Enhanced efficiency in nondoped, blue organic light-emitting diodes utilizing simultaneously local excition and two charge-transfer exciton from benzimidazole-based donor-acceptor molecules

Article abstract of DOI:10.1016/j.dyepig.2013.11.023

The simultaneous utilization of all charge-transfer excitons and local excitons is the pathway to obtain the high efficiency fluorescent organic light-emitting diodes (FOLEDs). Here, a twisted intramolecular charge transfer state (TICT-state), a planar intramolecular charge transfer state (ICT-state), and a locally excited state (LE-state) are demonstrated to enhance the occurrence of singlet excitons in the fluorescent emitters, which are based on benzimidazole and triphenylamine donor-acceptor derivatives. The synthesis, photophysics and electroluminescent (EL) performance are studied systematically. The fluorescence emitters (TPABBBI and TPABBI) with the special TICT and ICT characteristics realize the electron-hole (e-h) recombination via intramolecular conversion from charge-transfer excitons to radiative singlet exciton. The devices based on them show high efficiency (5.1 cd/A, 5.77 lm/W, 5.66% of EQE_m for TPABBBI and 3.56 cd/A, 3.11 lm/W, 4.23% for TPABBI), low efficiency roll-off at high luminance and stable blue emission.

Full text of DOI:10.1016/j.dyepig.2013.11.023

Dyes and Pigments 103 (2014) 39e49
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Enhanced efficiency in nondoped, blue organic light-emitting diodes
utilizing simultaneously local excition and two charge-transfer exciton
from benzimidazole-based donoreacceptor molecules
*
Xinhua Ouyang, Xingye Zhang, Ziyi Ge
Ningbo Institute of Material Technology and Engineering (NIMTE), Chinese Academy of Sciences (CAS), Ningbo 315201, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The simultaneous utilization of all charge-transfer excitons and local excitons is the pathway to obtain
the high efficiency fluorescent organic light-emitting diodes (FOLEDs). Here, a twisted intramolecular
charge transfer state (TICT-state), a planar intramolecular charge transfer state (ICT-state), and a
locally excited state (LE-state) are demonstrated to enhance the occurrence of singlet excitons in the
fluorescent emitters, which are based on benzimidazole and triphenylamine donoreacceptor de-
rivatives. The synthesis, photophysics and electroluminescent (EL) performance are studied system-
atically. The fluorescence emitters (TPABBBI and TPABBI) with the special TICT and ICT characteristics
realize the electronehole (eeh) recombination via intramolecular conversion from charge-transfer
excitons to radiative singlet exciton. The devices based on them show high efficiency (5.1 cd/A,
5.77 lm/W, 5.66% of EQEm for TPABBBI and 3.56 cd/A, 3.11 lm/W, 4.23% for TPABBI), low efficiency roll-
off at high luminance and stable blue emission.
Received 8 October 2013
Received in revised form
12 November 2013
Accepted 21 November 2013
Available online 1 December 2013
Keywords:
Local excition
Charge-transfer exciton
Blue
Organic light-emitting diodes
Luminescence
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Photochemistry
1. Introduction
Hence, it still remains a challenge to develop efficient luminescent
materials to generate high performance fluorescent OLEDs. Recent
Organic light-emitting diodes (OLEDs) have attracted exten-
sively attention for their potential applications in both full-color
display [1] and solid-state lighting [2]. In the past decades, much
effort has been paid to the development highly performance and
full-color OLEDs, nearly 100% internal electroluminescence (EL)
quantum efficiency (hint) has been achieve by employing transition
metal-centered phosphorescent emitters such as iridium (III) and
platinum (II) complexes, which can harvest both singlet and triplet
excitons by fast intersystem crossing (ISC) for phosphorescent
emission [3e5]. However, the efficiency and stability of blue elec-
troluminescent emitters are still lacking compared to the reported
green and red emitters. Moreover, a severe efficiency roll-off at high
current densities is observed in these devices owing to the triplet-
polaron or tripletetriplet exciton annihilation (TTA) [6].
works pointed to a significant role of intermediate charge transfer
(CT) state to form radiative exciton or used radiative CT excition
immediately for improving the efficiency of FOLED [8]. Segal et al.
demonstrated a device to improve the hint of Alq3 from 25% to 84%
by adding a mixing layer to change interaction of CT states [9]. Very
recently, Adachi and co-workers put forward a promising viable
mechanism to realize higher efficiency OLEDs by using the high
reverse intersystem crossing efficiency of the intermolecular CT
state between electron-donating and electron-accepting molecules
[10e12]. Ma et al. [13] reported a twisting donoreacceptor fluo-
rescent molecule that utilized energy of the CT-state and locally
excited state to obtain high-efficiency electroluminescence with a
maximum external quantum efficiency >5.0%, corresponding to the
hint >25%. Although the CT-state and locally excited state models
have been widespread acceptance and appear to be conclusion of
the results. However, there are still some points that arise leading
to doubt or contrary the interpretation. For example, both the
twisted angle necessary and excited-state CT equilibria are not
quite clear [14]. Herein, we choose benzimidazole containing tri-
phenylamine unit as emitters to realized radiative exciton with
high yield by intermediate CT-State in the exciton formation pro-
cess, and gained some new insights into the CT-States in the two
novel pretwisted D-A system. Commonly, imidazole-based donore
In view of this, OLEDs incorporating fluorescent emitters should
be given considerable concern due to their remarkably high reli-
ability and stability. However, the hint of fluorescent OLEDs
(FOLEDs) is limited 25% because of the spin statistical limit of 1:3
for the singlet to triplet excition ratio under electrical excitation [7].
