Polyphenylbenzene as a platform for deep-blue OLEDs: Aggregation enhanced emission and high external quantum efficiency of 3.98%

Article abstract of DOI:10.1021/acs.chemmater.5b00094

Great efforts have been devoted to seek novel approaches for the construction of efficient deep-blue fluorescent materials, one of the most important prerequisites for the commercialization of OLEDs. Here, we report a new way to utilize polyphenylbenzene as a platform to yield a series of efficient deep-blue emitters. Nondoped multilayer electroluminescence (EL) devices using these new luminogens as emitting layers are fabricated. Maximum current efficiency (CE) of 2.0 cd A^-1 is achieved and the Commission Internationale de lclairage (CIE) coordinates can stay at (0.15, 0.08), close to the saturated deep-blue (0.14, 0.08). Through rational design of the device structure, blue-violet emission with the CIE coordinates of (0.15, 0.06) can be realized. Furthermore, 10-based doped devices show deep-blue emission with improved CE as high as 4.51 cd A^-1, and the external quantum efficiency (EQE) of 3.98%, which are among the best EL performance for deep-blue emission.

Full text of DOI:10.1021/acs.chemmater.5b00094

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Article
Polyphenybenzene as a Platform for Deep-Blue OLEDs: Aggregation
Enhanced Emission and High External Quantum Efficiency of 3.98%
Xuejun Zhan, Ning Sun, Zhongbin Wu, Jin Tu, Lei Yuan, Xi Tang, Yujun
Xie, Qian Peng, Yong Qiang Dong, Qianqian Li, Dongge Ma, and Zhen Li
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00094 • Publication Date (Web): 08 Feb 2015
Downloaded from http://pubs.acs.org on February 18, 2015
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Polyphenybenzene as a Platform for Deep-Blue OLEDs: Aggregation
Enhanced Emission and High External Quantum Efficiency of 3.98%
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†
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Xuejun Zhan, Ning Sun, Zhongbin Wu, Jin Tu, Lei Yuan, Xi Tang, Yujun Xie, Qian Peng,
§
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Yongqiang Dong, Qianqian Li, Dongge Ma,*, and Zhen Li*,
†
Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University,
Wuhan 430072, China
‡
Changchun Institute of Applied Chemistry, the Chinese Academy of Sciences, Changchun 130022, China
§
Department of Chemistry, Beijing Normal University, Beijing 100875, China
‖
Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100190, China
ABSTRACT: Great efforts have been devoted to seek novel approaches for the construction of efficient deep-blue fluores-
cent materials, one of the most important prerequisites for the commercialization of OLEDs. Here we report a new way to
utilize polyphenylbenzene as a platform to yield a series of efficient deep-blue emitters. Non-doped multilayer electrolu-
minescence (EL) devices using these new luminogens as emitting layers are fabricated. Maximum current efficiency (CE)
of 2.0 cd A-1 is achieved and the Commission Internationale de l'Éclairage (CIE) coordinates can stay at (0.15, 0.08), close
to the saturated deep-blue (0.14, 0.08). Through rational design of the device structure, blue-violet emission with the CIE
coordinates of (0.15, 0.06) can be realized. Furthermore, 10-based doped devices show deep-blue emission with improved
CE as high as 4.51 cd A-1, and the external quantum efficiency (EQE) of 3.98%, which are among the best EL performance
for deep-blue emission.
with a maximum EQE of 4.3%, and a relatively high turn-
INTRODUCTION
on-voltage (Von) of 6.5 eV.9b Chien et al. reported an emit-
ter, 2-tert-butyl-9,10-bis[4´-(diphenylphosphoryl)phenyl]-
anthracene (POAn), with CIE coordinate of (0.15, 0.07)
and the maximum CE up to 3.2 cd A-1 (EQE of 4.5%),9c
while the degree of efficiency roll-off was obvious. Moor-
thy et al. even reported a pure deep-blue emitter with a
CIE of (0.14, 0.08) from 3,3´-bis-p-(2,2-diphenylvinyl)-
phenylbimesityl, with a moderate EQE of 1.51%.9d
Since the discovery of organic electroluminescence,
through the unremitting efforts of scientists, Organic
light-emitting devices (OLED) have been commercialized
for applications in displays and solid state lightings.1,2
However, unlike their red and green counterparts with
satisfactory performance, efficient and stable luminogens
with standard blue or deep blue were still scarce, because
of the intrinsic large bandgap and the encountered prob-
lems of stability.3,4 Through rational molecular design,
many efficient emitters such as fluorene,5 anthracene,6
pyrene7 and carbazole8 derivatives with deep-blue emis-
sion have been synthesized. However, most of them give a
light ranging within Commission Internationale de
l'Éclairage (CIE) (x < 0.15, y < 0.15).9a Actually, to further
reduce the power consumption and generate light of oth-
er colors by energy cascade, it is badly needed to develop
high efficient deep-blue emitters with CIEy close to 0.08.
