Stereoselective synthesis, efficient light emission, and high bipolar charge mobility of chiasmatic luminogens

Article abstract of DOI:10.1002/adma.201102804

Stereoregular tetraphenylethene derivatives (Z)-o-BCaPTPE and (Z)-o-BTPATPE featured with chiasmatic conformations and aggregation-enhanced emission characteristics are synthesized using a McMurry reaction. Both luminogens exhibit high hole and electron m

Full text of DOI:10.1002/adma.201102804

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Stereoselective Synthesis, Efficient Light Emission, and
High Bipolar Charge Mobility of Chiasmatic Luminogens
Zujin Zhao, Jacky W. Y. Lam, Carrie Y. K. Chan, Shuming Chen, Jianzhao Liu,
Ping Lu, Mario Rodriguez, José-Luis Maldonado, Gabriel Ramos-Ortiz, Herman H. Y. Sung,
Ian D. Williams, Huimin Su, Kam Sing Wong, Yuguang Ma, Hoi Sing Kwok, Huayu Qiu,
a n d Ben Zhong Tang*
Synthesis of fluorophores with novel molecular structures have
attracted much attention in the past two decades because such
materials may exhibit unique properties that enable them to
find an array of applications in optoelectronics and biological
science. In previous studies, we found that a series of propeller-
like luminogens that were non-fluorescent in solution became
strong emitters when aggregated in poor solvents or fabricated
as thin films, thus demonstrating a novel phenomenon of
aggregation-induced emission (AIE).[1] Systematic studies have
rationalized that restriction of intramolecular rotation (IMR)
is the main cause for the AIE effect, which blocks the nonra-
diative relaxation channels and populates the radiative decay.[2]
Among the AIE luminogens, tetraphenylethene (TPE) is a pro-
totype molecule. Studies have revealed that the IMR process
of TPE can be inhibited by many external factors, such as sol-
vent viscosity, temperature, pressure, etc.[3] However, there are
few reports on the internal control on the IMR process at the
molecular level.[2a,4] We envision that imparting a highly rigid
molecular structure to TPE may hinder its IMR process and
thus change the emission behavior of the luminogen accord-
ingly. Herein, we present the synthesis of two TPE derivatives
featured with cross-shaped (chiasmatic) molecular conforma-
tions and report their photoluminescence (PL), electrolumines-
cence (EL), and charge transport properties.
The chiasmatic luminogens (Z)-1,2-bis[4′-(9-carbazolyl)
biphenyl-2-yl]-1,2-diphenylethene [(Z)-o- BCaPTPE] and (Z)-
1,2-bis[4′-(diphenylamino)biphenyl-2-yl]-1,2-diphenylethene
[(Z)-o-BTPATPE]] were synthesized by McMurry reaction[5] of
2-arylbenzophenones, as illustrated in Scheme 1. For com-
parison, we also prepared their linear counterparts 1,2-bis[4′-
(9-carbazolyl)biphenyl-4-yl]-1,2-diphenylethene
(p-BCaPTPE
and 1,2-bis[4′-(diphenylamino)biphenyl-4-yl]-1,2-diphenylethene
(p-BTPATPE)) under similar experimental conditions. Detailed
synthetic procedures, characterization data, and 1H NMR
spectra are given in the Supporting Information and Figure
S1−S3 (Supporting Information). It is well known that McMurry
reaction of unsymmetrical diarylketones generally gives ster-
eorandom products with Z/E ratios of ≈50/50%.[6] In some
occasions, the Z and E-isomers can be separated by column
chromatography or recrystallization but the efficiency is nor-
mally quite low. On the contrary, we obtained o-BCaPTPE and
o-BTPATPE with solely Z stereostructure from their corre-
sponding benzophenones in moderate yields (52% and 59%
for (Z)-o-BCaPTPE and (Z)-o-BTPATPE, respectively). Their
structures were confirmed by single-crystal X-ray diffraction.
