Similar or totally different: The control of conjugation degree through minor structural modifications, and deep-blue aggregation-induced emission luminogens for non-doped OLEDs

Article abstract of DOI:10.1002/adfm.201202639

Four 4,4′-bis(1,2,2-triphenylvinyl)biphenyl (BTPE) derivatives, 4,4′-bis(1,2,2-triphenylvinyl)biphenyl, 2,3′-bis(1,2,2- triphenylvinyl)biphenyl, 2,4′-bis(1,2,2-triphenylvinyl)biphenyl, 3,3′-bis(1,2,2-triphenylvinyl)biphenyl and 3,4′-bis(1,2,2- triphenylvinyl)biphenyl (oTPE-mTPE, oTPE-pTPE, mTPE-mTPE, and mTPE-pTPE, respectively), are successfully synthesized and their thermal, optical, and electronic properties fully investigated. By merging two simple tetraphenylethene (TPE) units together through different linking positions, the π-conjugation length is effectively controlled to ensure the deep-blue emission. Because of the minor but intelligent structural modification, all the four fluorophores exhibit deep-blue emissions from 435 to 459 nm with Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of, respectively, (0.16, 0.14), (0.15, 0.11), (0.16, 0.14), and (0.16, 0.16), when fabricated as emitters in organic light-emitting diodes (OLEDs). This is completely different from BTPE with sky-blue emission (0.20, 0.36). Thus, these results may provide a novel and versatile approach for the design of deep-blue aggregation-induced emission (AIE) luminogens. Copyright

Full text of DOI:10.1002/adfm.201202639

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Similar or Totally Different: The Control of Conjugation
Degree through Minor Structural Modifications, and
Deep-Blue Aggregation-Induced Emission Luminogens
for Non-Doped OLEDs
Jing Huang, Ning Sun, Yongqiang Dong, Runli Tang, Ping Lu, Ping Cai, Qianqian Li,
Dongge Ma,* Jingui Qin, and Zhen Li*
applications in display and lighting.[1]
Despite the great success achieved by
some commercial products in this area,
a critical drawback to realizing full-color
displays is the poor performance of deep-
Four 4,4′-bis(1,2,2-triphenylvinyl)biphenyl (BTPE) derivatives, 4,4′-bis(1,2,2-
triphenylvinyl)biphenyl, 2,3′-bis(1,2,2-triphenylvinyl)biphenyl, 2,4′-bis(1,2,2-
triphenylvinyl)biphenyl, 3,3′-bis(1,2,2-triphenylvinyl)biphenyl and
3,4′-bis(1,2,2-triphenylvinyl)biphenyl (oTPE-mTPE, oTPE-pTPE, mTPE-mTPE,
blue OLEDs, since the intrinsic wide
and mTPE-pTPE, respectively), are successfully synthesized and their thermal,
optical, and electronic properties fully investigated. By merging two simple
tetraphenylethene (TPE) units together through different linking positions,
the π-conjugation length is effectively controlled to ensure the deep-blue
emission. Because of the minor but intelligent structural modification, all
the four fluorophores exhibit deep-blue emissions from 435 to 459 nm with
Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of,
respectively, (0.16, 0.14), (0.15, 0.11), (0.16, 0.14), and (0.16, 0.16), when
fabricated as emitters in organic light-emitting diodes (OLEDs). This is com-
pletely different from BTPE with sky-blue emission (0.20, 0.36). Thus, these
results may provide a novel and versatile approach for the design of deep-
blue aggregation-induced emission (AIE) luminogens.
bandgap makes it very difficult to inject
charges into the blue emitters.[2,3] During
the past decade, a number of excellent
fluorescent blue light-emitting materials,
composed of anthracence,[4] fl uorene,[5]
and styrylarylene[6] derivatives, have been
reported in the literature. Shu et al.[7]
and Cheng et al.[2a] developed efficient
non-doped deep-blue OLEDs with lumi-
nescence efficiencies of up to 5.3 and
5.6 cd A−1 . Generally, when fabricated
as thin solid films in practical applica-
tions, most fluorophores suffer from the
notorious aggregation-caused quenching
(ACQ) effect.[8] In 2001, Tang’s group
discovered an “abnormal” phenom-
1. Introduction
enon, termed aggregation-induced emission (AIE), which has
been proved to be an effective approach to tackle the ACQ
problem:[9] a series of organic molecules were found to be
non-luminescent in the solution state but highly emissive in
the aggregated state.[10] Among the typical AIE luminogens,
tetraphenylethene (TPE) and many of its derivatives (Chart
S1 in the Supporting Information) enjoy the advantages of
facile synthesis and outstanding AIE effect.[11] However, it is a
pity that none of them is an efficient emitter in the deep-blue
region, although TPE itself is a weak one. The most prom-
ising example, 4,4′-bis(1,2,2-triphenylvinyl)biphenyl (BTPE)
(Scheme 1), constructed by two simple TPE units, exhibited
outstanding improvement in its electroluminescence (EL) per-
formance, with current efficiency of up to 7.3 cd A−1 (0.45 cd A−1
for TPE), well demonstrating the proverb “two is better
than one”;[12] however, unfortunately, the maximum EL emis-
sion wavelength is red-shifted to a large extent, from 445
(deep-blue) to 488 nm (sky-blue). Is it possible to retain the
deep-blue emission of TPE, while improving its low efficiency
of 0.45 cd A−1 dramatically, just by modifying the subtle struc-
ture of the luminophors? If it is the case, many better AIE
emitters could be developed, and the properties of the present
Considerable interest has been attracted to the research field
of organic light-emitting diodes (OLEDs) due to their vast
J. Huang, R. Tang, Dr. P. Cai, Dr. Q. Li,
Prof. J. Qin, Prof. Z. Li
Department of Chemistry
Wuhan University
Wuhan 430072, China
E-mail: lizhen@whu.edu.cn; lichemlab@163.com
N. Sun, Prof. D. Ma
Institute of Applied Chemistry
The Chinese Academy of Sciences
Changchun 130022, China
E-mail: mdg1014@ciac.jl.cn
Prof. Y. Dong
Department of Chemistry
Beijing Normal University
Beijing 100875, China
Dr. P. Lu
Department of Chemistry
Jilin University, Jilin 130022, China
DOI: 10.1002/adfm.201202639
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Scheme 1. Chemical structures and EL performances of TPE and BTPE.
