Construction of efficient blue AIE emitters with triphenylamine and TPE moieties for non-doped OLEDs

Article abstract of DOI:10.1039/c3tc32207f

In this paper, by merging the hole-dominated triphenylamine (TPA) and tetraphenylethene (TPE) moieties together with different linkage positions, four derivatives of 1,2-bis[4′-(diphenylamino)biphenyl-4-yl]-1,2- diphenylethene (2TPATPE) were successfully synthesized with confirmed structures, and their thermal, optical and electronic properties were fully investigated. Thanks to the introduction of the meta-linkage mode on the TPE core, their π-conjugation length could be effectively restricted to ensure blue emission. The non-doped OLEDs based on these four emitters exhibit blue emissions from 443-466 nm, largely blue-shifted with respect to the green emission of 2TPATPE (514 nm). Meanwhile, good electroluminescence efficiencies with L_max, η_C,max, and η_P,max of up to 8160 cd m^-2, 3.79 cd A^-1, and 2.94 Im W^-1 respectively, have also been obtained, further validating our rational design of blue AIE fluorophores. The Royal Society of Chemistry.

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Construction of efficient blue AIE emitters with
triphenylamine and TPE moieties for non-doped
OLEDs†
Cite this: DOI: 10.1039/c3tc32207f
Jing Huang,a Yibin Jiang,b Jie Yang,a Runli Tang,a Ni Xie,c Qianqian Li,a
b
c
a
*
Hoi Sing Kwok, Ben Zhong Tang and Zhen Li
In this paper, by merging the hole-dominated triphenylamine (TPA) and tetraphenylethene (TPE) moieties
together with different linkage positions, four derivatives of 1,2-bis[40-(diphenylamino)biphenyl-4-yl]-1,2-
diphenylethene (2TPATPE) were successfully synthesized with confirmed structures, and their thermal,
optical and electronic properties were fully investigated. Thanks to the introduction of the meta-linkage
mode on the TPE core, their p-conjugation length could be effectively restricted to ensure blue
emission. The non-doped OLEDs based on these four emitters exhibit blue emissions from 443–466
nm, largely blue-shifted with respect to the green emission of 2TPATPE (514 nm). Meanwhile, good
electroluminescence efficiencies with Lmax, hC,max, and hP,max of up to 8160 cd mꢀ2, 3.79 cd Aꢀ1, and
2.94 Im Wꢀ1 respectively, have also been obtained, further validating our rational design of blue AIE
fluorophores.
Received 9th November 2013
Accepted 7th December 2013
DOI: 10.1039/c3tc32207f
www.rsc.org/MaterialsC
this problem as simply as possible, the use of molecules with
distinctive luminogenic characteristics, i.e. taking advantage of
Introduction
molecular aggregation, might be a viable solution.
Organic light-emitting diodes (OLEDs) have attracted consid-
erable attention owing to their promising applications in at-
panel displays and solid-state lighting resources.1 In order to
obtain full-color OLEDs, the hunt for highly efficient and stable
blue emitters has become a pressing issue.2 Because of the wide-
band-gap nature of blue emitters, their electroluminescence
(EL) performance is oen inferior to those of the other two
primary-color (red and green) emitters.3 Moreover, most
conventional uorophores suffer badly from the notorious
aggregation-caused quenching (ACQ) effect when fabricated as
thin solid lms in the devices.4 Although various chemical and
physical approaches have been utilized to alleviate the ACQ
effect, such as the attachment of bulky alicyclics and using
guest–host doped emitter systems, they have met with limited
success and there are also sometimes side effects, as the
formation of aggregates is an intrinsic process when uo-
rophores are located in close vicinity to one another.5 To solve
In 2001, Ben Zhong Tang's group observed an intriguing
phenomenon termed ‘aggregation-induced emission’ (AIE),
which is exactly the opposite of the ACQ effect: a series of
propeller-like molecules are nearly non-emissive in solution but
emit intensely in the aggregated state.6 They have also ratio-
nalized the restriction of intramolecular rotation (RIR) as the
main cause for the AIE effect based on their systematic
research.7 Consequently, this unique luminogenic characteristic
renders AIE-active molecules promising candidates for OLEDs.
Among the reported AIE uorophores, tetraphenylethene (TPE)
is an iconic molecule owing to its facile synthesis and splendid
AIE effect.8 But TPE itself is not a good emitter although its EL
emission is in the deep-blue region. Recently, by directly
attaching TPE to some classical ACQ chromophores, new AIE
emitters with outstanding EL performance have been gener-
ated.9 However, few blue AIE luminogens have been reported,
because it is difficult to retain both the good EL performance
and large band gaps of the molecules. For example, by just
linking two TPE blocks together through the para linkage mode,
the resultant 4,40-bis(1,2,2-triphenylvinyl)biphenyl (BTPE)
exhibits an electroluminescence (EL) performance with a
aDepartment of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic
Materials, Wuhan University, Wuhan 430072, China. E-mail: lizhen@whu.edu.cn;
lichemlab@163.com; Fax: +86-27-68755363; Tel: +86-27-68755363
bDepartment of Electronic and Computer Engineering, The Hong Kong University of
current efficiency of up to 7.3 cd Aꢀ1 9c much higher than that of
,
Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
TPE (0.45 cd Aꢀ1). But the EL emission is red-shied from deep
blue to sky blue (445 to 488 nm), as a result of the good conju-
gation effect between the two TPE moieties (Chart 1).
cDepartment of Chemistry, The Hong Kong University of Science & Technology, Clear
Water Bay, Kowloon, Hong Kong, China
† Electronic supplementary information (ESI) available: Chemical structures of
BTPE and its derivatives, AIE curves, optimized molecular structures and CV
plots for pTPE-2mTPA, mTPE-2oTPA, mTPE-2mTPA and mTPE-2pTPA. See DOI:
10.1039/c3tc32207f
Recently, in an attempt to overcome this problem, we have
reported an efficient approach of using different linkage modes
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devices. Then, is it possible to generate blue uorophores
through the minor and facile structural modication? Perhaps,
the meta linkage mode could realize this point.