Their ORTEP (Oak Ridge thermal ellipsoid plot) drawings are
shown in Figure 1, and the crystal data are provided in Table S1
(Supporting Information). Both (Z)-o-BCaPTPE and (Z)-
o-BTPATPE adopt a chiasmatic conformation, where intramo-
lecular π–π stackings with distances of 3.274 and 3.417 Å are
formed between phenyl rings. However, when 4-arylbenzophe-
nones were used as starting materials, only mixtures of Z- and
E-isomers were obtained. Although the detailed mechanism for
the formation of sole Z-products from 2-arylbenzophenones is
still under investigation, it is believed that the intramolecular
π–π interaction should account for the stereoselectivity of the
reactions. Such interaction helps rigidify and stabilize the inter-
mediate complex of titanium species, whose subsequent deoxy-
genation furnishes the Z-products (Scheme 2).[7]
Dr. Z. Zhao, Prof. H. Qiu
College of Material
Chemistry and Chemical Engineering
Hangzhou Normal University
Hangzhou 310036, China
Dr. J. W. Y. Lam, C. Y. K. Chan, Dr. J. Liu, Dr. H. H. Y. Sung,
Prof. I. D. Williams, Prof. B. Z. Tang
Department of Chemistry
The Hong Kong University of Science & Technology (HKUST)
Clear Water Bay, Kowloon, Hong Kong, China
E-mail: tangbenz@ust.hk
S. Chen, Prof. H. S. Kwok
Center for Display Research
HKUST, Clear Water Bay, Kowloon, Hong Kong, China
Dr. P. Lu, Prof. Y. Ma
State Key Laboratory of Supramolecular Structure and Materials
Jilin University
Changchun 130012, China
Dr. M. Rodriguez, Dr. J.-L. Maldonado, Prof. G. Ramos-Ortiz
Centro de Investigaciones en Óptica
A. P. 1-948, 37000 León, Gto., México
Dr. H. Su, Prof. K. S. Wong
Department of Physics
HKUST, Clear Water Bay, Kowloon, Hong Kong, China
The rigid chiasmatic conformations of (Z)-o-BCaPTPE and
(Z)-o-BTPATPE may strongly suppress the IMR process, which
may endow the luminogens with high PL efficiencies in both
DOI: 10.1002/adma.201102804
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Scheme 2. Proposed mechanism for the formation of (Z)-o-BCaPTPE and
(Z)-o-BTPATPE.
490 nm, respectively, while their crystals emit at shorter wave-
lengths of ≈456 nm (Figure S4, Supporting Information). A sim-
ilar phenomenon was also observed in other TPE derivatives.[3,8,9]
In the crystal state, the molecules of (Z)-o-BCaPTPE and (Z)-
o-BTPATPE may have conformationally adjusted themselves
by twisting their phenyl rings in order to fit into the crystalline
lattices, thus leading to hypsochromic shift in their PL spectra.
The absolute ΦF’s of (Z)-o-BCaPTPE and (Z)-o-BTPATPE films
measured using an integrating sphere are as high as 100 and
85%, respectively. Such values are more than two-fold higher
Scheme 1. Synthesis of TPE derivatives with chiasmatic and linear struc-
tures by McMurry reaction.
solution and aggregate states. This is indeed the case. (Z)-o-
BCaPTPE and (Z)-o-BTPATPE show intense emissions at 497
and 510 nm with fluorescence quantum yields (ΦF’s) of 43
and 38%, respectively, in dilute tetrahydrofuran (THF) solu-
tions (10 μM) (Table 1). The PL emissions of solid thin films
of (Z)-o-BCaPTPE and (Z)-o-BTPATPE are observed at 477 and
Figure 1. ORTEP drawings of (Z)-o-BCaPTPE (CCDC 825621) and (Z)-o-BTPATPE (CCDC 825622) and molecular orbital amplitude plots of their highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels calculated at the B3LYP/6-31G(d) level.