* Corresponding author.
E-mail address: geziyi@nimte.ac.cn (Z. Ge).
0143-7208/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.11.023

40
X. Ouyang et al. / Dyes and Pigments 103 (2014) 39e49
Table 1
acceptor molecules are made up an imidazole and arylamine units.
In these molecules, their N-substituted benzene rings were highly
twisted about the imidazole plane [15]. It can be expected to help
suppress fluorescence quenching caused by aggregation in the solid
state. Moreover, their donor units also twisted with the imidazole
plane with different dihedral angles. These would reduce the extent
of conjugation in the molecules and thus the amount of charge
transfer from the donor (arylamine) to the acceptor (imidazole)
[15]. In addition, a large difference in the dipole moment between
the ground states and excited states have been demonstrated [13].
These features lead to the emergence of dual or multiple fluores-
cence from two excited singlet states of imidazole-based donore
acceptor molecules, including LE-states and intramolecular charge
transfer states [16].
In this study, we report the synthesis, characterization, photo-
physical properties, and EL performances of benzimidazole con-
taining triphenylamine donoreacceptor derivatives (TPABBBI and
TPABBI). 1,2-diphenyl-1H-benzo[d]imidazole (BBI) is also synthe-
sized for the control experiments. The synthetic routes of them are
shown in Scheme 1. In these molecules, the triphenylamine moi-
eties are attached to benzimidazole to enhance the hole-transport
ability. By comparing with the BBI, TPABBBI and TPABBI exhibit the
particular photophysical properties due to the presence of torsional
motion in these molecules. The photoluminescent (PL) of TPABBBI
and TPABBI arises from a LE- and two CT-states in solution, while
the PL of them should be mainly considered as the contribution of
LE-state in film state. The nondoped EL devices using the emitters
(TPABBBI and TPABBI) show good performance and the hint of them
are over the limit of 25%, which can be attributed to generate the
newborn local excitons and CT excitons. We estimate the maximum
occurrence of singlet generation fraction is up to 75% in these de-
vices. Our results open a pathway for obtaining high efficiency
OLEDs.
The physical properties of TPABBI, and TPABBBI.
ox
red
Comp
Tm/Tg/Td (ꢀC)
E
E
HOMO (eV) LUMO (eV)
Energy
gap (eV)
onset
onset
V
V
Optical/
Optical/
Optical/
calculated
calculated
calculated
TPABBI 212/-/401
TPABBBI 257/-/410
0.89 ꢁ1.97 ꢁ5.25/ꢁ5.17 ꢁ2.39/ꢁ1.50 2.84/3.67
0.86 ꢁ1.88 ꢁ5.22/ꢁ5.17 ꢁ2.48/ꢁ1.61 2.74/3.56
corresponding to 5% weight loss) of them are 401 and 410 ꢀC for
TPABBI, and TPABBBI, respectively. No obvious glass transition
temperatures (Tg) are observed for them, while endothermic
melting transition temperatures (Tm) appear obviously at 212 and
257 ꢀC for TPABBI and TPABBBI, respectively. Such high Tm and Td
values indicate that these molecules are stable and have the po-
tential to be fabricated into devices by vacuum thermal evaporation
technology. Their highest occupied molecular orbitals (HOMO) and
lowest unoccupied molecular orbitals (LUMO) energy levels are
deduced by the results of cyclic voltammetry (CV) with three
electrodes system as shown in Fig. 2. The HOMO and LUMO energy
levels of TPABBI, and TPABBBI are estimated to be ꢁ5.25, ꢁ2.39,
and ꢁ5.22, ꢁ2.48 eV with regard to ferrocene (ꢁ4.36 eV [17]),
respectively. Obviously, the HOMO levels of these materials are
elevated significantly compared to that of TPBi (HOMO level
of ꢁ6.2 eV), revealing an enhanced hole injection ability. Likewise,
the levels of them are at the range of ꢁ2.39 eV to ꢁ2.48 eV, which
are close to that of TPBi (LUMO level of ꢁ2.7 eV), showing that the
electron injection abilities of them are similar with the TPBi.
2.2. Theoretical calculations
To understand electronic structures of these compounds, the
molecular configuration and frontier molecular orbitals were
optimized using DFT at the B3LYP/6-31G(d,p) level in the Gaussian
03 software. In the optimized configurations of TPABBI and TPABBBI
as shown in Fig. 3, a large dihedral angle (w70ꢀ) between the N-
substituted benzene and benzimidazole planes, this might prevent
2. Results and discussions
2.1. Thermal and electrochemical properties
The thermal properties of TPABBBI and TPABBI were determined
by differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) under a nitrogen atmosphere. Results are shown in
intramolecular extending of p-electron and suppress molecular
packing in the solid state effectively. In addition, these molecules
also show the dihedral angles between the triphenylamine moi-
eties and benzimidazole planes with the twisting angle of 36ꢀ and
Table
1 and Fig. 1. The decomposition temperatures (Td,
Scheme 1. The synthetic route of TPABBBI and TPABBI.

X. Ouyang et al. / Dyes and Pigments 103 (2014) 39e49
41
molecules. Subsequently, we also optimized the geometric struc-
tures of them with different twisting angles between the triphe-
nylamine moieties and benzimidazole planes from 0ꢀ to 90ꢀ.