Consequently, in this work, we have synthesized four
2TPATPE derivatives to further conrm our idea of generating
blue or deep-blue AIE emitters by changing the linkage modes.
As shown in Chart 2, all the four compounds possess denite
chemical structures without any isomers. First, we have
Chart 1 Schematic structures and maximum EL emissions of TPE, undertaken a facile synthesis to obtain pTPE-2mTPA. Compared
BTPE, 2TPATPE and their derivatives.
to 2TPATPE (514 nm), its EL emission is largely blue-shied to
466 nm. Inspired by this exciting result, three other 2TPATPE
derivatives, mTPE-2oTPA, mTPE-2mTPA and mTPE-2pTPA, have
also been synthesized, by attaching TPA moieties to the TPE
to construct blue or deep-blue AIE emitters.10 As the rst
example, a series of BTPE derivatives (Chart S1†), simply con-
structed from two TPE groups with ortho-, meta- or para-linkage
modes, have been successfully synthesized.10c The device data
demonstrate that all the molecules exhibit deep-blue emissions
ranging from 435 to 459 nm, with good electroluminescence
efficiencies of up to 2.8 cd Aꢀ1. Apparently, the p-conjugation
lengths of the four derivatives were effectively shortened by
using different linking positions of the two TPE units. In
addition, we have also found that it is more effective to attach
TPE to other chromophores through its meta-positions for both
reducing intramolecular conjugation and retaining good EL
performance. Is this a general rule for the design of blue or
deep-blue AIE luminogens, or only special case for the BTPE
system? With this question in mind, we tried to apply the meta
linkage mode to some other AIE luminogens. 2TPATPE
(Chart 2), constructed from hole-dominated triphenylamine
(TPA) and TPE units through the para linkage mode, exhibited
good hole-transporting ability and outstanding improvement in
EL performance with a current efficiency of up to 13.0 cd Aꢀ1
(cf. 0.45 cd Aꢀ1 for TPE).9g Nevertheless, its EL emission is
largely red-shied to 514 nm (cf. 445 nm for TPE), due to the
good conjugation between the TPA and TPE units. In addition,
owing to the synthetic method employed for 2TPATPE, it is a
mixture of E- and Z-isomers, which may inuence the molecular
array in the thin solid lms and the charge transfer of the
core through our favorite meta linkage mode. As a result, the
large band gaps resulting from the less conjugated linkage
mode are retained to ensure their emissions are still located in
the blue region. Gratifyingly, the device performances are
consistent with our expectations: when fabricated as emissive
layers in non-doped OLEDs, all these three emitters exhibit blue
emissions ranging from 443 to 454 nm, with Commission
Internationale de l'Eclairage (CIE) chromaticity coordinates of
(0.16, 0.13), (0.16, 0.16) and (0.16, 0.15). Although the device
congurations have not yet been optimized, the highest Lmax is
up to 3.79 cd Aꢀ1, which is much higher than that of TPE
(0.45 cd Aꢀ1), showing the superiority of this minor structural
modication. Herein, we would like to present the synthesis,
photophysical properties, theoretical calculations and electro-
luminescence of the four 2TPATPE derivatives in detail.
Results and discussion
Synthesis
Scheme 1 illustrates the synthetic routes to pTPE-2mTPA, mTPE-
2oTPA, mTPE-2mTPA and mTPE-2pTPA, and the detailed
Chart 2 Chemical structures of TPE-2TPA reported previously and its Scheme 1 Synthetic routes to pTPE-2mTPA, mTPE-2oTPA, mTPE-
derivatives investigated in this work.
2pTPA, and mTPE-2mTPA.
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procedures are presented in the experimental section. As the amorphous thin lms during the vaccum-deposition process,
key intermediate, 3,30-dibromobenzophenone, was synthesized ensuring the subsequent operating stability and good EL
from 1,3-dibromobenzene and 3-bromobenzaldehyde through performance of the resulting devices.