huge number of TPE-based luminophors should be re-evalu-
ated upon minor structural adjustments.
Very recently, we reported several TPE-based blue emitters
achieving a balance between emission efficiency and prolonged
π-conjugation[13] (Chart S2 in the Supporting Information). Our
synthetic ideas are the linkage of TPE to the classical and effi-
cient blue moiety spirofluorene by sharing one phenyl ring or
linking with carbazole through the single carbon–nitrogen bond
to ensure good EL performance and blue emission, or the uti-
lization of the twisted conformation to effectively decrease the
conjugation degree. Both of the methods have partially realized
our idea of generating blue AIE luminogens, however, efficient
emitters with deep-blue emission remain a challenge.[14] Ana-
lyzing the previous cases carefully, on one side the introduction
of the additional aromatic rotators could improve the LED effi-
ciency of TPE moieties; meanwhile, on another side, the bonded
aromatic moieties would extend the formed π-conjugation
system, leading to the bathochromic shift of the emission.
Our examples could decrease the π-conjugation between TPE
and the introduced aromatic rings to a large degree; thus, the
luminophors could emit blue light, similar to TPE itself. If the
π-conjugation degree between the aromatic blocks in the newly
designed TPE-based luminophors could be further decreased,
for example, to the extent that it would almost be negligible,
perhaps emission in the deep-blue region could be achieved.
Towards this goal, further structural modification should be
considered carefully, in addition to our previous two methods
mentioned above.
Scheme 2. The inductive and/or conjugative effect exists in m- and
p-nitrophenol.
Accordingly, four BTPE derivatives, oTPE-mTPE, oTPE-pTPE,
mTPE-mTPE, and mTPE-pTPE (Scheme 3) were synthesized
by utilizing the different conjugated effects of ortho-, meta-,
and para- linkages of biphenyl. In other words, when two TPE
units are merged with different linking positions, the pro-
longed π-conjugation length could be partially shortened for
bluer emission, compared to BTPE with sky-blue emission. The
experimental results largely confirmed our ideas: when fabri-
cated as emitters in OLEDs without optimization, all the four
fluorophores exhibit deep-blue emissions from 435 to 459 nm
with Commission Internationale de l’Eclairage (CIE) chroma-
ticity coordinates of (0.16, 0.14), (0.15, 0.11), (0.16, 0.14), and
(0.16, 0.16), totally different from BTPE with sky-blue emission
(0.20, 0.36). The highest Lmax is up to 2.8 cd A−1 , much higher
than that of TPE (0.45 cd A−1 ). Though the device performance
is inferior to those of the best deep-blue emitters, we believe
that excellent EL efficiencies could be obtained through further
device optimization and structure modification. Herein, we
present the synthesis, photophysical properties, and electrolu-
minescence of the four BTPE derivatives in detail.
Any textbook of basic organic chemistry
will indicate that different positions of nitro
and hydroxyl groups in nitrophenols, as
Scheme 2 illustrates, will surely affect their
pKa values. Interestingly, in m-nitrophenol,
the conjugated effect could be nearly ignored
between the two groups, while in p-nitroph-
enol, it cannot. Inspired by this point, it is
expected that just by changing the linkage
mode of two TPE moieties in BTPE, the
effect of “two is better than one” should be
still present; more importantly, the emis-
sion might possibly remain almost the same
as for TPE itself, as the result of the much
decreased conjugated effect between the
two TPE moieties, with the linkage mode
of meta-position possibly being the favorite. Scheme 3. Chemical structures of mTPE-pTPE, mTPE-mTPE, oTPE-mTPE, and oTPE-pTPE.
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TPE boronic esters(3 and 4) were key reactive
intermediate. It is not difficult to think that
some other AIE compounds containing TPE
blocks could be easily prepared from the TPE
boronic esters 3 and 4 with other aromatic
halides; in such AIE compounds, the con-
jugated effects between the TPE blocks and
the chosen aromatic rings, could perhaps be
controlled, to a large degree, to adjust their
light-emitting properties, especially the emis-
sion color. From this point, the syntheses
presented here might stimulate the prepara-
tion of more interesting AIE molecules; as a
good illustration, many other TPE derivatives
with different linkage modes are being tested
in our laboratory.