Interestingly, when the twisting angle is up to 90ꢀ, all of them have
almost complete separation of the HOMO and LUMO at benz-
imidazole and triphenylamine moieties. The complete separation is
preferable for efficient CT transition of HOMO (donor) / LUMO
(acceptor). However, all of them in such a 90ꢀ twisting conforma-
tions are unstable, because their energies are z0.1 eV higher than
that of them at the most stable states. Based on DFT total-energy
analysis, it can be found both the HOMO and the LUMO partially
delocalize to the whole molecules, instead of a fully separated (or
localized) HOMO and LUMO.
TPABBBI
TPABBI
100
80
60
40
20
0
(a)
401
410
2.3. Optical properties
100
200
300
400
500
600
Temperature (oC)
The absorption spectra of TPABBI and TPABBBI in solution and
the photoluminescence spectra in solution and film are given in
Fig. 4 and Table 2. By comparing with the BBI, the absorption
spectra of TPABBI and TPABBBI appear another peck at w360 nm,
indicating strong electron coupling between the benzimidazole
moiety (A) and triphenylamine (D) unit. Furthermore, the absorp-
tion spectra of them appear to be markedly alike because of the
presence of the same electronic transitions. As shown in Fig. 4, the
fluorescence spectra of TPABBI and TPABBBI reveal a single-band
emission, which gradually red-shift relative to BBI. This shows
that excited-state energy levels of these compounds from BBI to
TPABBBI are reduced, in other words, there is a gradual increase in
charge-separation degree or dipole moment.
(b)
TPABBI
TPABBBI
The fluorescence spectra of TPABBI and TPABBBI in various po-
larity solvents were recorded and are shown in Fig. 5 and Table 2.
With increase of the solvent polarity gradually from petroleum
ether to acetonitrile, the fluorescence spectra of them exhibit a
larger red-shifted, broader shape and longer fluorescence lifetime
50
100
150
200
250
Temperature (oC)
(s) (in Fig. 6). Obviously, a large difference in the dipole moment
between the excited state and the ground state, the CT state with a
large polarity is usually stabilized in polar solvents, and the change
is consistent with a variety of the excited state from the LE-state to
an excited state with the strong CT character [18]. The dipole
moment (me) of the fluorescent states can be estimated from the
energies of the fluorescence maxima (vf) against the solvent
parameter Df (shown in Fig. 7 and Table 2) and the values of them
are 24.2 D for TPABBBI and 24.6 D for TPABBI according to the
equation of LipperteMataga, respectively. [18]
Fig. 1. a) TGA thermograms of TPABBI and TPABBBI. b) DSC trance of TPABBI and
TPABBBI.
38ꢀ for TPABBI and TPABBBI, respectively. Hence, these benzimid-
azole derivatives are pretwisted D-A molecules. In HOMO, most of
-electrons located on the triphenylamine moieties and linkers,
p
while in LUMO, the
p-electrons delocalized on the whole
ꢀ
ꢁ
2
me me
ꢁ
mg
vf
¼
D
f þ C
(1)
(2)
(3)
4
p
ε0hca3
À
Á
0:5 n2 ꢁ 1
2n2 þ 1
ε ꢁ 1
D
f ¼
ꢁ
2ε þ 1
TPABBI
1
3
ꢂ
ꢃ
3M
4N
a ¼
p
d
TPABBBI
where mg is the ground-state dipole moment, a is the solvent cavity
(Onsager) radius, which was derived from Avogadro’s number (N),
molecular weight (M), and density (d ¼ 1), and ε, ε0, and n are the
solvent dielectric constant, vacuum permittivity, and solvent
refractive index, respectively. The value of mg was calculated at the
B3LYP 6-31G* level with Gaussian 03.
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Obviously, the large dipole moments of TPABBI and TPABBBI are
one of evidences for the formation of the twisted intramolecular
charge transfer state (TICT state) [19]. In order to confirm deeply the
TICT state and LE-state, we studied the temperature dependent
Potential (V vs SCE)
Fig. 2. Electrochemical properties of TPABBI, and TPABBBI.

42
X. Ouyang et al. / Dyes and Pigments 103 (2014) 39e49
Fig. 3. Optimized geometries and molecular orbital amplitude lots of HOMO and LUMO energy levels of TPABBI and TPABBBI at different twisting angles.
relationships of their fluorescence spectra as shown in Fig. 8.
Obviously, the fluorescent intensities of TPABBI and TPABBBI reduce
and lead to a broad and red-shifted spectrum, which can be attri-
bute to three main factors, (1) decreasing of temperature causes a
higher solvent polarity [20], improving the stability of TICT-state
and dominating a broad featureless and red-shifted TICT emis-
sion; (2) decreasing the fluctuation of surrounding medium (sol-
vent molecules) at low temperature, emission from the LE-state
starts to compete with the solvent reorganization time, then both
LE and TICT emission are observed simultaneously; (3) the twisted
equilibrium conformation of the TICT state results in low fluores-
cence transition moments, because the TICT-state always adjusts
the dihedral angle between the two planes by the torsional relax-
ation to accommodate with the change of the surrounding medium
polarity.
BBI
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
TPABBI
TPABBBI
300
400
500
600
700
Wavelength (nm)
Fig. 4. Normalized UV and PL spectra of BBI, TPABBI, and TPABBBI.
Table 2
Detailed parameters of photophysical properties of TPABBI and TPABBBI.
cmꢁ1
Ff
%
lem(FWHM) nm
labs nm
s
ns
em
Comp.