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T a b le 1 . Optical and thermal properties, fluorescence lifetimes, and energy levels of the luminogens. Abbreviations: Soln = solution (10 μM in THF),
Cryst = crystals, λem = PL maximum, αAIE = extent of emission enhancement (ΦFilm/ΦSoln), τ = fl uorescence decay time measured in THF solution;
excitation wavelength: 350 nm. ΦF = absolute fluorescence quantum yield measured using an integrating sphere, Tg = glass transition temperature,
Td = onset decomposition temperature, HOMO and LUMO are estimated from theoretical calculations, Eopt = energy bandgap determined from the
onset of absorption spectra.
Tg/Td
[°C]
HOMO/LUMO
[eV]
Eopt
[eV]
3.33
3.20
3.07
2.90
λ
em [nm]
Cryst
456
ΦF [%]
Film
100
τ
Soln
497
510
498
512
Film
477
490
487
509
Soln
43
[ns]
4.7
αAIE
2.3
2.2
200
55
(Z)-o-BCaPTPE
(Z)-o-BTPATPE
p-BCaPTPE
222/427
127/421
221/491
126/455
−5.25/−1.44
−4.85/−1.18
457
38
85
2.7
0.5
1.8
100
0.061
0.062
p-BTPATPEa)
100
a)From ref. [15].
than those in solutions (the AIE effect αAIE [1d,8a], a ratio of ΦF
in film to that in solution, is 2.3 for (Z)-o- BCaPTPE, and is 2.2
for (Z)-o- BTPATPE), demonstrating a feature of aggregation-
enhanced emission (AEE). However, their linear counterparts
show distinct emission behaviors. p-BCaPTPE and p-BTPATPE
exhibit negligible emissions in solutions with ΦF’s of merely
0.5 and 1.8%, respectively. Their solid films, however, emit
intensely with unity efficiencies, leading to much higher αAIE
values (Table 1).
a)
b)
fw (vol %)
fw (vol %)
99.5
80
70
0
99.5
80
70
0
Fluorescence lifetime is an important kinetic parameter for
the excited-state decay process and is closely associated with the
fluorescence efficiency. Generally, the shorter the lifetime is,
the lower the ΦF value is.[10] To gain insight into the emission
properties of the luminogens, we studied their dynamic behav-
iors in THF solutions (10 μM) using time-resolved fluorescence
spectroscopy. Whereas the singlet excited states of p-BCaPTPE
and p-BTPATPE decay rapidly with lifetimes as short as ≈60 ps
at room temperature, (Z)-o-BCaPTPE and (Z)-o-BTPATPE show
much higher values of 4.7 and 2.7 ns, respectively (Figure S5,
Supporting Information). Clearly, the active IMR process of the
TPE unit in p-BCaPTPE and p-BTPATPE has effectively con-
sumed the excited state energy, thus rendering the luminogens
weakly fluorescent in solutions. Thanks to the rigid chiasmatic
conformation, the IMR process is partially hampered in (Z)-
o-BCaPTPE and (Z)-o-BTPATPE. As a result, much stronger
emissions are observed in their solutions.[2a,10b]
To further confirm the AEE feature of (Z)-o-BCaPTPE and
(Z)-o-BTPATPE, we added water, a nonsolvent for both mole-
cules, into their THF solutions and measured the PL change.
As shown in Figure 2a, the emission of (Z)-o-BCaPTPE is
increased gradually when the water fraction (fw) in the solvent
mixture becomes higher. Due to its immiscibility with the
hydrophilic medium, the (Z)-o-BCaPTPE molecules must have
been aggregated in aquesous mixtures with high water fractions.
This restricts further the IMR process and hence enhances the
light emission. Evidently, (Z)-o-BCaPTPE is a luminogen that
exhibits AEE characteristic. It is noteworthy that the emission of
(Z)-o-BCaPTPE is blue-shifted in aqueous mixtures with high fw’s
due to the morphological change in the aggregates from amor-
phous to crystalline, as revealed by the transmission electron
microscopy (TEM) images and electron diffraction (ED) patterns
at fw’s of 70 and 99.5% (Figure S6, Supporting Information).
400
450
500
550
600
650 400
450
500
550
600
650
Wavelength / nm
Wavelength / nm
Figure 2. PL spectra of a) (Z)-o-BCaPTPE and b) p-BCaPTPE in THF/
water mixtures with different fw. Excitation wavelength: 350 nm.