The four BTPE derivatives, oTPE-mTPE,
oTPE-pTPE, mTPE-mTPE, and mTPE-pTPE,
1
were fully characterized by H and 13C NMR
spectroscopy, mass spectrometry, and ele-
mental analysis, well confirming their mole-
cular structures. Fortunately, single crystals
of oTPE-mTPE and oTPE-pTPE were cultured
Scheme 4. Synthetic routes to BTPE derivatives mTPE-pTPE, mTPE-mTPE, oTPE-mTPE, and
oTPE-pTPE.
from their dichloromethane/methanol solutions and crystallo-
graphically characterized.
2. Results and Discussion
2.1. Synthesis
2.2. Thermal Properties
Scheme 4 illustrates the synthetic routes to the four BTPE
derivatives of oTPE-mTPE, oTPE-pTPE, mTPE-mTPE, and
mTPE-pTPE. Unlike commercially available 4-bromoben-
zophenone, its two isomers, 3-bromobenzophenone (1) and
2-bromobenzophenone (2), were synthesized through two dif-
ferent approaches. While compound 1 was obtained from the
reaction between 1,3-dibromobenzene and benzoyl chloride,
the 2-aminobenzophenone was converted to compound 2
in two steps, with the diazo moieties as the reactive interme-
diate. Subsequently, the obtained compounds of 1 and 2, and
the purchased 4-bromobenzophenone, reacted with diphenyl-
methane, to yield the corresponding bromo-TPEs, including
pTPE-Br, mTPE-Br, and oTPE-Br. Next, these three bromo-TPEs
were utilized to synthesize their corresponding TPE boronic
esters. While the TPE boronic esters 3 and 4 were prepared
smoothly, the boronic ester with the substituted position in the
ortho-position could not be obtained, although we tried several
times with various efforts to optimize the reaction conditions;
the main reason might be the relatively too-large steric effect.
Next, the Suzuki coupling reactions, catalyzed by Pd(PPh3)4,
of TPE boronic esters with pTPE-Br, mTPE-Br, and oTPE-Br,
gave the final products of four BTPE derivatives: oTPE-mTPE,
oTPE-pTPE, mTPE-mTPE, and mTPE-pTPE. Without the cor-
responding boronic ester, the compound oTPE-oTPE could not
be prepared; however, the study of its isomers could well dem-
onstrate their almost totally different properties. The obtained
four BTPE derivatives, just like BTPE, contain two pieces of TPE
blocks, differing in their linkage modes, which directly leads to
their having different properties, as mentioned above and dis-
cussed in detail in the following. It should be pointed out that
the whole synthetic route was not too hard, and the designed
To be a good emitter, the luminogen should possess good
thermal properties to ensure the process of vaccum deposition
and subsequent operating stability of the devices. The thermal
properties of the four BTPE derivatives were investigated by
thermal gravimetric analyses (TGA; Figure 1) and differential
Figure 1. TGA thermograms of mTPE-mTPE, mTPE-pTPE, oTPE-mTPE,
and oTPE-pTPE recorded under N2 at a heating rate of 10 °C min−1
.
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T a b le 1 . The thermal, electrochemical and photophysical data of of mTPE-pTPE, mTPE-mTPE, oTPE-mTPE, and oTPE-pTPE.
a)
b)
c)
d)
e)
λmax,ab
PL λmax (aggr)f)
Td
Tg
Eg
EHOMO
ELUMO
[°C]
310
302
360
330
[°C]
[eV]
3.27
3.30
3.27
3.32
[eV]
[eV]
[nm]
[nm]
oTPE-pTPE
oTPE-mTPE
mTPE-pTPE
mTPE-mTPE
–
99
114
–
5.47
5.50
5.48
5.50
2.20
2.20
2.21
2.18
318
312
318
311
473
469
473
465
a)5% weight loss temperature measured by TGA under N2; b)bandgap estimated from optical absorption band edge of the solution; c)calculated from the onset oxidation
potentials of the compounds; d)estimated using the empirical equation ELUMO = EHOMO + Eg; e)observed from absorption spectra in dilute THF solution; f)determined in
THF/H2O = 1:99 solution.
scanning calorimetry (DSC). As listed in Table 1, the thermal
decomposition temperatures (Td, corresponding to 5% weight
loss) were determined to range from 302 to 360 °C. Due to the
bulky TPE unit and more planar structure, mTPE-pTPE pos-
sessed the highest thermal stability and exhibited higher glass
transition temperature (Tg) of 114 °C, compared to the other
three molecules with more twisted conformation. The good
thermal stability, with high Td and Tg values, should contribute
to the preparation of homogeneous and stable amorphous
emissive layers in OLED devices.
controlling the conjugated length in the AIE molecules through
the effective structural modification of different linkage posi-
tions. That is to say, really, we could adjust the electronic struc-
ture of the BTPE derivatives to a large extent simply by changing
the linkage positions of the two pieces of TPE blocks.