Solvent
ε
n
D
f
V
max
TPABBI
Petroleum ether
Hexane
Cyclohexane
Toluene
Chloroform
Ethyl acetate
Tetrahydrofuran
Methylene chloride
Acetonitrile
Film
Petroleum ether
Hexane
Cyclohexane
Toluene
Chloroform
Ethyl acetate
Tetrahydrofuran
Methylene chloride
Acetonitrile
Film
1.8
1.9
1.417
1.375
1.426
1.497
1.446
1.372
1.407
1.424
1.344
e
0.0734
0.0944
0.1004
0.1264
0.2534
0.2924
0.3146
0.3189
0.2929
e
24752
24691
24510
23529
22075
21978
21882
20920
19881
22222
25000
24938
24876
23810
22727
22573
22321
21008
19841
23310
64
68
65
78
88
93
83
80
45
84
52
56
54
66
72
81
71
63
36
68
404(43)
405(44)
408(45)
425(51)
453(70)
455(72)
457(68)
478(80)
503(100)
450(61)
400(43)
401(44)
402(46)
420(50)
440(61)
443(63)
448(61)
476(85)
504(72)
429(55)
353
354
353
354
355
354
355
355
353
353
352
354
353
354
352
355
354
355
353
352
e
e
e
e
e
e
2.02
2.38
4.81
6.02
7.58
8.93
2.8
3.5
3.9
3.4
e
37.5
e
TPABBBI
1.8
1.9
2.02
2.38
4.81
6.02
7.58
8.93
1.417
1.375
1.426
1.497
1.446
1.372
1.407
1.424
1.344
e
0.0734
0.0944
0.1004
0.1264
0.2534
0.2924
0.3146
0.3189
0.2929
e
e
e
e
e
e
3.5
1.9
4.5
1.8
37.5
e

X. Ouyang et al. / Dyes and Pigments 103 (2014) 39e49
43
Petroleum ether
Hexane
Cyclohexane
Toluene
Methylene chloride
Tetrahydrofuran
Ethyl acetate
Chloroform
Acetonitrile
Film
Film
(a)
TPABBBI
1.0
0.8
0.6
0.4
0.2
0.0
(a)
Fit curve
Acetonitrile
Fit curve
THF
1k
Fit curve
TPABBBI
100
400
450
500
550
600
650
700
15
20
25
30
35
40
45
Wavelength (nm)
Time (ns)
(b)
Petroleum ether
Hexane
Cyclohexane
Toluene
Methylene chloride
Tetrahydrofuran
Ethyl acetate
Chloroform
Acetonitrile
Film
1.0
Film
Fit curve
THF
Fit curve
Acetonitrile
Fit curve
TPABBI
(b)
0.8
0.6
0.4
0.2
0.0
1k
TPABBI
100
400
450
500
550
600
650
700
Wavelength (nm)
20
25
30
35
40
45
Time (ns)
Fig. 5. Fluorescence spectra of TPABBI and TPABBBI in various polarity solvents.
Fig. 6. The fluorescence decay profiles of TPABBI and TPABBBI at room temperature,
excitation wavelength ¼ 360 nm.
Additionally, the formation of a TICT state would generate a
significant charge separation between the DeA pair, which is a
nearly electronically decoupled DeA pair with forbidden radiative
transition to the ground state, thus a small TICT fluorescence
quantum yield is expected. Hence, the changes of fluorescence
quantum yields (Ff) of TPABBI and TPABBBI should be the other
evidences (Table 2). By comparison of the changes of Ff in the
various polarity solvents of TPABBI and TPABBBI, we can found Ff
improves firstly and then decreases quickly by the increase of the
polarities of the solvents. Obviously, the decrease process is
attributed to the appearing TICT state in these molecules, while the
increase of Ff may be produced by another CT process.
emission. With increasing the proportion of glycerol, the increase
in solvent polarity results in initial red-shifts of the TICT-emission
peak, and the increase in solution viscosity would suppress the
formations of the TICT state and then the ICT state, because
conformational transitions in the formation of the excited states
To understand the conformational change of the ICT state, we
selected two solvents, glycerol with high viscosity and high polarity
(ε, 42.5) and ethanol with low viscosity and relatively low polarity
(ε, 24.5) for binary solvents. The fluorescence spectra of TPABBI and
TPABBBI were measured in ethanoleglycerol (EeG) solvent mix-
tures, the ratio of E:G ranging from 100 : 0 to 10 : 90 as shown in
Fig. 9. The fluorescence spectrum of TPABBI and TPABBBI in neat
ethanol is characterized by a single long-band emission with a peak
at 469 and 478 nm, respectively. With increasing the proportion of
glycerol, the LE emission appears and rises by comparing with the
PL of them in hexane. The long-band peaks of them shift to 478 nm
for TPABBI and 499 nm for TPABBBI. Then a new peak appears at
w435 nm in the PL of TPABBI and TPABBBI. When E:G is 5:95, there
are only the LE-emission peaks. Besides the LE state, there are two
CT states of TPABBI and TPABBBI in the binary solvents. The emis-
sion peak from 469 to 498 nm may be assigned to the TICT-state
emission. The peak at w430 nm may be assigned to the ICT-state
Fig. 7. Solvent polarity (Df) dependence of the fluorescence maxima of TPABBI and
TPABBBI.

44
X. Ouyang et al. / Dyes and Pigments 103 (2014) 39e49
293 K
E:G=100:0
E:G=60:40
E:G=30:70
E:G=20:80
E:G=15:85
E:G=10:90
E:G=5:95
(a)
TPABBB
I
1.0
0.8
0.6
0.4
0.2
0.0
(a)
273 K
263 K
253 K
243 K
233 K
223 K
213 K
TPABBBI
450
500
550
600
650
400
500
600
700
Wavelength (nm)
Wavelength (nm)
293 K
263 K
243 K
223 K
213 K
203 K
193 K
(b)
E:G=100:0
E:G=30:70
E:G=20:80
E:G=15:85
E:G=10:90
E:G=5:95
1.0
0.8
0.6
0.4
0.2
0.0
(b)
TPABBI
400
450
500
550
600
650
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 8. The PL spectra of TPABBI and TPABBBI in dichloromethane measured at
different temperatures.