(Z)-o-BTPATPE also shows similar emission behavior (Figure
S7a, Supporting Information). On the other hand, the
PL spectra of p-BCaPTPE and p-BTPATPE in THF are almost
flat lines parallel to the abscissa. Intense signals, however, are
recorded when they are aggregated in THF/water mixtures
(Figure 2b and Figure S7b, Supporting Information).
In an effort to understand the mechanism for this AEE
system, we checked the geometric structures and the packing
arrangements of (Z)-o-BCaPTPE in the crystal state. Besides
intramolecular π–π interactions as discussed above, multiple
C−H···π hydrogen bonds with distances of 2.986 and 2.989 Å
are formed between the phenyl proton of one molecule and
the π cloud of the carbazole unit of adjacent molecule. Figure 3
illustrates parts of C−H···π hydrogen bonds formed in (Z)-
o-BCaPTPE crystals as an example. These multiple C–H···π
hydrogen bonds plus the π–π interactions help rigidify the
molecular conformation.[8c,d] The rotation of the phenyl rings
attached to the central vinyl stator is locked by such bonding in
the crystal state. As a result, the excited state energy consumed
by the IMR process is greatly reduced, making the luminogens
stronger emitters in the solid state.
We carried out theoretical calculations of (Z)-o-BCaPTPE
and (Z)-o-BTPATPE in order to understand their photophysical
properties at the molecular level. The density functional theory
with B3LYP hybrid functional at the basis set level of 6-31G(d)
was used for the calculation. As shown in Figure 1, the HOMOs
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0.09
0.06
0.03
a)
0.1
0.01
1E-3
t+ = 3.4 µs,
= 4.9 x 10−4 cm2/ Vs
µ
+
2
4
6
8 10
Electric field (V/cm)
0
Time / µs
4 x105
6 x105
0.00
0.04
0
10
20
0.1
30
Time / µs
40
50
60
b)
Figure 3. C −H···π hydrogen bonds formed between adjacent molecules
of (Z)-o-BCaPTPE in the crystal state. For clarity, only parts of C−H···π
hydrogen bonds are indicated as examples.
0.03
0.02
0.01
0.00
0.01
1E-3
1E-4
t− = 3.8 µs
− = 4.3 x 10−4 cm2/ Vs
of both luminogens are dominated by the orbitals from the
carbazole or triphenylamine units. In contrast, the electron
clouds of the LUMOs are mainly located on the TPE unit. Thus,
increasing the stiffness of the TPE conformation will hamper
the nonradiative decay of the excitons as they are more likely
to be confined in the central core during the PL process.[11] (Z)-
o-BTPATPE has higher HOMO and LUMO energy levels than
(Z)-o-BCaPTPE (Table 1). Both of them possess shorter effec-
tive conjugation lengths than their linear counterparts,[12] as
revealed by their blue-shifted absorption spectra (Figure S8,
Supporting Information) and wider energy bandgaps.
µ
2
4
6
8 10
Time / µs
Electric field (V/cm)
6 x105
0
10
20
30
Time / µs
40
50
60
Both (Z)-o-BCaPTPE and (Z)-o-BTPATPE are thermally stable.
As shown in Figure S9 (Supporting Information), they start to
decompose at temperatures (Td) as high as 427 and 421 °C. The
glass transition temperatures (Tg) are detected at 222 and 127 °C
for (Z)-o-BCaPTPE and (Z)-o-BTPATPE, respectively, revealing
that they also enjoy high morphological stability. The data are
close to those of their linear counterparts, suggesting that all
the luminogens share similar thermal properties, irrespective of
their conformations (Table 1). The Td and Tg of (Z)-o-BCaPTPE
are higher than those of (Z)-o-BTPATPE, probably due to the
higher rigidity of the carbazole unit.