the PCC (pyridinium chlorochromate) oxidation reaction in two
steps.11 Then 4,40-dibromobenzophenone and the obtained
compound 2 were reacted with diphenylmethane to yield
Optical properties
dibromo-TPE 3 and 4 without isomers, showing some advan- All of the four 2TPATPE derivatives are soluble in common
tages over the MucMurry reaction employed for the synthesis of organic solvents, such as tetrahydrofuran (THF), toluene,
2TPATPE. The corresponding TPE diboronic esters 5 and 6 were dichloromethane and chloroform, but insoluble in water. Fig. 2
successfully synthesized in good yields using Pd(dppf)2Cl2 as shows the UV-vis absorption spectra of the four luminogens in
the catalyst. Moreover, 2-bromotriphenyamine, 3-bromo- dilute THF (ꢂ1 mM). The maximum absorption wavelengths
triphenylamine and 4-bromotriphenylamine were obtained (labs,max) of pTPE-2mTPA, mTPE-2oTPA, mTPE-2mTPA and
according to procedures from the literature.13 Finally, the mTPE-2pTPA are 350, 300, 299 and 327 nm, respectively, which
desired products were successfully obtained through the Suzuki are more blue-shied, by as much as 61 nm, than that of
cross-coupling reactions of TPE diboronic esters 5–6 with bro- 2TPATPE (360 nm), partially conrming the efficient approach
motriphenylamine, in the presence of a catalytic amount of for conjugation adjustment by simply conveniently changing
Pd(PPh3)4, in 2M KOH aqueous solution, and a THF/H2O 3 : 1 the linkage modes. For mTPE-2oTPA and mTPE-2mTPA, their
mixed solution. All the products were puried by column maximum absorptions are almost identical to that of TPE
chromatography through a silica gel stationary phase using (299 nm), showing their limited intramolecular conjugation
petroleum ether and dichloromethane as eluent, and fully despite of the presence of two extra TPA units. mTPE-2pTPA has
characterized by 1H and 13C NMR, mass spectrometry, and the largest maximum absorption wavelength (327 nm), showing
elemental analysis.
its longer conjugation length. This could be explained by its
more planar optimized structure (Fig. S2†). While mTPE-2oTPA
has a much shorter conjugation length resulting from it having
the most twisted conformation. Importantly, in comparison
with pTPE-2mTPA, mTPE-2mTPA is less-conjugated upon the
introduction of the two meta-linked TPA groups to the TPE core,
shedding some light on the further construction of deep-blue
emitters by using the meta-TPE building blocks.
Thermal properties
The thermal properties of the four 2TPATPE derivatives were
characterized by thermal gravimetric analyses (TGA, Fig. 1) and
differential scanning calorimetry (DSC) under an atmosphere of
nitrogen, and the corresponding data is summarized in Table 1.
All of the four compounds exhibit high thermal decomposition
temperatures (Td, corresponding to 5% weight loss) ranging
from 411–457 ꢁC. Among them, mTPE-2oTPA possesses the
lowest Td value, probably due to its highly twisted conforma-
tion. We have only observed the glass transition temperatures
(Tg) for mTPE-2mTPA and mTPE-2pTPA with Tg values of 82 and
87 ꢁC, which are higher than that of the common blue uo-
rescent material 4,40-bis(2,20-diphenyl vinyl)-1,10-biphenyl
(DPVBi, 64 ꢁC).12 The good thermal stability with high Td and Tg
values will be benecial to the preparation of homogenous and
In order to study the AIE characteristic of the four 2TPATPE
derivatives, we chose water and THF as the solvent pair for their
miscibility, to investigate the photoluminescence (PL) proper-
ties of the obtained 2TPATPE derivatives. Fig. S1† shows the PL
spectra of the new uorophores in THF/water mixtures with
different water fractions (fw), which enabled ne-tuning of the
solvent polarity and the extent of solute aggregation. It can be
clearly seen that, when molecules were readily dissolved in pure
THF solutions, the PL curves are all practically a at line parallel
to the abscissa, showing their faint emission property in the
solution state. With the gradual addition of water, the PL
intensities of the four compounds remain low in aqueous
mixtures with a water content less than 60%, above which, they
increase swily with clear peaks becoming apparent in the
emission spectra. At a fw value of 99%, the PL spectra peaks are
at 484, 457, 458 and 460 nm for pTPE-2mTPA, mTPE-2oTPA,
mTPE-2mTPA and mTPE-2pTPA, respectively, which are all blue-
shied relative to those observed in aqueous mixtures with fw of
70% (500, 462, 493 and 484 nm, respectively). This should be
ascribed to the morphological change of the aggregates from
amorphous to crystalline state.8 That is to say, the molecules are
more easily crystallized when the water fraction is increased.
From pure THF solution to a THF/H2O mixture containing 95%
water, the emission intensities are increased over 300-fold,
which can also be veried by visual observations. When illu-
minated under 365 nm UV lamp, thei THF solutions emitted no
observable light, but intense emissions were clearly observed
from the THF/H2O mixture with 95% water content (Fig. 3A).
Fig. 1 TGA curves of Methyl-BTPE, Isopro-BTPE, Ph-BTPE and Cz-
BTPE recorded under N2 at a heating rate of 10 ꢁC min.
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Table 1 The thermal, electrochemical and photophysical data of the TPE-based luminogens
FF (%)
labs
PL lmax
Tgb ꢁC
Eg eV
EHOMO eV
ELUMOe eV
Aggr f nm
Film nm
Aggr f
Solng nm
a
Td ꢁC
c
d
pTPE-2mTPA
mTPE-2oTPA
mTPE-2mTPA
mTPE-2pTPA
422
411
438
457
—
—
82
87
3.21
3.36
3.45
3.33
ꢀ5.25
ꢀ5.27
ꢀ5.25
ꢀ5.19
ꢀ2.04
ꢀ1.91
ꢀ1.80
ꢀ1.86
484
457
458
460
483
458
459
461
64.4
40.4
47.8
35.2
350, 300
300
299
327
a
5% weight loss temperature measured by TGA under N2. b Glass-transition temperature measured by DSC under N2e. c Band gap estimated from
d
optical absorption band edge of the solution. Calculated from the onset oxidation potentials of the compounds. Estimated using empirical
equations ELUMO ¼ EHOMO + Eg. f Determined in THF : H2O ¼ 1 : 99 solution. g Observed from absorption spectra in dilute THF solution, 10 mM.
caused by their rigid molecular structures. The quantitative
enhancement of emission was evaluated by the PL quantum
yields (FF), using 9,10-diphenylanthracene as the standard.