Fig. 9. Fluorescence spectra of TPABBI and TPABBBI in various ethanol-glycerol binary
mixtures (100:0e5:95, v/v) at room temperature.
(TICT and ICT) are limited in high-viscosity solutions. Obviously, the
formation of TICT state is suppressed more easily than the ICT state.
In the above processes, there are two kinds of effects, one is from
polarity when increasing initially the proportion of glycerol, and
another is from viscosity at high levels of glycerol. In fluorescence
spectra of them in binary solvents, there are isoemissive points,
435 nm for TPABBBI and 430 nm for TPABBI, which can be inter-
preted in terms of two emission species with a parentedaughter
relationship as a proof for conversion from the TICT state to the ICT
state. In addition, the LE-states of TPABBI and TPABBBI are elec-
tronically excited to come into being the vertical nonpolar singlet
excited state. Then, the LE-state relaxes to the lowest singlet LE-
state (1LE1) by the geometry relaxation that needs a torsional
motion. After then, LE emission will be observed. When the com-
pounds are surrounded by the polar solvents, their LE-states will
vanish. The large geometry relaxation happens to form orthogonal
configuration that facilitates electron transfer from one unit to the
other and forms TICT-state. At last, the TICT emission can be
observed after a large variation of torsion angle. Therefore, upon
photoexcitation, it seems that there is a motheredaughter rela-
tionship from the TICT-state to ICT-state to LE-state [18].
emissions are from the properties of their triplet levels, Obviously,
the energy levels of 3LE-states from TPABBI and TPABBBI are higher
than that of the 1LE-states states (0.6 eV for TPABBI and 0.45 eV for
TPABBI). Hence, the principal components of the LE-states are
formed when electrons in the
p orbital of the benzimidazole or
triphenylamine ring are excited to the
p* orbital. The other
Phos of TPABBBI
Fluo of TPABBBI
Phos of TPABBI
Fluo of TPABBI
1.0
0.8
0.6
0.4
0.2
0.0
The phosphorescent spectra of TPABBI and TPABBBI at 77 K and
the corresponding LE spectra of them are shown in Fig. 10. The
fluorescence of them are mainly attributed to the contribution of
nonrelaxed LE-state in low polar solvent, suggesting the TICT
emission will becomes insignificant due to the limit of the torsional
motion and environment fluctuation, while the phosphorescent
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 10. The phosphorescent spectra and corresponding LE spectra of TPABBI and
TPABBBI.

X. Ouyang et al. / Dyes and Pigments 103 (2014) 39e49
45
component is the CT-state that involves the excitation of electrons
from the triphenylamine or linkers (donor) to the benzimidazole
group (acceptor). The LE-state of them should be a mixed state
containing the pp* state and some partial CT-states.
2.4. Electroluminescent properties
The OLEDs were fabricated to evaluate the EL performances of
TPABBI and TPABBBI, the construction of devices is ITO/MoO3
(10 nm)/NPB (80 nm)/TCTA (5 nm)/TPABBI or TPABBBI (20 nm)/TPBi
(40 nm)/LiF (1 nm)/Al (100 nm), where indium tin oxide (ITO) is
used as the anode, MoO3 as hole-injecting layer, N,N0-di-1-
naphthyl-N,N0-diphenylbenzidine (NPB) as hole-transporting
layer, 4,40,400-tri(N-carbazolyl)-triphenylamine (TCTA) as electron-
blocking layer, 1,3,5-tri(phenyl-2-benzimidazolyl)benzene (TPBi) as
electron- transporting and hole- blocking layer, and LiF used as the
electron injecting layer. The relative energy levels of the materials
employed in the devices are illustrated in Scheme 2.
Comparing the EL and PL spectra of them, they show the similar
results. Obviously, the EL spectra are identical with the PL spectra,
suggesting the electron and hole (eeh) recombination in the
emitting layers, and both EL and PL originate from the same radi-
ative decay process of singlet excitons. The maximum emission
peaks of device based on TPABBI and TPABBBI are located at 440 nm
and 460 nm with the FWHM of 60e62 nm, respectively. The EL
spectra of TPABBI and TPABBBI show little changes by increasing
voltages of devices. When the driving voltage is at 8 V, the CIE
coordinates (x, y) of TPABBI- and TPABBBI-based on devices are
found to be (0.151, 0.113) for TPABBI and (0.156, 0.132) for TPABBI,
respectively.
The current densityevoltageeBrightness (JeVeB) characteris-
tics of these devices are shown in Fig. 11, and the main performance
parameters of them are summarized in Table 3. As shown in Fig. 11,
the devices based TPABBI and TPABBBI show similar profiles of
current density-voltage owing to the same HOMO and LUMO levels.
The brightnesseefficiencies relationships are shown in Fig. 12.
Obviously, the efficiency of the device based on TPABBBI is much
higher than that of the device based on TPABBI, may be because of
the different fluorescence quantum yields, charge-transfer excitons
and carries recombination natures of TPABBBI and TPABBI. In this
regard, we also fabricated hole-only and electron-only devices with
the structures of ITO/MoO3 (10 nm)/NPB (80 nm)/TCTA (5 nm)/
TPABBI or TPABBBI (20 nm)/TPBI (40 nm)/MoO3 (40 nm)/Al
(100 nm), and ITO/TPBI (40 nm)/NPB (80 nm)/TCTA (5 nm)/TPABBI
or TPABBBI (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm). The
current density versus voltage curves of the devices are shown in
Fig. 11. The current densityevoltageeluminance (JeVeL) of TPABBBI and TPABBI.