Thanks to the great efforts of scientists, some deep-blue
emitters have been obtained. For example, Lin et al. re-
ported a deep-blue emitter of 3-[4-(1,1-dimesitylboryl)-
phenyl]-9-ethyl-9H-carbazole (CzPhB) at CIE (0.15, 0.09)
Another key point is the stability of the blue emission.
Generally, most of the luminogens could exhibit excellent
deep-blue or blue emission in solution, but in the solid
state (the real state in LED devices), due to the molecular
aggregation or π-π stacking, the emission would red-shift
to longer wavelengths, leading to the blue-green light and
lower efficiency, which was termed as “Aggregation
Caused Quenching (ACQ) effect”.10 Fortunately, through
introducing bulk substituted groups or/and making the
structure twisted, the problem of red-shifted emission
could be inhibited, by sacrificing the efficiency or/and
raising the turn-on-voltage,11,12 in most cases. Thus, if
some blue or deep blue luminogens could be prepared
conveniently with the characteristic of aggregation induc-
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Scheme 1. Synthetic routes of the luminogens 8-14.
NH2
NO2
Br
Br
4
5
6
7
8
9
NO2
NO2
NO2
3
CuCN
BuONO
1
Fe/HAc
Pd(PPh3)4
Br2
BuONO, CuBr2
CH3CN
2
Br2, CHCl3/HAc
DMSO
HAc
K2CO3
Br
Br
Br
Br
toluene/EtOH/H2O
NH2
1
NH2
Br
2
4
5
CN
Br
Br
Br
Br
10
11
12
13
14
15
16
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N
Ar1
=
N
6
7
Pd(PPh3)4
K2CO3
8
9
Pd(PPh3)4
K2CO3
N
Ar1
Ar1
Ar2
Ar2
Ar2
=
N
N
N
N
N
13
10
14
12
11
ed emission (AIE, a concept first proposed by Ben Zhong
Tang in 2001) but not ACQ, the results should be excit-
ed.13 Actually, with tetraphenyl ethylene (TPE), a typical
AIE luminogen with deep blue emission, as the basic
building block, we have successfully obtained some good
blue/deep blue AIE luminogens (which were recently
named as AIEgens), by utilizing different linkage modes
or/and increasing the twisting degree of the molecules in
the presence of additional groups.14 However, the conju-
gation length of TPE is a little longer, leading to the diffi-
culty to achieve good deep blue luminogens, and their
lowest CIEy value could only be 0.11, still higher than 0.08.
Is it possible to further optimize the structure of this kind
of promising AIEgens to design much better deep blue
AIE luminogens? To realize this point, the conjugation
length should be further shortened to some degree, while
the AIE properties are remained.
Inspired by all the points mentioned above, we de-
signed a series of hexaphenylbenzene derivatives, 8-14,
mainly through step-by-step Suzuki coupling reaction.
Carbazole and triphenylamine were introduced due to
their good hole-transporting ability. Furthermore, we
introduced cyano group to the center benzene so as to
form donor-π-acceptor (D-π-A) type fluorophores, with
the aim to make use of the effective radiative decay of
their excited intramolecular charge-transfer (ICT) state.20
For comparison, two molecules without cyano groups but
hydrogen atoms were also synthesized. Nondoped devices
based on 10 showed high performance with CE of 2.0 cd
A-1 and the CIE coordinates could stay at (0.15, 0.08), very
close to the saturated deep-blue (0.14, 0.08). Excitedly, 10-
based doped devices exhibited deep-blue emission with
more than doubled efficiency of 4.51 cd A-1. Herein, we
would like to present their syntheses, thermal, photo-
physical, electrochemical and electroluminescent (EL)
properties in detail.
Previously, we have designed a series of benzene-cored
luminophores (Chart S1), which were AIE active and ex-
hibited deep blue emission.15 Excitingly, even three carba-
zolyl groups were bonded to the benzene core, the maxi-
mum emission wavelength was as short as 401 nm, indi-
cating its very short molecular conjugation length. Inter-
estingly, once six phenyl groups were linked to a benzene
core, the formed hexaphenylbenzene, studied intensively
by Müllen et al., is also a deep blue luminogen with the
characteristic of aggregation enhanced emission (AEE),
conquering the unwanted ACQ effect.16 However, so far,
there were rarely reports concerning works using this star
molecule as a platform to tune electron structure and
apply them in OLEDs.17,18 The main reason might be the
limitation caused by the traditional approach with the
usage of alkyne and tetraphenylcyclopentadienone
through the Diels-Alder reaction.19 To fully excavate the
potential of hexaphenylbenzene for OLED applications,
new methods of functionalization should be explored.
RESULTS AND DISCUSSION
The seven new luminogens were obtained similarly
through six synthetic steps with good yields (Scheme 1).
Compound 4 was prepared through threefold Suzuki re-
action between 2 and the commercial available 3 (4-tert-
butylphenylboronic acid), then it was reduced by iron
powder and followed by bromination reaction to yield 5.