From pure solution in THF to the aggregated state in a 99%
aqueous mixture, the FF values of pTPE-2mTPA increased from
0.5% to 64.4%. Similar phenomena were also observed for the
other three luminogens. The quantum yields were up to 40.4%,
47.8% and 35.2%, for mTPE-2oTPA, mTPE-2mTPA and mTPE-
2pTPA in 99% aqueous mixture, respectively. Evidently, these
four 2TPATPE derivatives are all AIE-active.
Furthermore, we have also investigated the PL properties of
the four uorophores in the solid state, as luminescent mate-
rials are oen fabricated as thin solid lms for their practical
applications. As shown in Fig. 3B, for mTPE-2oTPA, mTPE-
2mTPA and mTPE-2pTPA, they all exhibit blue emissions in the
Fig. 2 UV spectra of pTPE-2mTPA, mTPE-2oTPA, mTPE-2mTPA and
mTPE-2pTPA in THF solution. Concentration (mM): ꢂ10.
range of 458–461 nm, which are similar values to those observed
in aqueous mixtures. However, the solid emission of pTPE-
2mTPA is signicantly red-shied with a PL peak at 483 nm, in
the sky-blue region. This is in consistent with the visual
observations of the four compounds under identical conditions,
indicating the more conjugated molecular structure of pTPE-
2mTPA than those of the other three molecules.
Among them, pTPE-2mTPA in the mixed solvent emits almost
green light, which is signicantly red-shied in comparison to
the other analogues, indicating its better conjugated structure.
However, when more water is added (fw ¼ 99%), the emissions
of the four uorophores in the aqueous mixtures decreased to
Electrochemical properties
some extent, probably due to their relative poor solubility
Cyclic voltammetry (CV) was carried out to investigate the elec-
trochemical properties of the four 2TPATPE derivatives. The
highest occupied molecular orbital (HOMO) energy levels were
estimated from the onset oxidation potentials according to the
equation: HOMO ¼ ꢀ(4.8 + Eox) eV, while the lowest unoccupied
molecular orbital (LUMO) energy levels were obtained from
optical band-gap energies (estimated from the onset wave-
lengths of the UV absorptions) and HOMO values. For pTPE-
2mTPA, mTPE-2oTPA, mTPE-2mTPA and mTPE-2pTPA (Fig. S3†),
their HOMO values are calculated to be ꢀ5.25, ꢀ5.27, ꢀ5.25 and
ꢀ5.19 eV, respectively, which are very close to that of N,N0-bis-
(naphthalen-1-y)-N,N0-bis(phenyl)benzidine (NPB, ꢀ5.30 eV). The
small energy gap between the hole-transporting layer (NPB) and
mTPE-2oTPA, mTPE-2mTPA and mTPE-2pTPA determined in THF/ emissive layers suggests efficient charge transfer in the OLEDs,
Fig. 3 (A) Plots of fluorescence quantum yields of pTPE-2mTPA,
H2O solutions using 9,10-diphenylanthracene (F ¼ 90% in cyclo-
hexane) as standard versus water fractions. Inset: photos of the
luminogens in THF/water mixture (fw ¼ 99%) taken under the illumi-
nation of a 365 nm UV lamp; (B) the PL spectra of the films of the four
compounds. The thin films were spin-coated onto ITO glass from
thus a low turn-on voltage and high luminescence of the device.
Moreover, the smaller band-gap energy of pTPE-2mTPA (3.21 eV)
has demonstrated its longer effective conjugation, which will
lead to a more red-shied EL emission. However, all of the four
derivatives possess much larger band gaps than that of 2TPATPE
dilute THF solution with concentrations of ꢂ1 mg mLꢀ1
.
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(1.8 eV), further proving the powerful structural adjustment for conguration of ITO/MoO3 (10 nm)/NPB (60 nm)/EML (15 nm)/
controllable emissions by simply changing the linkage modes. As TPBi (35 nm)/LiF (1 nm)/Al (100 nm) under identical conditions.
a result, the EL emissions of the new luminogens should be In these OLED devices, MoO3, NPB, and TPBi worked as the
largely blue-shied. Accordingly, their LUMO energy levels are hole-injection, hole-transporting, and hole-blocking layers
calculated to be ꢀ2.04, ꢀ1.91, ꢀ1.80 and ꢀ1.86 eV, respectively. respectively, and pTPE-2mTPA, mTPE-2oTPA, mTPE-2mTPA and
mTPE-2pTPA served as emitters. Fig. 6 shows the current
density–voltage–brightness (J–V–L) characteristics, current effi-
ciency versus current density curves and EL spectra of the
OLEDs, and the EL data are listed in Table 2. The device based
on mTPE-2pTPA exhibited the lowest turn-on voltage of 2.4 V
owing to its higher HOMO energy level, compared to those of
pTPE-2mTPA, mTPE-2oTPA and mTPE-2mTPA (4.0, 4.0 and 5.0 V,
respectively). As shown in the current density–voltage–lumi-
nance (J–V–L) curves (Fig. 6a), the current densities increase
rapidly with increasing voltage. Better EL performance was
observed for pTPE-2mTPA with Lmax, hC,max, and hP,max of 8160
cd mꢀ2, 3.79 cd Aꢀ1, and 2.94 Im Wꢀ1 respectively. pTPE-
2mTPA, has a smaller band gap (3.21 eV) and lower LUMO
energy level (ꢀ2.04 eV) than those of the other three molecules
(3.36 to 3.45 eV, ꢀ1.91 to ꢀ1.80 eV), which is benecial for the
charge injection from the adjacent layers (NPB and TPBi) to the
emissive layer. As a result, pTPE-2mTPA shows a higher device
performance. As we have reported previously,10 there is a
balance between good electroluminescence performance and
controllable EL emission. Although the EL data are not as good
as those of 2TPATPE, to our surprise, the EL emission of pTPE-
2mTPA is dramatically blue-shied from 514 to 466 nm, with
CIE coordinates of (0.17, 0.22) in the sky-blue region. More
importantly, pTPE-2mTPA was conveniently synthesized with
conrmed chemical structure, and without a mixture of E- and
Z-isomers as is the case for 2TPATPE, and the two TPA units
were linked to the TPE core through their meta-positions. Thus,
this successful attempt indeed veries the effectiveness of the
minor structural modication of changing linking positions.