Fig. 13. Obviously, a slight difference between the hole and electron
current densities based on TPABBI and TPABBBI devices are found.
This indicated the efficient carries reconstruction and recombina-
tion can be formed in the layer of TPABBI or TPABBBI. Additionally,
with the same voltage, the current densities of TPABBBI are better
than that of TPABBI. It can also be explained from another aspect
why the performance of TPABBBI is better than that of TPABBI. It is
worthwhile to note that the EQE of the TPABBBI- and TPABBI-based
devices only show a little decrease at high current densities, indi-
cating TPABBBI and TPABBI are promising candidates for OLED
emitters.
2.5. Discussions about local excition and two kinds of charge-
transfer excitons
Noteworthy, the external quantum efficiencies (EQE) (hext) of
these FOLEDs are 4.23% for TPABBI and 5.66% for TPABBBI. Natu-
rally, the EQE is relative on the light out-coupling efficiency (hPh
and internal quantum efficiency hint with the formula of
hext Ff hint hPh, where the hPh is 20% by measuring the glass
)
(
)
¼
ꢂ
ꢂ
substrate with an index of refraction n ¼ 1.5, Ff is the fluorescent
quantum yield of the emitters, and the hint is limited to 0.25. Ideally,
when the values of their Ff were assumed to be 100%, the maximal
of hext is calculated to only be 4.2% for TPABBBI and 3.4% for TPABBI,
respectively, which is significantly lower than the values of 5.66%
and 4.23%. However, the TTA does not play a role in the high EQEs of
these devices because the brightness increases linearly with the
increase of the current density at low current injection and less
than linearly at higher current injection [21]. Therefore, the
improved EL performance is related to the intrinsic factors of
TPABBBI and TPABBI, which can capture eeh pair enthusiastically
and convert it into the emissive exciton efficiently.
In order to analyze deeply the appeared results, we proposed
two assumptions demonstrated as follows, (1) The benzimidazole
and triphenylamine moieties in TPABBI and TPABBBI keep orthog-
onal structure in solid film, thus the two moieties have the inde-
pendent electronic configuration in ground state; (2) the eeh
capture is predominantly intramolecular. In these OLED devices,
Table 3
The Electroluminescence characteristics of TPABBI, and TPABBBI.
Emitter CIE (x, y) LEmax PEmax EQE Lmax
Devices performance at
(cd/A) (lm/W) (%) (cd/m2) 100 cd mꢁ2
LE (cd/A) PE (lm/W) EQE (%)
TPABBI 0.15,0.11 3.56 3.11
TPABBBI 0.15,0.13 5.1 5.77
4.23 7895 3.27
5.66 20680 5.09
2.72
4.26
4.19
5.62
Scheme 2. Energy level diagrams for devices.

46
X. Ouyang et al. / Dyes and Pigments 103 (2014) 39e49
on benzimidazole moiety and the linkers (benzene and biphenyl) of
one molecule respectively, which will appear a bound ICT exciton
(ICTE). In this process, the electron and hole are stabilized at the
special dihedral angles. Similarly, The ICT state also produces a
mixing state of singlet CT-state (1CT-state) and triplet CT-state (3CT-
state), and the transition of 1CT 4 3CT is also allowed. Thus, three
type excitons will appear in TPABBBI- and TPABBI-based device, LE,
ICT, and TICT, comprising 1LE-state, 3LE-state, 1CT-state and 3CT-
state. However, the efficient radiative decay pathway mainly cen-
ters on 1LE-state, while the radiationless transitions might take
place in other states (3LE-state, 1CT-state, 3CT-state) owing to the
presence of the spin-forbidden and space-forbidden [27].
The CT-states containing TICT and ICT that lie energetically be-
tween the free carrier continuum and the final states, can be
regarded as the immediate precursor to the final, strongly bound
states (singlet states or triplet states) [28]. The formation rate of the
triplet exciton is considerably smaller than that of the singlet
exciton due to a consequence of more ionic nature and smaller
binding energy of the singlet exciton [29]. For TPABBI and TPABBBI,
the same results are found that the formation rate of 3LE is much
smaller formation rate of 1LE. In addition, the donor and acceptor
groups in TPABBI and TPABBBI are linked by benzene or biphenyl
Fig. 12. The brightness-efficiencies curves of TPABBBI and TPABBI.
electron and hole encounter and capture each other with Coulomb
interaction to form bound eeh pair [22], which results in the
neutral excitons or charge transfer (CT) states [23]. The bound eeh
pair is often localized on a molecule or single conjugated segment
due to relatively low dielectric constant of organic molecule [24].