The key intermediate 6 and 7 could be gotten in one step,
when 5 was treated with CuCN and t-BuONO. Finally,
through palladium (0)-catalyzed Suzuki cross-coupling
reaction between 6/7 and 4-(diphenylamino)phenylboro-
nic
(diphenylamino)phenylboronic
yl)phenyl boronic
acid/4-(9H-carbozol-9-yl)phenylboronic
acid/3-
acid/3-(9H-carbozol-9-
acid/(9-phenyl-9H-carbazol-3-
yl)boronic acid, compounds 8-14 could be synthesized
with preferable yield. All these new compounds were
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effectively suppress the possible intermolecular interac-
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tions.
3
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5
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8
9
10
11
12
Figure 1. TGA curves of 8-14 recorded under N2 at a heating
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rate of 10 oC/min.
purified by column chromatography on silica gel using
petroleum ether-chloroform as eluent and fully character-
1
ized by H and 13C NMR, mass spectrometry, and ele-
mental analysis.
Generally, good thermal stability of the emitters is ben-
eficial to the process of vacuum deposition and operating
stability of OLED devices. The thermal properties of all
these new compounds were investigated by thermograv-
imetric analysis (TGA) and differential scanning calorime-
try (DSC) measurements. As depicted in Figure 1 and Ta-
ble 1, all the luminogens are thermally stable with Td (cor-
responding to 5% weight loss) values in the range of 390-
473 oC. Owing to good thermal stability of carbazole,
compounds contain carbazole units (9, 11, 13, and 14)
show better thermal stability than those bearing TPA
units (8, 10, and 12). Generally, compounds with para-
linkage mode show better thermal stability than the ones
with meta-linkage way, owing to their more rigid confor-
Figure 2. (A) UV spectra and (B) PL spectra of 8-14 in THF
solution (~10 μM).
o
mations. Thus, among them, 9 and 11 (473 and 468 C)
possess higher decomposition temperatures than others.
The glass transition temperatures (Tg) for 8, 10, 11, 12, 13
o
and 14 are 131, 128, 137, 101, 113 and 112 C, respectively, fur-
ther demonstrating their better thermal stability. Thus,
the good thermal properties would benefit to the good
performance of the corresponding LED devices.
All these compounds have good solubility in common
organic solvents, such as dichloromethane, chloroform
and tetrahydrofuran (THF) et al.. Figure 2A and 3A show
their absorption spectra in diluted THF solutions and thin
solid films. In their solutions, compound 9, 11, 13, 14
demonstrate similar absorptions, where the bands at
about 330 and 290 nm should be ascribed to the intramo-
lecular charge transfer (ICT) transition (Figure S2) and
the carbazole-centered n-π* transition, respectively, while
the bands around 340 nm are caused by the π-π* transi-
tion. 8, 10, 12 also exhibit similar absorptions, the bands
around 300 nm could be attributed to the TPA-centered
n-π* transition, and the shoulder band around 330 nm of
10 is mainly caused by the ICT transition. These absorp-
tion bands do not obviously red-shift in their thin solid
films, since the periphery tert-butyl phenyl groups could
Figure 3. (A) UV and (B) PL spectra of 8-14 of the films.
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Table 1. The thermal, electrochemical and photophysical data of the luminogens.
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5
PL λem
ФF (%)
λmax,abs
solvh
nm
a
c
d
e
Td
Tgb
oC
Eg
EHOMO
eV
ELUMO
solvf filmg
solvf
filmg
nm
oC
eV
eV
nm
nm
6
7
8
9
TPA-TPA/8
442
473
454
468
390
131
---
128
137
101
113
3.33
-5.20
-1.87
-2.03
-2.09
-2.15
-1.86
-2.08
406
367
443
406
448
412
396
374
439
401
424
394
58.7
34.3
68.6
80.3
7.2
316
341
323
342
Cz-Ph-Ph-Cz/9
TPA-CN-TPA/10
Cz-Ph-CN-Ph-Cz/11
mTPA-CN-mTPA/12
3.50 -5.53
3.17 -5.26
302(s330) 295(s338)
10
11
12
13
14
15
16
17
18
19
20
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31
32
33
34
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36
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38
39
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44
45
46
47
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49
50
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53
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60
3.48 -5.63
3.40 -5.26
3.54 -5.62
340
297
340
345
342
298
339
345
Cz-mPh-CN-mPh-Cz/13 445
6.4
Ph-Cz-CN-Cz-Ph/14 460
112
3.41
-5.60
-2.19
400
410
45.7
a 5% weight loss temperature measured by TGA under N2. b Glass-transition temperature measured by DSC under N2. c Band gap
estimated from optical absorption band edge of the solution. d Calculated from the onset oxidation potentials of the compounds.
e Estimated using empirical equations ELUMO = EHOMO + Eg. f Determined in THF solution. g On glass. h Observed from absorption
spectra in dilute THF solution, 10 μM.