In contrast to many organic materials, where the charge
transport is dominated by either holes or electrons, the chias-
matic conformation of (Z)-o-BCaPTPE and (Z)-o-BTPATPE has
endowed them with interesting bipolar charge transport capa-
bility. We used time-of-flight (TOF) transient photocurrent tech-
nique to investigate their charge mobility. (Z)-o-BCaPTPE- or
(Z)-o-BTPATPE-doped poly(styrene) (PS) films (10 μm) with or
without C60 were prepared and sandwiched between two indium
tin oxide (ITO)-coated glass slides for the measurement. The
efficient PL emissions of (Z)-o-BCaPTPE and (Z)-o-BTPATPE in
the solid state suggest that the probability for exciton generation
is very high in their photophysical process. Thus, the introduc-
tion of C60 to the PS film is to facilitate the exciton dissociation
and hence increases the efficiency for charge carrier genera-
tion owing to its strong electron-accepting capability.[13] The
Figure 4. Transient photocurrents in a) hole transport and b) electron
transport configurations for film (10 μm) of (Z)-o-BCaPTPE:PS:C60
(50:48.5:1.5 wt%) composite. Inset: log–log plot of the photocurrent as
a function of time.
(Z)-o-BCaPTPE:PS:C60 composite exhibits a higher photocur-
rent, which gives a better plot from which the charge mobility
can be measured. The photogeneration and charge transport
are confirmed by the follows: i) Dark current is found to be neg-
ligible under the bias configuration. In this case, under photo-
excitation but with zero external electric field, no photocurrent
is detected. ii) When an external electric field is applied to the
sample in the absence of photoexcitation, no charge transport
occurrs. In contrast, photocurrents are readily detected when
a biased sample is photoexcited. Figure 4 shows the transient
photocurrents of (Z)-o-BCaPTPE:PS:C60 (50:48.5:1.5 wt%)
film at applied fields of 40 and 60 V μm−1 as an example.. The
detected photocurrent is dispersive. Since the charge transpor-
tation in films fabricated from organic materials occurs via a
hopping process, the dispersive behavior can be explained by
the presence of traps such as oxygen and water that may arise
during the film preparation.[14]
As shown in Figure 4a, the transient photocurrents for
holes increase as the bias voltage becomes higher. The tran-
sient time determined from the logarithmic plot is 3.4 μs at an
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applied electric field of 60 V μm−1 , from which a hole mobility
of 4.9 × 10−4 cm2 V−1 s−1 is determined. This value is approxi-
mately five-fold higher than that of N,N′-diphenyl-N,N′-bis(3-
methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) under the
same measurement conditions.[13a]
T a b le 2 . EL performances of (Z)-o-BCaPTPE and (Z)-o-BTPATPE. Device
configuration: a) ITO/(Z)-o-BCaPTPE or (Z)-o-BTPATPE (80 nm)/TPBi
(10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm), b) ITO/NPB (60 nm)/
(Z)-o-BCaPTPE or (Z)-o-BTPATPE (60 nm)/LiF (1 nm)/Al (100 nm), and
c) ITO/NPB (60 nm)/(Z)-o-BCaPTPE or (Z)-o-BTPATPE (20 nm)/TPBi
(10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm). Abbreviations: λEL = EL
maximum, Von = turn-on voltage at 1 cd m−2, Lmax = maximum lumi-
nance, ηC,max = maximum current efficiency, and ηext,max = maximum
external quantum efficiency.
Interestingly, photocurrent for electrons was also detected
when the polarity of the applied bias was switched (Figure 4b).
The transient time determined from the logarithmic plot is
3.8 μs at 60 V μm−1 , corresponding to an electron mobility of 4.3 ×
10−4 cm2 V−1 s−1 for (Z)-o-BCaPTPE. This value is close to that
of hole mobility and is more than one-order higher than that
of tris(8-hydroxyquinolinolato)aluminium (Alq3), a widely used
electron-transporting material.[14b] (Z)-o-BTPATPE also shows
similar bipolar charge transport property (Figure S10, Supporting
Information) and its hole and electron mobilities are 3.7 ×
10−4 and 3.3 × 10−4 cm2 V−1 s−1 , respectively. Since p-BTPATPE
shows a hole mobility of 5.2 × 10−4 cm2 V−1 s−1 [15] but exhibits
no electron transport property, this suggests that the chiasmatic
conformation of (Z)-o-BCaPTPE and (Z)-o-BTPATPE plays a
key role for the high electron mobility of the molecules. (Z)-
o-BCaPTPE exhibits higher hole and electron mobilities than
(Z)-o-BTPATPE, presumably due to its more rigid conformation
and stronger intramolecular π−π interaction.