For TPABBI and TPABBBI, two channels might bring about the
bound eeh pair. When the eeh pair localizes at single conjugated
segments (benzimidazole planes of TPABBI and TPABBBI) (Fig. 14),
then forms neutral, tightly bound local exciton (LE). This exciton
has large exciton binding energy owing to the result of both the
Coulomb interaction and image potential [25]. Commonly, the
random recombination of carries will produce only one singlet local
exciton (1LE) for every three triplet local excitons (3LE) in this
process. Moreover, the electron and hole of a bound eeh pair may
reside on benzimidazole and triphenylamine moiety of one mole-
cule respectively, and then give rise to a loosely bound TICT exciton
(TICTE). For this exciton, the electron and hole are stabilized by a
twist in the molecule of up to 90ꢀ. Since the electron and hole wave
function are orthogonal in perpendicular conformation, the two
electrons of TICTE are neither parallel (YY or [[) nor antiparallel
([Y), just remain nearly perpendicular to each other. The TICT state
that will undergo geometry relaxation of the TPA and BBI moieties
by a slight torsion, results in a mixing state of singlet CT-state (1CT-
state) and triplet CT-state (3CT-state). Furthermore, it is possible
that the transition of 1CT 4 3CT is allowed due to the small energy
splitting between them [26]. Additionally, the eeh pair may be held
and having nearly similar
p systems. The TICT- and ICT-states are
one of spineorbit, charge-transfer intersystem crossing mechanism
(SOCTeISC). This SOCTeISC system from CT-state may take place
via a spineorbit mechanism to produce the neutral LE-state
involved in the spin flip that is coupled to a significant change in
orbital angular momentum [30]. Moreover, the SOCTeISC rate is
enhanced with an approximately perpendicular orientation be-
tween the donor and acceptor. For the TPABBI and TPABBBI, the
electron transferring happens between the donor and acceptor in
the CT-states to form neutral LE due to the delocalization of the
electron wave function in the imidazole and triphenylamine or
linkers units (Fig. 15). Therefore, it is possible that the transitions
are responsible for the newborn 1LE from CTE, e.g., 1CT / 1LE and
3CT / 1CT / 1LE.
According to the above discussions, we put forward two hy-
potheses, (1) mixing equal proportions of LE, ICT, and TICT; (2)
ideally, converting all of the ICT and TICT to 1LE. A maximum
generation rate of 1LE can be estimated, e.g., 1/3 ꢂ 1/4(1LE) þ 1/
3(TICT) þ 1/3(ICT) ¼ 75%. The result is agreed with experimental
data for the TPABBI- and TPABBBI-based devices. Our results
deduce that increasing TICT and ICT in the EL device can increase
efficiently the 1LE, because it is a significantly intermediate state in
the newborn 1LE formation. Significantly, the EQE of the TPABBBI
and TPABBI-based devices only show a little decrease at high cur-
rent densities (Fig. 13). This suggests that TPABBBI and TPABBI are
the promising candidates for blue emitters.
80
hole
electron
TPABBBI
TPABBI
70
60
50
40
30
20
10
0
3. Conclusions
In conclusion, we have demonstrated a method to enhance the
occurrence of singlet excitons in the fluorescent emitters by uti-
lizing simultaneously a twisted intramolecular charge transfer state
(TICT-state), a planar intramolecular charge transfer state (ICT-
state), and a locally excited state (LE-state) based on benzimidazole
donoreacceptor derivatives. The EL properties of them were
investigated with the device structure of ITO/MoO3/NPB/TCTA/
TPABBI or TPABBBI/TPBi/LiF/Al. The devices based them present a
stable and high efficiency blue emissions of 5.1 cd/A, 5.77 lm/W,
5.66% for TPABBBI and 3.56 cd/A, 3.11 lm/W, 4.23% for TPABBI.
Interestingly, the devices show a significant characteristic of
increasing the radiative exciton generation efficiency due to the
influence of TICT-states and ICT states. Our results reveal that
TPABBBI and TPABBI are good candidates to serve as highly-
12
14
16
18
20
22
24
Voltage (V)
Fig. 13. Current density versus voltage characteristics of the hole-only and electron-
only devices for TPABBBI and TPABBI.

X. Ouyang et al. / Dyes and Pigments 103 (2014) 39e49
47
Fig. 14. The formation process of exciton in EL device based on TPABBI and TPABBBI.
Fig. 15. The electron transfers between orbitals oriented in the three planes and the processes of intramolecular exciton reorganization.

48
X. Ouyang et al. / Dyes and Pigments 103 (2014) 39e49
effective blue OLEDs. This opens an avenue for designing and
synthesizing highly-efficient blue OLEDs by employing the emitters
with the twisted D-p-A and planar linkers.
organic molecule gas deposition system. Different organic mate-
rials were deposited on the ITO-coated glass substrate according to
the designed structure. LiF buffer layer and Al were deposited as a
co-cathode under a pressure of 5 ꢂ 10ꢁ4Pa. Electroluminescent
spectra and commission international De L0 Eclairage(CIE) coordi-
nation of these devices were measured by a PR655 spectra scan
spectrometer. The luminescent brightness (L)-current (I)-voltage
(V) characteristics were recorded simultaneously with the mea-
surement of the EL spectra by combining the spectrometer through
a Keithly model 2400 programmable voltageecurrent source. The
layer thicknesses of the deposited materials were monitored in situ
using a model FTM-V oscillating quartz thickness monitor made in
Shanghai, China. All the measurements were carried out at room
temperature under ambient conditions.
4. Experimental
4.1. Materials and characterization
All starting materials were purchased from TCI, the reagents
were obtained from J&K Chemical company and used without
further purification. 1H NMR and 13C NMR spectra were determined
in CDCl3 with a Bruker DRX 400 MHz spectrometer. Chemical shifts
(d) were given relative to tetramethylsilane (TMS). The coupling
constants (J) were reported in Hz. Elemental analyses were recor-
ded with a PerkineElmer 2400 analyzer. ESI-Ms spectra were
performed with a FINNIGAN Trace DSQ mass spectrometer at 70 eV.
Fluorescence emission spectra of these samples were measured
with a FLSP920 spectrophotometer. Experiment course was moni-
tored by TLC. Column chromatography was carried out on silica gel
(100e200 mesh).