In the PL spectra, their emissions in diluted solutions
mainly locate at 400-450 nm except compound 9 (367
nm). As a result of the intramolecular charge transfer de-
rived from the introduced cyan group, the emissions of 10
(443 nm) and 11 (406 nm) red-shift 37 and 39 nm, in com-
parison with those of 8 (406 nm) and 9 (367 nm), respec-
tively. In our previous study, the conjugation length of the
whole molecules could be partially controlled by utilizing
the different linkage mode of the different construction
aromatic blocks. For example, just by changing the link-
age mode of two TPE pieces from the para positions to
meta ones, the maximum electroluminescent emission
wavelengths of the resultant two BTPE derivatives in their
LED devices could be adjusted from 488 nm to much
blue-shifted one, 452 nm, as the result of the different
conjugation effect between the two TPE moieties (Chart
S2). However, when the linkage mode is changed from the
para positions in 10 to meta ones in 12, their maximum
emission wavelengths are not with a big difference, but
similar. Almost the same phenomenon is observed in the
cases of 11 and 13, disclosing the minor influence of the
different linkage mode. This is reasonable. As demon-
strated in Figure 5, all the luminogens are highly twisted,
which directly lead to the weak conjugation effect be-
tween the constructing aromatic blocks. Thus, the differ-
ent conjugation effects caused by the different linkage
modes should be weakened by the highly twisted confor-
mation, unlike those present in the cases of BTPE deriva-
tives any longer.
these blue shifted emissions are beneficial to achieve good
performance. However, unlike most of the luminogens
exhibit blue-shifted emissions in films compared to those
in solution, 9 and 14 exhibit a little red shifted emission in
solid state, indicating their relatively good coplanar con-
formation.
To quantitatively know the fluorescent properties of
these luminogens, their absolute fluorescent quantum
yields were measured (Table 1). The fluorescent quantum
yields of 10 and 11 surpass 65%, owing to the suitably
modified D-π-A molecular structures and their relatively
good coplanarity, which could provide highly emissive
ICT excited states upon excitation (Figure S2).20a For 12
and 13, due to their too highly twisted conformations,
lower quantum yields were found.
In order to study the AEE characteristic of the seven
luminogens, we chose cyclohexane and THF as the sol-
vent pair for their miscibility, and recorded the photolu-
minescence (PL) change. Figure S3 show the PL spectra of
the new fluorophores in THF/cyclohexane mixtures with
different cyclohexane fractions (fw), which enable fine-
tuning of the solvent polarity and extent of solute aggre-
gation. With gradual addition of cyclohexane, the PL in-
tensities of all the seven compounds increase when cyclo-
hexane content higher than 20%. Further increasing the
cyclohexane content to 99%, in most cases, the emission
intensities do not show decrease. Also, for comparison,
the quantum yields of the luminogens were tested at dif-
ferent concentrations in THF solutions. From the absolute
quantum yields in THF solutions and the mixtures of THF
and cyclohexane (Figure S4), most of these luminogens
demonstrate AEE characteristic.16b Figure S4b show the
absolute PLQYs of 8-14 in the THF/cyclohexane mixtures
with different cyclohexane fractions (0, 20, 50, 80 and
99%). When the cyclohexane fraction was 20%, most of
the PLQYs show some increase. This trend can even be
maintained to 80% and 99% for compound 14 and 13,
In the PL spectra of their thin films, most of the lu-
minogens exhibit narrow emissions with fwhm (full width
at half-maximum) of about 50 nm. Compared to para-
linkage 10 (439 nm), a slightly blue-shifted emission is
found in the PL spectra of meta-linkage 12 (424 nm). Simi-
lar phenomenon could also be found in the cases of 11 and
13 (Table 1). As we aim to obtain blue/deep-blue OLEDs,
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Figure 5. Calculated molecular orbital amplitude plots of HOMO and LUMO levels and optimized molecular structure.
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respectively. Further increasing the cyclohexane fraction
induced some decrease of PLQYs. This is not contradicto-
ry to our perspective of AEE and may be reasonable. At
high concentration of cyclohexane, due to the poor solu-
bility of the luminogens, the percentage of molecules in-
side the nanoparticles (which do not contribute to the
PLQY) will increase.15,16b Moreover, except 14, the PL peaks
show blue-shifted when the cyclohexane content is in-
creased, probably due to the solvation effect (Figure S2)
or/and the morphological change of the aggregates from
amorphous to crystalline state. This is beneficial to the
fabrication of deep blue light-emitting devices.
