFPPI
435/412
451/428
447/422
0.359
0.352
0.287
3.26
3.18
3.22
0.54
0.53
0.53
-5.44/-2.18
-5.43/-2.25
-5.43/-2.21
-5.03/-1.20
-5.04/-1.37
-5.10/-1.52
439
476
419
3
2
3
2
3
2
2
b Solid state thin-film samples on quartz substrate.
c Measured in CH Cl using 9,10-diphenylanthracene as a standard (Ф=0.90 in cyclohexane).
2
2
d Measured by the onset absorption wavelength of solution state.
e HOMO was determined from the onset of oxidation potentials with ferrocene as reference; LUMO was
determined from E and HOMO level.
g
f Values obtained using Gaussian 09 at the B3LYP/6-31G(d) level.
g Measured from TGA.

ACCEPTED MANUSCRIPT
Table 2
Key characteristics of devices based on DPACTPI, DPACPhTPI and DPACFPPI and other
recently reported non-doped blue emitting devices with CIE < 0.08
y
a
b
c
d
e
Emitters
λEL (nm)
Von (V)
CE (cd/A)
PE (lm/w)
EQE (%)
CIE (x, y)
Ref
f
f
f
f
DPACTPI
DPACPhTPI
DPACFPPI
DPACPhTPI
BPA-BPI
TPAXAN
mTPA-PPI
CPhBzIm
PIBCz
428
436
424
428
428
428
404
426
414
426
396
-
3.6
3.3
3.6
3.0
3.1
3.4
3.2
2.5
2.7
3.5
4.0
5.5
4.75
-
1.848
3.842
1.506
1.379
0.65
-
1.758
4.022
1.403
1.403
0.68
-
2.31
3.51
3.03
3.50
1.37
4.62
3.33
3
(0.165, 0.068)
(0.160, 0.078)
(0.162, 0.057)
(0.156, 0.047)
(0.15, 0.05)
Device I
Device II
Device III
Device VII
[25]
(0.155, 0.049)
(0.161, 0.049)
(0.16, 0.05)
[29]
0.84
1.6
0.48
1.07
1.12
1.58
-
[42]
[43]
1.28
1.89
0.74
0.75
2.30
1.14
1.99
1.35
2.74
4.21
2.2
(0.15, 0.05)
[44]
CzS1
(0.157,0.055)
(0.160, 0.041)
(0.160, 0.054)
(0.166, 0.064)
(0.161, 0.046)
(0.16, 0.06)
[45]
DPPP
[46]
CzB-S
-
2.03
-
[47]
CzB-PIM
TPA-PIM
m-BBTPI
PPI-F-TPA
420
420
428
425
1.52
0.79
1.81
1.0
[48]
3.28
3.63
3.11
[49]
3.2
4.2
[50]
(0.16, 0.05)
[51]
2
a λEL is the emission wavelength at 20 mA/cm .
2
b V is the voltage at 1 cd/m .
on
c Maximum current efficiency.
d Maximum power efficiency.
e Maximum external quantum efficiency.
2
f CIE at 20 mA/cm .