Von
[V]
Lmax
[cd m−2
6250
8010
5890
7070
9060
6420
λEL
[nm]
489
492
488
488
492
492
ηC,max
[cd A−1
4.8a)
ηext,max
[%]
2.1
active layer
]
]
a) (Z)-o-BCaPTPE
b) (Z)-o-BCaPTPE
c) (Z)-o-BCaPTPE
a) (Z)-o-BTPATPE
b) (Z)-o-BTPATPE
c) (Z)-o-BTPATPE
6.8
4.5
6.8
3.4
8.0
4.3
7.9b)
3.1
5.8c)
2.6
5.1d)
3.3e)
2.4
1.3
4.7f)
2.0
a)At 30 cd m−2
; ; ; ; ;
b)At 100 cd m−2 c)At 60 cd m−2 d)At 50 cd m−2 e)At 80 cd m−2 f)At
28 cd m−2
.
The efficient solid-state PL efficiency and bipolar charge
mobility of (Z)-o-BCaPTPE and (Z)-o-BTPATPE inspire us to
explore their multifunctionality in optoelectronic devices. We
fabricated three kinds of organic light-emitting diodes (OLEDs)
with configurations of a) ITO/(Z)-o-BCaPTPE or (Z)-o-BTPATPE
(80 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm),
in which (Z)-o-BCaPTPE and (Z)-o-BTPATPE serve as both
hole-transporting and light-emitting layers and 1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (TPBi) and Alq3 function as
hole-blocking and electron-transporting layers, respectively;
b) ITO/NPB (60 nm)/(Z)-o-BCaPTPE or (Z)-o-BTPATPE (60 nm)/
LiF (1 nm)/Al (100 nm), in which (Z)-o-BCaPTPE and (Z)-o-
BTPATPE serve as both light-emitting and electron-transporting
layers and N,N′-di(1-naphthyl)-N,N′-diphenyl-benzidine (NPB)
acts as hole-transporting layer; and c) ITO/NPB (60 nm)/(Z)-
o-BCaPTPE or (Z)-o-BTPATPE (20 nm)/TPBi (10 nm)/Alq3
(30 nm)/LiF (1 nm)/Al (100 nm), in which (Z)-o-BCaPTPE and
(Z)-o-BTPATPE serve as light-emitting layers only. The per-
formance data of the devices are summarized in Table 2 and
the characteristic curves are given in Figure S11,S12 (Sup-
porting Information). Both luminogens show sky blue EL at
488–492 nm. The EL spectra are similar to the PL spectra of
their films and change little with the applied voltage, indicating
that the EL originates from the same radiative decay of the sin-
glet excitons. The maximum current efficiencies attained by the
devices are observed at low current densities and decay quicky
with increasing current density. This is in some sense under-
standable because the device structures are not yet optimized.
Exciton leakage to NPB or TPBi layers and quenching under
high electric filed, accumulation of holes, etc., may cause the
efficiency drops quickly at high current density.
(ηext,max) of 2.1%. Much better performances are observed in
device (b) fabricated in the absence of TPBi/Alq3 layers. The
ηC,max and ηext,max reach 7.9 cd A−1 and 3.1%, respectively,
which are even higher than those of device (c) that contains
both NPB and TPBi/Alq3 layers. These results prove again
that (Z)-o-BCaPTPE possesses outstanding hole- and electron-
transporting capabilities in addition to excellent light-emitting
properties. (Z)-o-BTPATPE can also serve as a multifunctional
material in OLEDs but its EL performances are inferior to those
of (Z)-o-BCaPTPE due to its comparatively lower solid-state PL
efficiency and bipolar charge mobility. Unlike (Z)-o-BCaPTPE,
the device (a) of (Z)-o-BTPATPE shows a lower turn-on voltage
and higher efficiencies than those of devices (b) and (c) (Table 2).