structures are similar to some of Adachi’s molecules
(Chart S4), we also explored the potential TADF emission
of our luminogens 10-14. However, in their transient PL
decay characteristics, all these luminogens exhibit first-
order exponential decays, indicating no TADF emission
present (Figure S5). Typically, the energy distance (∆EST)
between the lowest singlet state and the lowest triplet
state should be small enough. To understand the absence
of TADF emission, we tested the low-temperature fluores-
cence and phosphorescence (Figure S6). For 10, 11, 12, 13
and 14, their ∆EST values are calculated to be 0.43, 0.58,
0.41, 0.66 and 0.58 eV, respectively. As a result, in our case,
these ∆EST values may not be small enough to guarantee
the T1 → S1 reverse intersystem crossing (ISC) process,
while that of similar molecules in Adachi’s work can be as
small as 0.083 eV.2c
Thermally activated delayed fluorescence (TADF), de-
veloped by Adachi and co-workers, can lead to potentially
100% internal singlet yields through up-conversion of tri-
plet to singlet states,2c which opens up a new mind for
electroluminescence. Considering that the highly twisted
Cyclic voltammetry (CV) measurements were carried
out to investigate the electrochemical properties of these
luminogens. The highest occupied molecular orbital
(HOMO) energy levels were estimated from the onset
oxidation potentials according to the equation: HOMO = -
(4.8 + Eox) eV, while the lowest unoccupied molecular
orbital (LUMO) energy levels were obtained from optical
band-gap energies (estimated from the onset wavelengths
of the UV absorptions) and HOMO values. As shown in
Figure 4 and Table 1, for 8, 9, 10, 11, 12, 13 and 14, their
HOMO values are calculated to be -5.20, -5.53, -5.26, -5.63,
-5.26, -5.62 and -5.60 eV, respectively. Some of them are a
light higher than that of NPB (-5.30 eV), which could con-
tribute to the device performance, due to the relatively
easy transfer of holes. Their corresponding LUMO energy
levels are calculated to be -1.87, -2.03, -2.09, -2.15, -1.86, -
2.08 and -2.19 eV, respectively. The small energy gap be-
tween the emissive and hole-transporting layers suggests
the efficient charge transfer in the OLEDs, thus low turn-
Figure 4. Cyclic voltammograms of compounds 8-14 record-
ed in dichloromethane.
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Table 2. EL performances of 8 (Aa), 9 (B), 10 (C, H, I, J, K, L, M, N, O), 11 (D), 12 (E), 13 (F) and 14 (G).
1
2
3
4
5
6
7
8
device
λEL (nm)
420
448
445
450
451
Vonb (V)
3.5
Lmax (cd m-2)
734
ηP, max (lm w-1)
0.35
ηC, max (cd A-1) ηext, max (%)
CIEc (x, y)
0.16, 0.08
0.16, 0.08
0.15, 0.07
0.15, 0.08
0.15, 0.11
A
B
C
D
E
0.44
0.30
0.84
0.35
0.34
0.19
0.38
2.00
1.49
1.28
1.74
2.06
3.03
3.98
4.51
0.72
0.35
1.04
0.39
0.33
0.22
0.43
2.30
2.18
1.44
1.98
2.35
2.83
3.85
3.98
4.3
3.3
709
0.20
2090
1088
922
0.76
3.5
0.26
3.7
.0.27
0.15
9
F
448
449
431
3.7
826
0.15, 0.09
0.15, 0.09
0.15, 0.08
0.15, 0.06
0.16, 0.10
0.16, 0.09
0.15, 0.09
0.16, 0.11
0.16, 0.11
0.16, 0.11
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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49
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59
60
G
H
I
3.7
1105
0.29
1.60
3.3
3907
1498
3334
4121
441
3.3
1.37
J
424
427
430
431
4.5
4.1
0.90
1.33
K
L
3.7
5993
4653
5063
4746
1.66
M
N
O
3.6
3.6
3.4
2.60
3.47
432
428
3.95
a Device configuration: ITO/MoO3 (10 nm)/X/TPBi (30 nm)/LiF (1 nm)/Al; for device A-G: X = NPB(60 nm)/8-14 (30 nm);
for device H: X = NPB (60 nm)/mCP (10 nm)/10 (30 nm); for device I: X = NPB (40 nm)/mCP (10 nm)/10 (15 nm); for de-
vice J-O: X = NPB (60 nm)/mCP (10 nm)/BmPyPb: x% 10 (20 nm)/BmPyPb (10 nm), x = 5%(J), 10%(K), 20%(L), 30%(M),
40%(N), 50%(O). b Abbreviations: Von = turn-on voltage at 1 cd m-2, Lmax = maximum luminance, ηC, max, ηC, max and ηext, max
c
= maximum power, current and external efficiencies, respectively. CIE = Commission International de l'Éclairage coor-
dinates at 6 V.
on voltages and high luminescence of the devices could be
expected. Moreover, the large band-gaps of 12 (3.40 eV), 13
(3.54 eV), 14 (3.41 eV) demonstrate their relatively poor
conjugation, owing to more twisted conformation by
adopting meta-linkage mode or/and more crowd substitu-
tions.
ties and the phenyl core, directly leading to the designed
deep blue emissions.
To investigate the EL performance of the new lumino-
gens, seven nondoped fluorescent OLEDs were fabricated
with a configuration of ITO/MoO3 (10 nm)/NPB (60
nm)/X (30 nm)/TPBi (30 nm)/LiF (1 nm)/Al (Figure 6), in
which MoO3, NPB and TPBi worked as hole-injection,
hole-transporting and hole-blocking layers, respectively.