In order to investigate the AIE properties of the new com-
pounds, PL spectra were recorded; THF and water were chosen
as the solvent pair for their miscibility. Figure S1 (Supporting
Information) clearly demonstrates the PL change and fluores-
cent image of mTPE-mTPE in THF and THF/water mixtures as
an example. It is easily seen that, in dilute THF solution, the PL
curve is practically a flat line parallel to the abscissa, confirming
the nearly non-emissive property in the solution state. How-
ever, when a large amount of water is added, intense emission
is observed. As shown in Figure S1 (Supporting Information),
when the water fraction is over 80%, the PL intensity increases
swiftly, indicating the formation of aggregates. At a fw value
of 99%, the PL intensity is 274-fold higher than that in pure
THF. Interestingly, the PL spectrum peak is blue-shifted from
480 to 465 nm as the water fraction increases from 80% to 99%
This could be explained by the morphological change of the
aggregates from amorphous to crystalline state; that is, crystal-
lization causes blue-shifted emission, similar to those reported
in the literature.[11a,15] In our previous work,
2.3. Optical Properties
All four BTPE derivatives are soluble in common solvents, such
as tetrahydrofuran (THF), chloroform, and dichloromethane,
but insoluble in water. Figure 2A shows the absorption spectra
of mTPE-pTPE, mTPE-mTPE, oTPE-pTPE, and oTPE-mTPE
in dilute THF solution, with the maximum absorption wave-
lengths (λmax,abs) at 318, 311, 318, and 312 nm, respectively. In
comparison with that of BTPE (340 nm), the much blue-shifted
λ
max,abs, as much as 29 nm, indicates the shorter conjugation
in these four luminogens, which is consistent with our idea of
the more twisted the conformation the AIE
molecule, the easier it is to crystallize. For
example, Ph3TPE is more easily crystallized
than the other two compounds, PhTPE and
Ph2TPE, owing to its twisted conformation
(Chart S2 in the Supporting Information).
Furthermore, in its LED device, Ph3TPE
exhibits much blue-shifted (ca. 27 nm)
maximum emission than that in the solu-
tions with the mixture solvents of THF and
water. Here, in comparison with BTPE, these
four derivatives are more twisted, thus, they
are liable to crystallize in the solid states. The
blue-shifted emission from 480 to 465 nm, is
just a hint for their further blue-shifted emis-
sion in LED devices. The EL spectrum will
Figure 2. A) UV spectra in THF solution. Concentration (μM): 12.7, 10.3, 11.6, and 11.2 for
mTPE-mTPE, mTPE-pTPE, oTPE-mTPE, and oTPE-pTPE, respectively. B) Plots of fluorescence
quantum yields determined in THF/H2O solutions using 9,10-diphenylanthracene (Φ = 90%
in cyclohexane) as standard versus water fractions; inset: photos of mTPE-mTPE, mTPE-pTPE,
oTPE-pTPE, and oTPE-mTPE in THF/water mixtures (fw = 0% and 99%) taken under the illumi-
nation of a 365 nm UV lamp.
be discussed later. The quantitative enhance-
ment of emission was evaluated by the PL
quantum yields (ΦF), using 9,10-diphenylan-
thracene as the standard. From pure solution
in THF to aggregate state in 99% aqueous
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Figure 4. Calculated molecular orbital amplitude plots of HOMO and
LUMO levels of mTPE-pTPE, mTPE-mTPE, oTPE-pTPE, and oTPE-mTPE.
similar, indicating that the calculated structures could somehow
be used to know some molecular information.
According to the single crystal data of oTPE-pTPE and oTPE-
mTPE, the Csp2=Csp2 double bonds of oTPE-pTPE are 1.342 and
1.355 Å, apparently shorter than those in oTPE-mTPE, which
are 1.351 and 1.360 Å. Analyzing the bond lengths of oTPE-
pTPE and oTPE-mTPE carefully, as shown in Figure S6 (Sup-
porting Information), in comparison with the bond lengths of
TPE, the length changes in oTPE-mTPE are smaller than those
of oTPE-pTPE. This discloses that the oTPE-pTPE is more con-
jugated than oTPE-mTPE, owing to the different conjugated
effect of para- and meta-linkage modes. This is reasonable:
with lesser or negligible electronic conjugated effect between
the two TPE blocks in oTPE-mTPE and oTPE-pTPE, each TPE
block should possess similar structure to the TPE molecule
itself; thus, the TPE blocks in the BTPE derivatives should have
similar bond lengths as in the TPE molecule. It is easily seen
that in oTPE-mTPE, the bond lengths are more approximate to
those of TPE, indicating that the two TPE blocks retain almost
the same electronic properties as TPE, no matter that they are
linked together through a single carbon–carbon bond. Thus, its
emission should be close to that of TPE. Actually, the maximum
emission wavelength in its LED device (435 nm) is in the deep-
blue region, blue-shifted even compared to TPE, as a result of
the more twisted conformation.
Although we have not obtained single crystals of mTPE-pTPE
and mTPE-mTPE, we may estimate that the conjugation length
of mTPE-pTPE is longer than mTPE-mTPE, for the same reason
just discussed. These results are well consistent with the meas-
ured optical data and realize our idea of tuning the conjuga-
tion length for bluer emission by simply changing the linking
positions.
Moreover, we carried out theoretical calculations of the four
luminogens in order to further understand the structure–
property relationship at the molecular level. Figure 4 shows the
orbital distributions of HOMO and LUMO energy levels of the
four BTPE derivatives. The electron clouds of the HOMOs and
LUMOs are almost dominated by the whole molecules, showing
the weak intramolecular charge transfer of these luminophores.
Thus, the nonpolar properties of these AIE molecules partially
ensure their deep-blue emissions in their LED devices.