This suggests that (Z)-o-BTPATPE is in favor of hole injection,
while (Z)-o-BCaPTPE prefers electron injection due to its rela-
tively lower HOMO energy level.
For comparison, control OLEDs with configurations of
a) ITO/p-BCaPTPE or p-BTPATPE (80 nm)/TPBi (10 nm)/Alq3
(30 nm)/LiF (1 nm)/Al (100 nm) and b) ITO/NPB (60 nm)/p-
BCaPTPE or p-BTPATPE (60 nm)/LiF (1 nm)/Al (100 nm) were
fabricated, in which the linear luminogens take the same roles
as those of chiasmatic luminogens mentioned above. Their
performance curves are given in Figure S13 (Supporting Infor-
mation). p-BCaPTPE and p-BTPATPE function well as both
hole-transporting and light-emitting layers in device (a). How-
ever, the performances of device (b) based on both luminogens
are very poor, revealing that p-BCaPTPE and p-BTPATPE are
not promising electron-transporting materials. These results
further evidence that the high electron mobilities of (Z)-o-
BCaPTPE and (Z)-o-BTPATPE stem from their novel chiasmatic
conformations.
Because of the high bipolar charge mobility, the EL devices
of the chiasmatic luminogens without NPB or TPBi/Alq3
layers perform well and show good EL efficiency. Device (a) of
(Z)-o-BCaPTPE without NPB shows a maximum current effi-
ciency (ηC,max) of 4.8 cd A−1 and an external quantum efficiency
In summary, stereoregular TPE derivatives (Z)-o-BCaPTPE
and (Z)-o-BTPATPE were first synthesized by McMurry reaction
in moderate yields. Although most TPE derivatives with linear
structures such as p-BCaPTPE and p-BTPATPE are weakly
fluorescent in solutions, (Z)-o-BCaPTPE and (Z)-o-BTPATPE,
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Commun. 2009, 4332; d) Y. Hong, J. W. Lam, B. Z. Tang, Chem. Soc.
Rev. 2011, DOI: 10.1039/c1cs15113d.
with their chiasmatic conformations, can emit intensely in
the solution state due to the intramolecular π–π interactions,
which restricts the IMR process and hence promotes radiative
decay of the excitons. (Z)-o-BCaPTPE and (Z)-o-BTPATPE emit
as efficient as their linear counterparts in the solid state and
enjoy similarly high morphologically and thermally stabilities.
Their chiasmatic conformation coupled with intramolecular
π−π stacking endows them with fascinating material proper-
ties. p-BCaPTPE and p-BTPATPE with linear structures exhibit
hole-transporting property only, but (Z)-o-BCaPTPE and (Z)-o-
BTPATPE show bipolar charge mobility, although they are con-
structed from hole-transporting building blocks. High hole and
electron mobilities up to 4.9 × 10−4 and 4.3 × 10−4 cm2 V−1 s−1 ,
respectively, have been detected in (Z)-o-BCaPTPE. The chias-
matic luminogens can serve as multifunctional materials in
OLEDs. The device fabricated using (Z)-o-BCaPTPE as both the
light-emitting and electron-transporting layers exhibits high
efficiencies up to 7.9 cd A−1 and 3.1%, although the device
structure is not yet optimized. Such attributes make the chias-
matic luminogens a new type of promising materials for opto-
electronic devices.
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Acknowledgements
The authors thank Dr. Chunmei Deng and Prof. Zhigang Shuai for
theoretical calculation. This project was partially supported by the
Research Project Competition of HKUST (RPC11SC09), Research
Grants Council of Hong Kong (604711, 603509, and HKUST2/CRF/10),
the University Grants Committee of Hong Kong (AoE/P-03/08). Z.J.Z.
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Zhejiang Province (Y4110331). J.-L.M. and G.R.-O. acknowledge grants
from CONACYT (55250 and J-49512).
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Received: July 21, 2011
Revised: August 27, 2011
Published online: October 21, 2011
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