Performances of all these devices are shown in Table 2
(device A-G) and Figure S7. All the devices show deep-
blue emission. Devices based on D-π-A type emitters turn
on with the devices at lower voltage when compared with
the devices based on 8 or 9, indicating the smaller injec-
tion barriers between transporting layers and emitters.
When the emitters adopt meta-linkage way (12 and 13) or
connect carbazole unit at 3-position (14) so as to yield
To further verify the structure-property relationship, we
have carried out Density Functional Theory (DFT) calcu-
lations (B3LYP/6-31g*) to obtain the optimized structures
and orbital distributions of HOMO and LUMO energy
levels of these luminogens. As demonstrated in Figure 5,
the electron clouds of HOMO energy levels are all mainly
located at the peripheral TPA or carbazole units, due to
the good electron-donating abilities. For 10-14, the LU-
MOs of the five luminogens are dominated by orbitals
from the central cyano substituted benzene. And there is
obvious separation of the HOMO and LUMO, which
should benefit to the transfer of both holes and electrons.
While for 8 and 9, because of the week electron-
withdrawing ability of benzene, the electron clouds of
LUMO energy levels are more delocalized and HOMO-
LUMO overlaps can be observed. In the optimized molec-
ular structures of 8-14, it is clearly seen that all the mole-
cules adopt highly twisted structure and meta-linkage 12
and 13 are even more twisted than the para-linkage ones.
Just because of the highly twisted conformations of these
luminogens, the conjugation effect is largely decreased
between the peripheral TPA and tert-butylphenyl moie-
1.9
1.9
2.0
2.1
2.1
2.2
2.2
2.3
2.3
2.7
3.1
LiF/Al
i
B
ITO
4.7
P
T
5.2
5.3 5.3
5.3
5.3
5.5
5.6
5.6 5.6
6.2
Figure 6. Energy level diagram of the nondoped devices.
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should be beneficial to the transfer of holes. Considering
700
600
500
400
300
200
100
0
device H
device I
1
2
3
4
5
6
7
8
A
smaller gap between the emitter and EML can yield better
injection of electrons.18 But the LUMO levels of 8 and 12
are a little bit higher (0.2 eV) than that of compound 10.
As a result, lower efficiencies were obtained. Compared to
the devices based on 10, lower EL efficiencies for 11 with
maximum luminance (Lmax) of 1088 cd m-2 and maximum
current efficiency (ηC, max) of 0.35 cd A-1 may be ascribed to
the different hole-transporting ability between carbazole
and triphenylamine unit.
103
102
101
100
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Next, we further optimized the device based on 10. And
additional layer of 1,3-di(9H-carbazol-9-yl)benzene (mCP)
-100
2
4
6
8
10
12
14
Voltage (V)
was added between NPB and EML with a configuration of
ITO/MoO3 (10 nm)/NPB (60 nm)/mCP (10 nm)/10 (30
nm)/TPBi (30 nm)/LiF (1 nm)/Al in which mCP with wide
band gap and high triplet state energy (ET) functioned as
the blocking layer to confine excitons in the EML.21 As
shown in Figure 7 and Figure S8, compound 10 exhibit
better efficiencies with CE of 2.0 cd A-1, while the CIEy
could still remain 0.08.
B
2.0
1.6
1.2
0.8
0.4
device H
device I
Otherwise, it should be noted that the device I (Table 2)
with a configuration of ITO/MoO3 (10 nm)/NPB (40
nm)/mCP (10 nm)/10 (15 nm)/TPBi (30 nm)/LiF (1 nm)/Al
with compound 10 as EML can even exhibit blue-violet
emission with the CIE coordinates of (0.15, 0.06) (Figure
S8). And compared to device H, the decreased thickness
of the layer of NPB and emitter do not show any influence
on Von. The maximum CE and EQE are 1.49 cd A-1 and
2.18%, respectively. This result is among the best perfor-
mance for nondoped OLEDs with CIEy < 0.06 with the
CIEx remaining 0.15.18,20b All these characteristics suggest
that 10 is promising for the deep-blue OLED application.
1
10
100
1000
Luminance (cd/m2)
Figure 7. Change in current efficiency with the luminance in
multilayer EL devices of 10.
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29
30
31
32
33
34
35
36
37
38
39
40
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60
From the low temperature phosphorescence character-
istics, ET value of 10 was estimated to be 2.63 eV. To fur-
ther confine excitons confine excitons in the EML and
enhance the device performances of 10-based devices,
commercial
available
1,3-bis(3,5-dipyrid-3-yl-
phenyl)benzene (BmPyPb) (ET = 2.70 eV) was selected to
be the host for the fabrication of doped devices. Also, an
additional layer of BmPyPb was inserted to enhance the
transfer of electrons. Devices with the configuration of
ITO/MoO3 (10 nm)/NPB (60 nm)/mCP (10 nm)/BmPyPb:
x% 10 (20 nm)/BmPyPb (10 nm)/TPBi (30 nm)/LiF (1
nm)/Al (Figure S13B) were constructed and tested (Figure
8, S11 and S12). The doped concentration is changed from
5 to 50%. At the doping concentration of 20%, the CIE
coordinate of (0.15, 0.09) is obtained with good stability.