Figure 3. Optimized molecular structures of: a) mTPE-pTPE, b) mTPE-
mTPE, c) oTPE-pTPE, and d) oTPE-mTPE. ORTEP drawings and bond
lengths of: e) oTPE-pTPE, and,f) oTPE-mTPE.
mixture, the ΦF values of mTPE-mTPE increased from 0.02% to
20.5%. Similar behavior was observed for the other three mol-
ecules (Figure 2B and Figure S2–S4 in the Supporting Informa-
tion). The quantum yields are up to 32.4%, 32.6%, and 23.4%,
respectively, for mTPE-pTPE, oTPE-pTPE, and oTPE-mTPE in
99% aqueous mixture, from being nearly nonluminescent in
the solution state. In other words, all the compounds are AIE-
active.
2.4. Theoretical Calculations and Crystal Structures
To better understand the correlation between structure and
physical properties, as well as the device performance, density
functional theory (DFT) calculations (B3LYP/6-31g∗) were car-
ried out to obtain orbital distributions of highest occupied mole-
cular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels and optimized molecular structures. As
shown in Figure 3, the optimized molecular structures of these
four BTPE derivatives are highly twisted. As mentioned above,
the highly twisted conformations make them change easily
from the amorphous to the crystalline state. This is another
reason for the much blue-shifted emission of the four BTPE
derivatives in their LED devices, in comparison with BTPE,
in addition to the controlled conjugated effect. Fortunately, we
could further examine the geometric structures of oTPE-pTPE
and oTPE-mTPE in their ORTEP drawings, since we obtained
their single crystals. As shown in Figure 3, both of these two
molecules adopt twisted conformations and no intermolecular
interactions exist in the packing arrangement, which is benefi-
cial to prevent the formation of species detrimental to emission.
It seems that their twisted extents are larger than those of TPE
and BPTPE (Figure S5 in the Supporting Information). Com-
paring the ORTEP drawings of oTPE-pTPE and oTPE-mTPE
with their calculated optimized structures (Figure 3), they seem
2.5. Electrochemical Properties
The electrochemical properties of the four BTPE derivatives
were investigated by cyclic voltammetry (CV). The HOMO
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energy levels were calculated to be about 5.5 eV, according to
the following equation: HOMO = –(4.8 + Eox) eV. Their LUMOs
could be obtained from optical bandgap energies (estimated
from the onset wavelength of their UV absorptions) and
HOMO values, which were about 2.2 eV. The slight difference
of their energy levels suggests that these four luminogens pos-
sess almost the same transporting properties.
Accompanying the increase of the voltage, the luminance
increases rapidly. Among them, mTPE-pTPE exhibits the best EL
performance, with a maximum luminance (Lmax) of 3266 cd m−2 ,
a maximum current efficiency (ηC, max) of 2.8 cd A−1 and a
maximum power efficiency (ηP, max) of 2.0 lm W−1 , which are
more than 1.5-fold higher than the values achieved by oTPE-
mTPE. Good EL efficiencies were obtained from devices of
oTPE-pTPE and mTPE-mTPE, with Lmax, ηC, max, and ηP, max
of 3166 cd m−2 , 2.2 cd A−1 , and 1.8 lm W−1 ; and, 1718 cd m−2 ,
2.1 cd A−1 , and 1.7 lm W−1 , respectively. The EL emission of
the original pTPE-pTPE (BTPE) reported in the literature, was
tuned from the sky-blue to deep-blue in our new BTPE deriva-
tives, realizing the control of light emission through a very
simple strategy with minor structural modification. In detail, all
the four luminophors exhibit deep-blue emission (435–459 nm)
when fabricated as emissive layers (Figure 6), compared to
BTPE with sky-blue emission peaked at 488 nm. Especially, the
EL spectrum of oTPE-mTPE is peaked at 435 nm with CIE chro-
maticity coordinates of (0.16, 0.14), which is even 10 nm blue-
shifted compared to TPE (445 nm). This should be ascribed to
its highly twisted geometric structure as mentioned above, in
addition to the controlled conjugated effect derived from the dif-
ferent linkage positions. The highly twisted geometric structure
makes oTPE-mTPE more easily crystallized when serving as
emitter; that is, crystallization will cause blue-shifted emission.
In addition, as mTPE-pTPE owns more planar conformation,
its EL spectrum is more red-shifted, with EL peak at 459 nm
and CIE chromaticity coordinates of (0.16, 0.16), than those
of oTPE-pTPE and mTPE-mTPE, with values of 454 nm, (0.16,
0.14); and, 452 nm, (0.15, 0.11), respectively. The satisfactory EL
efficiencies and deep-blue emission should be ascribed to the
different conjugation degree of ortho-, meta-, or para- linkage,
which could effectively tune the conjugated effects of two
simple TPE units, though the device configuration is yet to be
optimized. As mTPE-pTPE, which is more conjugated than the
other three emitters, possesses the best EL efficiencies while
still emitting in the deep-blue region, it further indicates that
the balance between high efficiencies and effective conjugation
length could be adjusted. On one hand, once the conjugation is
almost disrupted, such as oTPE-mTPE, it may result in poor EL
performance; on the other hand, if the conjugation is prolonged
too much, like BTPE, the EL spectrum will be red-shifted to a
large extent; our preliminary results confirmed that, by intel-
ligently modifying the subtle structure of AIE luminophors, a
balance can be partially achieved.