With the purpose of constructing deep-blue AIE luminogens,
a TPE core with m, m-linkage modes has also subsequently been
Theoretical calculations
To further understand the structure–property relationship at
the molecular level, Density Functional Theory (DFT) calcula-
tions (B3LYP/6-31g*) of the four luminogens were carried out to
obtain the optimized structures and orbital distributions of
HOMO and LUMO energy levels of pTPE-2mTPA, mTPE-2oTPA,
mTPE-2mTPA and mTPE-2pTPA, respectively. As demonstrated
in Fig. 4, the electron clouds of HOMO energy levels are all
mainly located on the TPA moieties, due to the good electron-
donating and hole-transporting abilities of TPA, while the
LUMOs of the four chromophores are dominated by orbitals
from the TPE core. Such electron distributions disclose their
weak intramolecular charge transfer property, which will lead to
their controllable blue or almost deep-blue emissions. As
reported in the literature, 2TPATPE is a mixture of E- and
Z-isomers. For E-2TPATPE, its molecular orbitals are more
delocalized, explaining its much higher intramolecular conju-
gation and redder emission than these four derivatives with
certain Z-congurations presented in this paper. This is in good
accordance with the optical properties discussed above and
theoretically fulls our design idea of generating blue AIE
emitters by changing the linking positions. Moreover, for mTPE-
2oTPA, the dihedral angle between the adjacent phenyl blades
of TPE and TPA is ꢂ52ꢁ, much higher than those of 2TPATPE
and the other three chromophores (ꢂ36ꢁ). It shows that mTPE-
2oTPA possesses the most twisted conformation among the four
2TPATPE derivatives, which can also be easily seen from their
optimized structures (Fig. 5).
Electroluminescence
The good thermal stability and efficient solid-state emission of
these materials prompt us to investigate their electrolumines-
cence properties. We have fabricated non-doped devices with a
Fig. 4 Calculated molecular orbital amplitude plots of HOMO and Fig. 5 The dihedral angles of the phenyl blades between TPE and TPA
LUMO levels and optimized molecular structures of pTPE-2mTPA, moieties according to the optimized structures of E-2TPATPE and
mTPE-2oTPA, mTPE-2mTPA and mTPE-2pTPA.
mTPE-2oTPA.
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Fig. 6 (a) Current density–voltage–luminance characteristics of multilayer EL devices of pTPE-2mTPA, mTPE-2oTPA, mTPE-2mTPA and
mTPE-2pTPA. (b) Change in current efficiency with the current density in multilayer EL devices. (c) EL spectra of the luminogens. Device
configurations: ITO/NPB(60 nm)/EML(20 nm)/TPBi(40 nm)/LiF (1 nm)/Al.
benzene derivatives, there is nearly no conjugation between the
Table 2 EL performances of the TPE-based luminogensa
two substituted groups in the meta-positions, leading to the
shorter conjugation length of the resultant molecule. However,
when the two groups are in the ortho-positions, although there
are some conjugation effects, the highly twisted structure will
decrease the conjugation length. Surely, a good conjugation
effect with the longest conjugation length will be achieved for
the para-positions.
lEL
Von Lmax
(cd mꢀ2
hC,max
(cd Aꢀ1
hP,max
(Im Wꢀ1
CIE
(x,y)
(nm) (V)
)
)
)
pTPE-2mTPA 466
4.0 8160
3.79
1.51
1.92
1.57
2.94
1.15
1.49
1.12
0.17, 0.22
0.16, 0.13
0.16, 0.16
0.16, 0.15
mTPE-2oTPA 443
mTPE-2mTPA 454
mTPE-2pTPA 445
4.0 4880
5.0 5480
2.4 5020
As shown in Chart 3, there are four meta-positions in a TPE
unit. The pleasing results obtained by using one and two meta-
Commission International de substituted TPE, inspire us to expand our blue and deep-blue
AIE systems by taking three or all meta-substituted TPE as
a
Abbreviations: Von ¼ turn-on voltage at 1 cd mꢀ2, Lmax ¼ maximum
luminance, hP,max and hC,max
efficiencies, respectively. CIE
l'Eclairage coordinates.