The CIEy suffers a modest increase when further increas-
ing the doping concentration. Lower turn-on-voltages and
better current efficiencies are observed when increasing
the doping concentration. At a lower doping concentra-
tion of 5%, the current efficiency is 1.28 cd A-1, then sur-
passes 2.0 cd A-1 when the concentration is increased to
20%. When the concentration is further increased, the
efficiency does not show some drop, disclosing the ad-
vantages of polyphenybenzene and the designed twist
Figure 8. Change in current efficiency with the luminance in
doped devices of 10 with different doping concentrations (5,
10, 20, 30, 40 and 50%).
highly twisted structure, Von are raised, which are caused
by bad conjugation. Among devices fabricated, the device
based on compound 10 show the highest efficiencies with
maximum EQE of 1.04%, CE of 0.84 cd A-1, and PE of 0.76
lm W-1. Meanwhile, the CIE coordinates of (0.15, 0.07),
very close to the standard deep blue, was observed. And
the EL spectra are almost identical at different driving
voltages, indicating the stability of compound 10 as the
emitting material layer (EML). Actually, the best perfor-
mance of 10 may be reasonable. From the energy diagram
(Figure 6), the HOMO levels of compound 8, 10 and 12
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Research Program (2013CB834701, 2013CB834805) for finan-
cial support.
structure. Among the doped devices fabricated, device O
1
2
3
4
5
6
7
8
employing 50% doped concentration as the EML could
show deep-blue emission with maximum EQE of 3.98%,
CE of 4.51 cd A-1, and PE of 3.95 lm W-1, which is among
the best EL performance for deep-blue emission.18,20b Gen-
erally, efficiency roll-off is significant for OLED devices. In
these doped devices, similar to literature results,22 when
the doping concentration is above 30%, the efficiencies
show a relatively large degree of roll-off. For 100 mA cm-²,
the current efficiencies are decreased by 30, 32, 46, 65, 71
and 73%, for 5, 10, 20, 30, 40 and 50% doping concentra-
tion, respectively, showing that the roll-off is doping con-
centration dependent. This should be attributed to the
singlet-singlet annihilation (SSA) and singlet-triplet anni-
hilation (STA) processes,23 or/and the carrier imbalance.22a
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CONCLUSIONS
In summary, through a new synthetic approach, by using
polyphenybenzene as a platform, novel deep-blue lu-
minogens with D-π-A structure have been designed and
synthesized. Due to the AEE properties of polyphenyben-
zene, these luminogens exhibit enhanced emission. And
most of the luminogens exhibit blue-shifted emissions in
films compared to those in solution. All fabricated devices
show deep-blue emission. Nondoped devices based on 10
display high performance with EQE, CE, and PE of 2.3%,
2.0 cd A-1, and 1.6 lm W-1, respectively, while the CIE coor-
dinates can stay at (0.15, 0.08). Through rational design of
the device structure, CIE coordinates of (0.15, 0.06) can be
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proved performance with EQE and CE of 3.98% and 4.51
cd A-1. Combined with CIE, all these characteristics sug-
gest that 10 is a promising candidate for the deep-blue
OLED application. Thus, the obtained experimental re-
sults might open up a new avenue to utilize polyphe-
nybenzene as a platform for the further development of
efficient deep-blue luminogens.
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ASSOCIATED CONTENT
Supporting Information
Experimental section, optimized structures, PL spectra in
various solutions, low-temperature luminescence and phos-
phorescence and detailed devices performance. This material
is available free of charge via the Internet at
http://pubs.acs.org/ .
AUTHOR INFORMATION
Corresponding Authors
* (“D. M.”). E-mail: mdg1014@ciac.jl.cn.
* (“Z. L.”). E-mail: lizhen@whu.edu.cn or lichemlab@163.com.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the National Science Foundation of China
(no. 21325416, 51333007), and the National Fundamental Key
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NH2
NO2
Br
Br
NO2
1
2
3
4
5
6
NO2
NH2
NO2
Br
3
CuCN
BuONO
1
Fe/HAc
Pd(PPh3)4
Br2
BuONO, CuBr2
CH3CN
2
Br2, CHCl3/HAc
DMSO
HAc
K2CO3
Br
Br
Br
Br
toluene/EtOH/H2O
NH2
1
2
4
5
CN
Br
Br
Br
Br
7
8
9
N
N
Ar1
=
10
11
12
13
14
15
16
17
18
19
20
21
22
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60
6
7
Pd(PPh3)4
K2CO3
8
Pd(PPh3)4
K2CO3
9
N
Ar1
Ar1
Ar2
Ar2
N
Ar2
=
N
N
N
N
13
10
14
12
11
Scheme 1. Synthetic routes of the luminogens 8-14.