2.6. Electroluminescence
The good thermal property and outstanding AIE effect of
the four BTPE derivatives prompted us to investigate their
electroluminescence properties. Non-doped OLED devices
with a configuration of ITO/MoO3 (10 nm)/NPB (60 nm)/
emission layer (EML) (15 nm)/TPBi (35 nm)/LiF (1 nm)/Al
(100 nm) were fabricated. In these OLED devices, MoO3, NPB,
and TPBi worked as the hole-injection, hole-transporting, and
hole-blocking layers, respectively, and oTPE-mTPE, oTPE-pTPE,
mTPE-mTPE, or mTPE-pTPE served as emitters.
Figure
5 shows the current density–voltage–brightness
(J–V–L) characteristics, current efficiency versus current density
curves, and EL spectra of the OLEDs. As listed in Table 2, all
the devices turn on at a low voltage from 3.7 to 4.1 V, showing
small injection barriers from transporting layers in the devices.
3. Conclusions
In summary, four BTPE derivatives, namely oTPE-mTPE,
oTPE-pTPE, mTPE-mTPE, and mTPE-pTPE, were success-
fully obtained with the aim of generating deep-blue emitters
in OLED devices. By utilizing the different conjugated effect
of ortho-, meta-, and para- linkage of two TPE units, the pro-
longed π-conjugation length could be partially shortened,
realizing the control of light emission through a very simple
strategy with minor structural modification. When fabricated as
emissive layers in OLEDs, all the four luminogens exhibit deep
blue emissions from 435 to 459 nm with Lmax and ηC, max up to
Figure 5. a) Current density–voltage–luminance characteristics of
multilayer EL devices of mTPE-pTPE, mTPE-mTPE, oTPE-mTPE, and
oTPE-pTPE. b) Change in current efficiency with the current density in
multilayer EL devices. Device configurations: ITO/MoO3 (10 nm)/NPB
(60 nm)/EML (15 nm)/TPBi (35 nm)/LiF (1 nm)/Al.
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T a b le 2 . EL performances of mTPE-pTPE, mTPE-mTPE, oTPE-mTPE, and oTPE-pTPE.
a)
b)
c)
c)
c)
Von
Lmax
CIEd)
(x, y)
λEL
[nm]
ηP, max
ηC, max
ηext, max
[%]
[cd m−2
]
[Im W−1
1.7
]
[cd A−1
2.1
]
[V]
3.9
4.1
3.7
3.9
mTPE-mTPE
mTPE-pTPE
oTPE-mTPE
oTPE-pTPE
452
459
435
454
1718
1.9
1.9
1.4
1.7
(0.15, 0.11)
(0.16, 0.16)
(0.16, 0.14)
(0.16, 0.14)
3266
2.0
2.8
2883
1.4
1.8
3166
1.8
2.2
a)
b)
V
on
is the turn-on voltage at 1 cd m−2
;
L
the maximum luminance; c)ηC, max, ηC, ma, and ηext, max the maximum power, current, and external efficiencies, respectively;
max
d)CIE coordinates at 100 mA cm−2
.
American) equipped with a 337 nm nitrogen laser and a 1.2 m linear
flight path in positive ion mode, or a GCT premier CAB048 mass
spectrometer. UV-vis absorption spectra were recorded on a Shimadzu
UV-2500 recording spectrophotometer. Photoluminescence spectra
were recorded on a Hitachi F-4500 fluorescence spectrophotometer.
Differential scanning calorimetry (DSC) was performed on a Mettler
Toledo DSC 822e at a heating and cooling rate of 15 °C min−1 from room
temperature to 180 °C under argon. The glass transition temperature
(Tg) was determined from the second heating scan. Thermogravimetric
analysis (TGA) was undertaken with a NETZSCH STA 449C instrument.
The thermal stability of the samples under a nitrogen atmosphere was
determined by measuring their weight loss while heating at a rate of
10 °C min−1 from 25 to 600 °C. Cyclic voltammetry (CV) was carried out
on a CHI voltammetric analyzer in a three-electrode cell with a Pt counter
electrode, a Ag/AgCl reference electrode, and a glassy carbon working
electrode at a scan rate of 100 mV s−1 with 0.1 M tetrabutylammonium
perchlorate (purchased from Alfa Aesar) as the supporting electrolyte, in
anhydrous dichloromethane solution purged with nitrogen. The potential
values obtained in reference to the Ag/Ag+ electrode were converted to
values versus the saturated calomel electrode (SCE) by means of an
Figure 6. EL spectra of mTPE-pTPE (1), mTPE-mTPE (4), oTPE-mTPE (3),
oTPE-pTPE (2), and BTPE. Inset: CIE chromaticity coordinates of the four
luminogens.
internal ferrocenium/ferrocene (Fc+/Fc) standard.
Computational Details: The geometrical and electronic properties
were optimized at B3LYP/6-31g(d) level using Gaussian 09 program.
The molecular orbitals were obtain at the same level of theory.