¼
¼
maximum power and current
building blocks. Thus, in addition to the small blue AIE mole-
cules, linear or hyperbranched polymers could also be conve-
niently constructed with controllable EL emissions, and the
related work is under way in our lab.
utilized. As expected, devices based on the other three emitters,
mTPE-2oTPA, mTPE-2mTPA and mTPE-2pTPA, all exhibit almost
deep-blue emissions ranging from 443–454 nm with CIE coor-
dinates of (0.16, 0.13), (0.16, 0.16) and (0.16, 0.15), respectively.
Among them, mTPE-2mTPA possesses the best EL performance
with Lmax, hC,max, and hP,max of 5480 cd mꢀ2, 1.92 cd Aꢀ1, and
1.49 Im Wꢀ1, compared to mTPE-2oTPA and mTPE-2pTPA with
Lmax, hC,max, and hP,max of 4880 and 5020 cd mꢀ2, 1.51 and 1.57
cd Aꢀ1, and 1.15 and 1.12 Im Wꢀ1, respectively. For pTPE-
2mTPA, when the linking positions of the TPE core are changed
from para to meta, the EL peak of the resultant luminogen
mTPE-2mTPA is blue-shied from 466 (sky-blue) to 454 nm
(almost deep-blue), further realizing our idea of generating blue
emitters and demonstrating that TPE with m, m-linkage mode
should be an effective and promising building block for AIE
molecules with limited conjugation.
Conclusions
In this work, we have successfully synthesized four 2TPATPE
derivatives, namely pTPE-2mTPA, mTPE-2oTPA, mTPE-2mTPA
and mTPE-2pTPA, in an effort to generate blue or deep-blue AIE
emitters. By employing the TPE core with m, m-linkage mode,
the torsion degree between the TPE and TPA units could be
signicantly increased, and as a result, the p-conjugation
lengths of the four luminogens have been effectively shortened.
When fabricated as emissive layers in OLEDs, all of the four
emitters exhibit blue to almost deep-blue emissions ranging
Compared to 2TPATPE which exhibits a green emission
(514 nm), light-emitting devices based on the four new deriva-
tives exhibit blue or almost deep-blue emissions, conrming
the control of light-emission through a very simple strategy with
a minor structural modication. The ingenious employment of
a TPE core with m, m-linkage mode and o-, m-, or p-TPA has
greatly decreased the intramolecular conjugation of 2TPATPE,
resulting in their blue EL emissions and good device perfor-
mance. This is consistent with our previous reports10c and also
gives some insight into further molecular design. Thus, the
above results once again conrmed that in the disubstituted Chart 3 Meta-substituted positions of TPE.
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from 443 to 466 nm with Lmax and hC,max up to 8160 cd mꢀ2 and the pretreatment chamber. The samples were transferred to the
3.79 cd Aꢀ1, and CIE coordinates of (0.17, 0.22), (0.16, 0.13), organic chamber with a base pressure of 7 ꢃ 10ꢀ7 Torr for
(0.16, 0.16) and (0.16, 0.15) respectively, but not the green the deposition of N,N-bis(1-naphthyl)-N,N-diphenylbenzidine
emission of 2TPATPE. In other words, the minor structural (NPB), emitters, 2,20,200-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-
modications have led to signicant blue-shied emissions, benzimidazole) (TPBi), which served as hole-transporting, light-
which is proven to be an efficient approach to tune the conju- emitting, hole-blocking, and electron-transporting layers,
gation of the whole luminogens. Considering the previous respectively. The samples were then transferred to the metal
studies, our further work will focus on the construction of more chamber for cathode deposition which composed of 1 nm
excellent small blue and deep-blue AIE molecules and polymers lithium uoride (LiF) capped with 100 nm aluminum (Al). The
with enhanced EL efficiencies, by further application of our idea light-emitting area was 4 mm2. The current density–voltage
of utilizing the different linkage mode.
characteristics of the devices were measured by a HP4145B
semiconductor parameter analyzer. The forward direction
photons emitted from the devices were detected by a calibrated
UDT PIN-25D silicon photodiode. The luminance and external
quantum efficiencies of the devices were inferred from the
photocurrent of the photodiode. The electroluminescence
spectra were obtained using a PR650 spectrophotometer. All the
measurements were carried out under air at room temperature
without device encapsulation.
Experimental section
Characterization
1H and 13C NMR spectra were measured on a MECUYRVX300
spectrometer. Elemental analyses of carbon, hydrogen, and
nitrogen were performed on a CARLOERBA-1106 microanalyzer.
Mass spectra were measured on a ZAB 3F-HF mass spectro-
photometer. UV-vis absorption spectra were recorded on a
Shimadzu UV-2500 recording spectrophotometer. Photo-
luminescence spectra were recorded on a Hitachi F-4500 uo-
rescence spectrophotometer. Differential scanning calorimetry
(DSC) was performed on a Mettler Toledo DSC 822e at a heating
and cooling rate of 10 ꢁC minꢀ1 from room temperature to
250 ꢁC under nitrogen. The glass transition temperature (Tg)
was determined from the second heating scan. Thermogravi-
metric analysis (TGA) was undertaken using a NETZSCH STA
449C instrument. The thermal stability of the samples under a
nitrogen atmosphere was determined by measuring their
weigꢁht loss while heating at a rate of 10 C min from 25 to
600 C. Cyclic voltammetry (CV) was carried out on a CHI vol-
tammetric analyzer in a three-electrode cell with a Pt counter
electrode, an Ag/AgCl reference electrode, and a glassy carbon
working electrode at a scan rate of 100 mV sꢀ1 with 0.1 M tet-
rabutylammonium perchlorate (purchased from Aldrich) as the
supporting electrolyte, in anhydrous dichloromethane solution
purged with nitrogen. The potential values obtained in refer-
ence to the Ag/Ag+ electrode were converted to values versus the
saturated calomel electrode (SCE) by means of an internal fer-
rocenium/ferrocene (Fc+/Fc) standard.