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Figure 1. TGA curves of 8-14 recorded under N2 at a heating rate of 10 oC/min.
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Figure 2. (A) UV spectra and (B) PL spectra of 8-14 in THF solution (~10 μM).
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Figure 3. (A) UV spectra and (B) PL spectra of 8-14 of the films.
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Table 1. The thermal, electrochemical and photophysical data of the luminogens.
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5
PL λem
ФF (%)
λmax,abs
solvh
nm
a
c
d
e
Td
Tgb
oC
Eg
EHOMO
eV
ELUMO
solvf filmg
solvf
filmg
nm
oC
eV
eV
nm
nm
6
7
8
9
TPA-TPA/8
442
473
454
468
390
131
---
128
137
101
113
112
3.33
-5.20
-1.87
-2.03
-2.09
-2.15
-1.86
-2.08
-2.19
406
367
443
406
448
412
396
374
439
401
424
394
410
58.7
34.3
68.6
80.3
7.2
316
341
323
342
Cz-Ph-Ph-Cz/9
TPA-CN-TPA/10
Cz-Ph-CN-Ph-Cz/11
mTPA-CN-mTPA/12
3.50 -5.53
3.17 -5.26
302(s330) 295(s338)
10
11
12
13
14
15
16
17
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19
20
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23
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29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3.48 -5.63
3.40 -5.26
3.54 -5.62
340
297
340
345
342
298
339
345
Cz-mPh-CN-mPh-Cz/13 445
Ph-Cz-CN-Cz-Ph/14 460
6.4
3.41
-5.60
400
45.7
a 5% weight loss temperature measured by TGA under N2. b Glass-transition temperature measured by DSC under N2. c Band gap
estimated from optical absorption band edge of the solution. d Calculated from the onset oxidation potentials of the compounds.
e Estimated using empirical equations ELUMO = EHOMO + Eg. f Determined in THF solution. g On glass. h Observed from absorption
spectra in dilute THF solution, 10 μM.
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Figure 4. Cyclic voltammograms of compounds 8-14 recorded in dichloromethane.
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Figure 5. Calculated molecular orbital amplitude plots of HOMO and LUMO levels and optimized molecular structure.
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Table 2. EL performances of 8 (Aa), 9 (B), 10 (C, H, I, J, K, L, M, N, O), 11 (D), 12 (E), 13 (F) and 14 (G).
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8
device
λEL (nm)
420
448
445
450
451
Vonb (V)
3.5
Lmax (cd m-2)
734
ηP, max (lm w-1)
0.35
ηC, max (cd A-1) ηext, max (%)
CIEc (x,y)
0.16, 0.08
0.16, 0.08
0.15, 0.07
0.15, 0.08
0.15, 0.11
A
B
C
D
E
0.44
0.30
0.84
0.35
0.34
0.19
0.38
2.00
1.49
1.28
1.74
2.06
3.03
3.98
4.51
0.72
0.35
1.04
0.39
0.33
0.22
0.43
2.30
2.18
1.44
1.98
2.35
2.83
3.85
3.98
4.3
3.3
709
0.20
9
2090
1088
922
0.76
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11
12
13
14
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60
3.5
0.26
3.7
.0.27
0.15
F
448
449
431
3.7
826
0.15, 0.09
0.15, 0.09
0.15, 0.08
0.15, 0.06
0.16, 0.10
0.16, 0.09
0.15, 0.09
0.16, 0.11
0.16, 0.11
0.16, 0.11
G
H
I
3.7
1105
0.29
1.60
3.3
3907
1498
3334
4121
441
3.3
1.37
J
424
427
430
431
4.5
4.1
0.90
1.33
K
L
3.7
5993
4653
5063
4746
1.66
M
N
O
3.6
3.6
3.4
2.60
3.47
432
428
3.95
a Device configuration: ITO/MoO3 (10 nm)/X/TPBi (30 nm)/LiF (1 nm)/Al; for device A-G: X = NPB(60 nm)/8-14 (30 nm);
for device H: X = NPB (60 nm)/mCP (10 nm)/10 (30 nm); for device I: X = NPB (40 nm)/mCP (10 nm)/10 (15 nm); for de-
vice J-O: X = NPB (60 nm)/mCP (10 nm)/BmPyPb: x% 10 (20 nm)/BmPyPb (10 nm), x = 5%(J), 10%(K), 20%(L), 30%(M),
40%(N), 50%(O). b Abbreviations: Von = turn-on voltage at 1 cd m-2, Lmax = maximum luminance, ηC, max, ηC, max and ηext, max
= maximum power, current and external efficiencies, respectively. CIE = Commission International de l'Éclairage coor-
dinates at 6 V.
c
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Figure 6. Energy level diagram of the nondoped devices.
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Figure 7. Change in current efficiency with the luminance in multilayer EL devices of 10.
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Figure 8. Change in current efficiency with the luminance in doped devices of 10 with different doping concentrations (5%, 10%,
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20%, 30%, 40% and 50%).
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