OLED Device Fabrication and Measurement: The hole-transporting
material NPB (1,4-bis(1-naphthylphenylamino)-biphenyl) and electron-
transporting material 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene
(TPBI) were obtained from a commercial source. The EL devices were
fabricated by vacuum deposition of the materials at a base pressure of
10−6 Torr onto glass precoated with a layer of indium tin oxide (ITO) with
a sheet resistance of 25 Ω per square. Before deposition of an organic
layer, the clear ITO substrates were treated with oxgen plasma for
2 min. The deposition rate of organic compounds was 1–2 Å s−1 . Finally,
a cathode composed of lithium fluoride (1 nm) and aluminum (100 nm)
was sequentially deposited onto the substrate in the vacuum of
10−6 Torr. The L–V–J of the devices was measured with a Keithey
2400 Source meter and a Keithey 2000 Source multimeter equipped
with a calibrated silicon photodiode. The EL spectra were measured by
JY SPEX CCD3000 spectrometer. All measurements were carried out at
room temperature under ambient conditions.
3266 cd m−2 and 2.8 cd A−1 , and CIE chromaticity coordinates
of (0.16, 0.14), (0.15, 0.11), (0.16, 0.14), and (0.16, 0.16), respec-
tively. In comparison with BTPE (pTPE-pTPE), the structures of
our newly prepared BTPE derivatives, oTPE-mTPE, oTPE-pTPE,
mTPE-mTPE, and mTPE-pTPE, are similar. But, their EL per-
formance, especially the emission color, is different. Actually,
a minor or even apparently negligible structural modification
is an intelligent utilization of the basic conjugative effect; thus,
the four luminophors reported here are totally different from
the original pTPE-pTPE reported in the literature, although
their structures are similar. It is hoped that this design idea may
stimulate the flourishing development of AIE luminophors with
controllable emission, and many previously reported TPE-based
emitters should be re-investigated, just changing the linkage
positions of the aromatic blocks as described here.
Preparation of Compounds: All other chemicals and reagents were
obtained from commercial sources and used as received without further
purification. Solvents for chemical synthesis were purified according to
the standard procedures. Compound 1, pTPE-Br and Compound 4 were
synthesized according to the literature.[12]
Synthesis of Compound 2: 2-Aminobenzophenone (3.92 g, 20.0 mmol)
was dissolved in HBr (20 mL, 47% in H2O), cooled to 0 °C, and an
aqueous solution (5 mL) of NaNO2 (1.52 g, 22 mmol) was added
dropwise. After 30 min, the reaction mixture was allowed to warm to
room temperature, stirred for another hour. This suspension was then
added with vigorous stirring to CuBr (3.59 g, 25 mmol) dissolved in 15 mL
of HBr. The reaction mixture was heated to 60 °C for 1 h, afterwards
4. Experimental Section
Characterization: 1H and 13C NMR spectra were measured on a
MECUYRVX300 spectrometer. Mass spectra were measured on a ZAB
3F-HF mass spectrophotometer. Matrix-assisted laser desorption
ionization time-of-flight mass spectra were measured on a Voyager-
DESTR MALDI-TOF mass spectrometer (MALDI-TOF MS; ABI,
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poured on iced water (100 mL) and extracted with dichloromethane.
The combined organic fractions were washed with brine (100 mL), dried
over MgSO4, filtered and concentrated in vacuo. The crude product was
purified by chromatography using petroleum ether/dichloromethane
(3:1 v/v) as eluent to obtain a yellow oil in the yield of 65% (3.4 g). H
NMR (300 MHz, CDCl3) δ (ppm): 7.83−7.1 (m, 2H), 7.66-7.61 (m, 2H),
7.47-7.36 (m, 5H).
oTPE-pTPE: White powder. Yield: 48%. 1H NMR (300 MHz, CDCl3)
δ (ppm): 7.13-6.80 (m, 36H), 6.46-6.43 (m, 2H). 13C NMR (75 MHz,
CDCl3)
δ (ppm): 144.2, 143.4, 142.2, 141.1, 139.9, 138.9, 133.0,
131.6, 130.9, 130.7, 128.5, 128.0, 127.6, 127.2, 127.0, 126.7. MS (EI),
m/z: 662.40 ([M+], calcd. for C52H38: 662.86). HRMS: m/z for C52H38
[M+NH4+], calcd.: 680.3312; found: 680.3334.
1
oTPE-mTPE: White powder. Yield: 40%. 1H NMR (300 MHz, CDCl3)
δ (ppm): 7.08-6.88 (m, 33H), 6.81-6.78 (m, 2H), 6.64-6.61 (m, 3H). 13C
NMR (75 MHz, CDCl3) δ (ppm): 144.0, 141.2, 139.8, 137.5, 132.2, 131.6,
131.3, 130.3, 130.0, 129.1, 128.1, 127.9, 126.9, 126.7, 122.7. MS (EI),
m/z: 662.42 ([M+], calcd. for C52H38: 662.86). HRMS: m/z for C52H38
[M+NH4+], calcd.: 680.3312; found: 680.3319.