Preparation of nanoaggregates
Stock THF solutions of the uorophores were prepared with a
concentration of 10ꢀ3 mol Lꢀ1. Aliquots of the stock solution
were transferred to 10 mL colorimetric cylinders. Then appro-
priate amounts of THF and water were added successively under
vigorous shaking to furnish 10ꢀ5 M solutions with different
water fractions (0–99.9 vol%). The PL measurements of the
resultant solutions were then performed immediately.
Preparation of compounds
ꢀ1
ꢁ
All other chemicals and reagents were obtained from commer-
cial sources and used as received without further purication.
Solvents for chemical synthesis were puried according to the
standard procedures.
Synthesis of compound 1
A solution of 1.3 M iPrMgCl$LiCl in THF (4.5 mL) was added
dropwise to 1,3-dibromobenzene (1.39 g, 5.9 mmol) in anhy-
drous THF (20 mL) under N2 at ꢀ20 ꢁC. Aer 0.5 h, the mixture
was warmed to room temperature, stirred for 2 h and cooled to
ꢀ20 ꢁC again. Then, 3-bromobenzaldehyde (0.6 mL, 5 mmol)
Computational details
ꢁ
was added to the mixture at ꢀ20 C. The reaction mixture was
stirred for another 2 h at ꢀ10 ꢁC and then allowed to reach room
temperature overnight. Aer hydrolysis with saturated NH4Cl
solution, the mixture was extracted with CH2Cl2 (3 ꢃ 50 mL).
The combined organic layers were dried over Na2SO4 and
evaporated under reduced pressure. The crude product was
used for next step without further purication.
The geometrical and electronic properties were optimized at
B3LYP/6-31g(d) level using Gaussian 09 program. The molec-
ular orbitals were obtained at the same level of theory.
OLED device fabrication and measurement
The devices were fabricated on 80 nm ITO-coated glass with a
sheet resistance of 25 U ,ꢀ1. Prior to loading into the
pretreatment chamber, the ITO-coated glass was soaked in
Synthesis of 3,30-dibromobenzophenone (2)
ultrasonic detergent for 30 min, followed by spraying with de- To a solution of compound 1 (5 mmol) in CH2Cl2 (50 mL),
ionized water for 10 min, soaking in ultrasonic de-ionized water pyridinium chlorochromate complex (1.08 g, 5 mmol) was
for 30 min, and oven-baking for 1 h. The cleaned samples were added with several portions. The reaction mixture was stirred at
treated by peruoromethane (CF4) plasma with a power of room temperature for 3 h. Aer vacuum ltration, the ltrate
100 W, gas ow of 50 sccm, and pressure of 0.2 Torr for 10 s in was evaporated under reduced pressure. The crude product was
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puried by ash chromatography using petroleum ether/ethyl potassium hydroxide (560 mg, 10 mmol) in THF (15 mL) and
acetate as eluent to obtain a white solid in the yields of 52%. 1H distilled water (5 mL) was reuxed for 2 days under N2 in a
NMR (300 MHz, CDCl3) d (ppm): 7.93 (s, 2H), 7.76–7.68 (m, 4H), 50 mL schlenk tube. The mixture was extracted with dichloro-
7.41–7.36 (t, 2H).
methane. The combined organic extracts were dried over
anhydrous Na2SO4 and concentrated by rotary evaporation. The
crude product was puried by column chromatography on silica
gel using dichloromethane/petroleum ether as eluent to afford
the product as white powder in the yield of 75%. 1H NMR (300
MHz, CDCl3) d (ppm): 7.30–7.24 (m, 21H), 7.11–7.09 (m, 13H),
7.04–7.02 (m, 12H). 13C NMR (100 MHz, CDCl3) d (ppm): 148.4,
148.0, 143.9, 143.1, 141.9, 141.4, 140.2, 138.7, 132.0, 131.5,
129.7, 129.4, 128.0, 126.7, 126.4, 124.3, 123.4, 123.3, 122.9,
121.6. MS (EI), m/z: 818.52 ([M+], calcd for C62H46N2, 818.37).
Anal. calcd for C62H46N2: C, 90.92; H, 5.66; N, 3.42. Found: C,
90.55; H, 5.75; N, 3.15.
Synthesis of compound 3
A 2.1 M solution of n-butyllithium in hexane (6 mmol, 2.9 mL)
was added to a solution of diphenylmethane (1.26 g, 7.5 mmol)
in anhydrous tetrahydrofuran (20 mL) at 0 ꢁC under an N2
atmosphere. Aer stirring for 1 h at this temperature, 4,40-
dibromobenzophenone (1.69 g, 5 mmol) was added. Aer 2 h,
the mixture was slowly warmed to room temperature. Then, the
reaction was quenched with an aqueous solution of ammonium
chloride and the mixture was extracted with dichloromethane.
The organic layer was evaporated aer drying with anhydrous
sodium sulfate and the resultant crude product was dissolved in
toluene (20 mL). The p-toluenesulfonic acid (0.17 g, 1 mmol)
was added, and the mixture was reuxed overnight and cooled
to room temperature. The mixture was evaporated and the
crude product was puried by silica gel column chromatog-
raphy using petroleum ether as eluent to obtain a white powder
(3) in the yield of 62%. 1H NMR (300 MHz, CDCl3) d (ppm): 7.36–
7.21 (m, 5H), 7.14–7.12 (m, 6H), 7.01–6.99 (m, 4H), 6.88–6.85
(m, 3H).