4.4. Computational details
The geometric and electronic structure of them were optimized
by Density functional theory (DFT) level of theory with the three-
parameter Becke-style hybrid functional (B3LYP). The HOMO and
LUMO energy levels are predicated by 6-31G (d,p) basis set. All of
calculations about the molecule have been performed on the huge
computer origin 2000 server center using the Gaussian 03 program
package [31]. The compositions of molecular orbits were analyzed
using the GaussView 3.0 program.
4.2. Synthesis
2-(4-bromophenyl)-1-phenyl-1H-benzo[d]imidazole,
40-
bromo-N,N-diphenyl-[1,10- biphenyl]-4-amine, and 1-phenyl-2-(4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) phenyl)-1H-benzo
[d]imidazole (BPPE) were synthesis following the methods of
published papers [17].
4.5. Fluorescence, phosphorescence and lifetime measurements
The fluorescence and phosphorescence spectrum was recorded
using a Hitachi F-4500 fluorescence spectrometer. Excitation and
emission slit width were set at 10 nm and 10 nm, respectively.
Compounds were dissolved in tetrahydrofuran with the concen-
tration of w10ꢁ7 mol/L. For the phosphorescence, the solution was
then inserted into a Dewar vessel containing liquid nitrogen (77 K).
The liquid column in the quartz tube was solidified and appeared to
be a transparent glass. The F-4500 phosphorescence accessory was
used to measure phosphorescence spectra. The measurement was
performed at 77 K using a chopper to remove shorter-lived singlet
emissions.
The fluorescence lifetimes were measured by a time corre-
lated single photon counting spectrometer from Edinburgh In-
struments (FLS920) with a nanosecond hydrogen flash lamp as
the excitation source (repetition rate 40 kHz) at room tempera-
ture. The instrument response (FWHM ca. 1 ns) was determined
by measuring the light scattered by a Ludox suspension (solu-
tion) or solid film itself (film). The data were analyzed by itera-
tive convolution of the luminescence decay profile with the
instrument response function using software package provided
by Edinburgh Instruments.
4.2.1. General method for the synthesis of TPABBI and TPABBBI
BPBE (3.0 mmol), 4-bromo-N,N-diphenylanilineor 40-bromo-
N,N-diphenyl- [1,10-biphenyl]-4-amine (3.5 mmol) and Pd(PPh3)4
(0.03 mmol) were suspended in toluene (12 mL) and Et4NOH (6 mL
of a 20% aqueous solution), the reaction was heated to reflux for
18 h. After cooled, the solution was extracted with CH2Cl2 (30 mL),
washed with water (2 ꢂ 50 mL), dried by MgSO4 and evaporated to
dryness. After drying under vacuum, then, it was purified by ethyl
acetate/petroleum ether (1:12) as an eluant to afford white solid.
N,N-diphenyl-40-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,10-
biphenyl]-4-amine (TPABBI): yield: 1.05 g, 68%. 1H NMR(CDCl3,
400 MHz, ppm)
d
: 7.94 (d, 1H, J ¼ 8.4 Hz), 7.66 (d, 2H, J ¼ 8.4 Hz),
7.54 (d, 5H, J ¼ 8.0 Hz), 7.49 (d, 2H, J ¼ 8.0 Hz), 7.38 (d, 3H,
J ¼ 7.64 Hz), 7.35e7.27 (m, 6H), 7.14 (d, 6H, J ¼ 8.0 Hz), 7.06 (t, 2H,
J ¼ 7.6 Hz). 13C NMR (CDCl3, 100 MHz, ppm)
d: 152.2, 147.7, 147.5,
142.9, 141.5, 137.3, 137.1, 133.7, 130.0, 129.8, 129.3, 128.6, 128.1, 127.7,
127.5, 126.3, 124.6, 123.6, 123.4, 123.2, 123.1, 119.8, 110.5. ESI-MS (m/
z): 513.2 (Mþ). Anal. calcd for C37H27N3: C, 86.52; H, 5.30; N, 8.18;
Found: C, 86.54; H, 5.22; N, 8.23.
N,N-diphenyl-400-(1-phenyl-1H-benzo[d]imidazol-2-yl)-
[1,10:40,100-terphenyl]-4-amine (TPABBBI): yield: 1.53 g, 65%.1H
NMR(CDCl3, 400 MHz, ppm)
d
:7.92 (d, 1H, J ¼ 8.4 Hz), 7.69e7.65 (m,
Acknowledgments
6H), 7.61e7.57 (m, 3H), 7.55e7.50 (m, 5H), 7.39e7.36 (m, 3H), 7.31e
7.28 (m, 5H), 7.15 (d, 6H, J ¼ 8.0 Hz), 7.04 (t, 2H, J ¼ 7.6 Hz). 13C NMR
This work was Financial supported from National Natural Sci-
ence Foundation of China (21074144, 51273209), Ningbo Interna-
tional Cooperation Foundation (2012D10009), the Open Fund of the
State Key Laboratory of Luminescent Materials and Devices (South
China University of Technology) and the External Cooperation
Program of the Chinese Academy of Sciences (No. GJHZ1219).
(CDCl3, 100 MHz, ppm) d: 152.1,147.6, 147.4, 143.1,141.5,140.1, 138.4,
137.4,137.1,134.2,129.9,129.8,129.3,128.8,128.6,127.6,127.5,127.4,
127.0, 126.7, 124.5, 123.8, 123.4, 123.0, 119.8, 110.4. ESI-MS (m/z):
589.1 (Mþ); Anal. calcd for C37H27N3: C, 87.58; H, 5.30; N, 7.13;
Found: C, 87.62; H, 5.17; N, 7.24.
4.3. Devices fabrication
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