Synthesis of mTPE-Br: A 2.3 M solution of n-butyllithium in hexane
(4.09 mmol, 1.78 mL) was added to a solution of diphenylmethane
(0.86 g, 5.12 mmol) in anhydrous tetrahydrofuran (40 mL) at 0 °C
under an atmosphere of argon. After stirring for 1 h at this temperature,
3-bromobenzophenone (0.89 g, 3.41 mmol) was added. After 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 after drying with anhydrous sodium sulfate, and the resulting
crude product was dissolved in toluene (25 mL). The p-toluenesulfonic
acid (0.12 g, 0.68 mmol) was added, and the mixture was refluxed
overnight and cooled to room temperature. The mixture was evaporated
and the crude product was purified by silica gel column chromatography
using petroleum ether as eluent to obtain a white powder in the yield
of 50% (0.7 g). 1H NMR (300 MHz, CDCl3) δ (ppm): 7.20-7.11 (m,
11H), 7.02-7.01 (m, 5H), 6.96-6.95 (m, 2H). 13C NMR (75 MHz, CDCl3)
δ (ppm): 146.0, 143.4, 143.3, 143.1, 142.2, 139.5, 134.2, 131.3, 130.1,
129.6, 129.3, 128.0, 127.8, 126.9, 126.8, 121.9. MS (EI), m/z: 412.13
([M+], calcd. for C26H19Br: 411.33).
Crystallographic data (excluding structure factors) for the structure(s)
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos. CCDC-
894519 and 894520.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
Synthesis of oTPE-Br: Prepared according to a similar procedure
for mTPE-Br from Compound 2. White powder. Yield: 30%. 1H
NMR (300 MHz, CDCl3) δ (ppm): 7.17-7.03 (m, 19H). MS (EI), m/z:
412.10 ([M+], calcd. for C26H19Br: 411.33).
Z.L. is grateful to the National Science Foundation of China (no.
21161160556), the National Fundamental Key Research Program
(2011CB932702, 2013CB834700), and the Open Project of State Key
Laboratory of Supramolecular Structure and Materials (sklssm201219)
for financial support. D.M. is grateful to the Science Fund for Creative
Research Groups of NSFC (20921061), and the National Natural Science
Foundation of China (50973104, 60906020) for financial support.
Synthesis of Compound 3: A 2.3 M solution of n-butyllithium in
hexane (15.0 mmol, 6.5 mL) was added to a solution of mTPE-Br
(4.12 g, 10.0 mmol) in anhydrous tetrahydrofuran (60 mL) at –78 °C
under an atmosphere of argon. After stirring for 4 h, 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6.1 mL) was added. After 2 h,
the mixture was slowly warmed to room temperature. After stirring
overnight, the reaction was terminated by the added brine. The mixture
was extracted with dichloromethane, and the combined organic layer
was dried over anhydrous sodium sulfate. After filtration and solvent
evaporation, the crude product was purified by silica gel column
chromatography using petroleum ether/dichloromethane (5:1 v/v) as
eluent. White powder of 3 was obtained in the yield of 60% (2.30 g).
Received: September 12, 2012
Published online: December 4, 2012
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1H NMR (300 MHz, CDCl3)
δ (ppm): 7.53-7.48 (m, 2H),
7.09-7.02 (m, 17H), 1.28 (s, 12H). 13C NMR (75 MHz, CDCl3) δ (ppm):
143.6, 143.3, 142.9, 141.1, 140.8, 137.5, 134.2, 132.7, 131.3, 131.2,
127.5, 127.0, 126.3, 126.2, 83.6, 24.7. MS (EI), m/z: 458.35 ([M+], calcd
for C32H31BO2, 458.40).
General Procedure for the Synthesis of Compounds mTPE-pTPE, mTPE-
mTPE, oTPE-mTPE and oTPE-pTPE: mTPE-mTPE: A mixture of mTPE-Br
(412 mg, 1 mmol), compound 3 (459 mg, 1 mmol), Pd(PPh3)4 (0.10 g,
4% mmol), and potassium hydroxide (236 mg, 5 mmol) in 15 mL of
THF and 3 mL of distilled water in a 50 ml Schlenk tube was refluxed
for 2 d under argon. The mixture was extracted with dichloromethane.
The combined organic extracts were dried over anhydrous Na2SO4 and
concentrated by rotary evaporation. The crude product was purified by
column chromatography on silica gel using dichloromethane/petroleum
ether (1:5 v/v) as eluent to afford the product as a white powder in the
yield of 61.2% (320 mg). 1H NMR (300 MHz, CDCl3) δ (ppm): 7.09-7.04
(m, 32H), 6.97-6.92 (m, 6H). 13C NMR (75 MHz, CDCl3) δ (ppm): 144.0,
143.9, 143.7, 141.5, 141.2, 140.5, 131.6, 130.6, 130.2, 128.1, 127.9, 126.7,
125.4. MS (EI), m/z: 662.70 ([M+], calcd. for C52H38: 662.86). HRMS: m/z
for C52H38 [M+NH4+], calcd.: 680.3312; found: 680.3307.
mTPE-pTPE: White powder. Yield: 65%. 1H NMR (300 MHz, CDCl3) δ
(ppm): 7.09-6.98 (m, 38H). 13C NMR (75 MHz, CDCl3) δ (ppm): 144.2,
144.1, 143.9, 143.8, 142.8, 141.5, 141.3, 141.1, 140.8, 140.1, 139.1, 131.8,
131.6, 130.5, 130.4, 128.3, 128.0, 127.9, 126.7, 126.4, 125.3. MS (EI),
m/z: 662.79 ([M+], calcd. for C52H38: 662.86). HRMS: m/z for C52H38
[M+NH4+], calcd.: 680.3312; found: 680.3322.
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2336 wileyonlinelibrary.com
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Adv. Funct. Mater. 2013, 23, 2329–2337
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