Synthesis of mTPE-2oTPA
Prepared following the similar procedure to pTPE-2mTPA from
compound 6 and 2-bromotriphenylamine. White solid. Yield:
45%. 1H NMR (300 MHz, CDCl3) d (ppm): 7.26–7.15 (m, 9H),
7.10–7.03 (m, 13H), 6.90–6.69 (m, 20H), 6.58–6.47 (m, 4H). 13C
NMR (100 MHz, CDCl3) d (ppm): 147.6, 144.6, 144.2, 143.1,
141.1, 141.0, 140.5, 140.4, 140.2, 138.8, 132.3, 131.6, 130.2,
129.6, 129.3, 128.9, 127.8, 126.9, 126.7, 126.5, 126.4, 125.8,
122.2, 121.4. MS (EI), m/z: 818.27 ([M+], calcd for C62H46N2,
818.37). Anal. calcd for C62H46N2: C, 90.92; H, 5.66; N, 3.42.
Found: C, 90.59; H, 5.43; N, 3.12.
Synthesis of compound 4
Prepared by following the similar procedure to compound 3
from 3,30-dibromobenzophenone (2). White solid. Yield: 65%.
1H NMR (300 MHz, CDCl3) d (ppm): 7.25–7.23 (m, 2H), 7.14–7.11
(m, 8H), 7.00–6.95 (m, 8H).
Synthesis of mTPE-2mTPA
Prepared following the similar procedure to mTPE-2mTPA from
compound 6 and 3-bromotriphenylamine. White solid. Yield:
61.1%. 1H NMR (300 MHz, CDCl3) d (ppm): 7.24–7.22 (m, 18H),
Synthesis of compound 5
A mixture of compound 3 (4.90 g, 10 mmol), 4,4,40,40,5,5,50,50- 7.15–7.07 (m, 8H), 6.98–6.85 (m, 20H). 13C NMR (100 MHz,
octamethyl-2,20-bi(1,3,2-dioxaborolane) (6.35 g, 25 mmol), CDCl3) d (ppm): 148.2, 148.0, 143.8, 142.4, 141.9, 140.8, 140.1,
potassium acetate (6.87 g, 70 mmol), and Pd(dppf)Cl2 (0.15 g, 131.3, 130.9, 130.4, 129.6, 129.4, 128.3, 128.0, 126.7, 125.4,
0.2 mmol) in anhydrous 1,4-dioxane (80 mL) were reuxed 124.1, 123.6, 123.3, 122.8, 122.0. MS (EI), m/z: 818.54 ([M+], calcd
under N2 for 12 h, and then water (20 mL) was added. The crude for C62H46N2, 818.37). Anal. calcd for C62H46N2: C, 90.92; H,
product was extracted into ethyl acetate, washed with water, and 5.66; N, 3.42. Found: C, 91.09; H, 5.17; N, 3.04.
dried over anhydrous sodium sulfate. Aer removing solvent
under reduced pressure, the residue was puried by column
Synthesis of mTPE-2pTPA
chromatography using petroleum ether 60–90 ꢁC and ethyl
Prepared following the similar procedure to mTPE-2pTPA from
acetate (v/v 10/1) as eluent. A white powder of 5 was obtained in
compound 6 and 4-bromotriphenylamine. White solid. Yield:
1
the yield of 68.9% (4.2 g). H NMR (300 MHz, CDCl3) d (ppm):
40.1%. 1H NMR (300 MHz, CDCl3) d (ppm): 7.29–7.22 (m, 11H),
7.53–7.47 (m, 4H), 7.11–7.09 (m, 4H), 7.07–7.05 (m, 6H), 7.00–
6.98 (m, 4H), 1.26 (s, 12H).
7.18–7.07 (m, 26H), 7.04–6.99 (m, 9H). 13C NMR (100 MHz,
CDCl3) d (ppm): 147.8, 147.1, 144.0, 143.9, 141.6, 141.1, 140.0,
135.3, 131.5, 130.4, 130.0, 129.6, 129.4, 128.3, 128.0, 127.9,
Synthesis of compound 6
126.7, 125.0, 124.4, 124.2, 123.0. MS (EI), m/z: 818.57 ([M+], calcd
Prepared by following the similar procedure to compound 5
from 3,30-dibromotetraphenylethene (4). White solid. Yield:
57%. 1H NMR (300 MHz, CDCl3) d (ppm): 7.52–7.47 (m, 4H),
7.11–7.10 (m, 14H), 1.27 (s, 12H).
for C62H46N2, 818.37). Anal. calcd for C62H46N2: C, 90.92; H,
5.66; N, 3.42. Found: C, 90.43; H, 5.46; N, 3.33.
Acknowledgements
Synthesis of pTPE-2mTPA
We are grateful to the National Fundamental Key Research
A mixture of compound 5 (520 mg, 0.89 mmol), 3-bromo- Program (2013CB834701), the National Natural Science Foun-
triphenylamine (667 mg, 1.80 mmol), Pd(PPh3)4 (30 mg) and dation of China (no. 21161160556) and Open Project of State
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Key Laboratory of Supramolecular Structure and Materials
(sklssm201302) for nancial support.
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