Benzene-cored fluorophors with TPE peripheries: Facile synthesis, crystallization-induced blue-shifted emission, and efficient blue luminogens for non-doped OLEDs

Article abstract of DOI:10.1039/c2jm31855e

In this paper, three benzene-cored fluorophors with TPE moiety peripheries (PhTPE, Ph2TPE and Ph3TPE), are synthesized and their thermal, optical and electronic properties are investigated. All of them are nearly non-emissive when readily dissolved in solution but become highly emissive in the aggregate state, making them promising candidates for optoelectronic materials. Non-doped OLEDs with these luminogens as emissive layers exhibit sky-blue to deep blue emissions from 488 to 457 nm with L_max, η_{C, max} and η_{P, max} up to 3966 cd m^-2, 5.0 cd A^-1 and 3.87 lm W^-1, respectively, owing to the twisted conformation between the benzene core and TPE units.

Full text of DOI:10.1039/c2jm31855e

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PAPER
Benzene-cored fluorophors with TPE peripheries: facile synthesis,
crystallization-induced blue-shifted emission, and efficient blue luminogens for
non-doped OLEDs†
a
b
a
a
a
b
Jing Huang, Ning Sun, Jie Yang, Runli Tang, Qianqian Li, Dongge Ma, Jingui Qin and Zhen Li
a
a
*
*
Received 25th March 2012, Accepted 20th April 2012
DOI: 10.1039/c2jm31855e
In this paper, three benzene-cored fluorophors with TPE moiety peripheries (PhTPE, Ph2TPE and
Ph3TPE), are synthesized and their thermal, optical and electronic properties are investigated. All of
them are nearly non-emissive when readily dissolved in solution but become highly emissive in the
aggregate state, making them promising candidates for optoelectronic materials. Non-doped OLEDs
with these luminogens as emissive layers exhibit sky-blue to deep blue emissions from 488 to 457 nm
with Lmax, hC, max and hP, max up to 3966 cd mꢀ2, 5.0 cd Aꢀ1 and 3.87 lm Wꢀ1, respectively, owing to the
twisted conformation between the benzene core and TPE units.
for the AIE effect.7 Their unique luminescent properties make
AIE luminophors promising candidates for OLEDs. Specifically,
lots of research work on tetraphenylethene (TPE)-based lumi-
nogens has been reported with excellent electroluminescent (EL)
performance.8 For example, pyrene or triphenylamine-cored
fluorophors with TPE peripheries (Chart S1, ESI†) could achieve
current efficiencies up to 12.3 and 8 cd Aꢀ1, respectively.
Unfortunately the emission of the devices was not deep blue,
though the emission of TPE itself is.9 And so far, high perfor-
mance AIE-active blue-emitting materials are still very scarce,
because linking TPE with other large aromatic rings will
certainly extend the p-conjugation length and as a result, the
maximum EL emission wavelengths would be much red-shifted,
beyond the blue emission region. Thus, it is still a challenge to
develop new TPE-based luminophors with both blue-emission
and good EL performance.
Introduction
Intrigued by the pioneering achievements by Tang and Vanslyke
in 1987,1 great effort has been devoted to the design and synthesis
of luminogenic materials for their potential applications in
organic light-emitting diodes (OLEDs) and solid-state lighting.2
To acheive full color displays, it is critical to hunt for good blue-
emitting materials, as it is very difficult to inject charges into blue
emitters as a result of their intrinsic wide band gap.3 Also, since
most organic luminescent materials are used as solid films and
suffer from the aggregation-caused quenching effect (ACQ
effect) due to the formation of delocalized excitons or excimers in
the solid state,4 much research work has been done to mitigate
the ACQ effect,5 for example, doping luminophors into host
materials, the attachment of bulky alicyclics and blending with
transparent polymers. However, these methods might also bring
side effects such as a more complicated fabrication process or
jeopardizing the electronic conjugation in the luminophores.
In 2001, Tang’s group observed an unusual phenomenon and
termed it as aggregation-induced emission (AIE): a series of
organic molecules were found to be non-luminescent in solution
but emissive in the aggregated state.6 They have also identified
that the restriction of intramolecular rotation is the main cause
We have recently reported two TPE-based blue emitters,
SFTPE and TPE-2Cz,10 in which the TPE moiety is linked to
the twisted spirofluorene by sharing either one phenyl ring or
carbazole through the single carbon–nitrogen bond. The non-
doped EL devices based on these two luminophors exhibit
blue emissions at ꢁ460 nm and enhanced efficiencies up to
3.33 cd Aꢀ1. These results partially realized our idea of
adjusting the balance between blue-emission and prolonged
p-conjugation, and encouraged us to design and synthesize
more AIE-active blue-emitting materials through various
approaches. As pointed out by Tang et al.,7,9 crystalline AIE
molecules could demonstrate much more blue-shifted emission
than those in the amorphous state. Can we therefore utilize this
important feature to construct blue TPE-based luminophors? If
it works, it will be another new strategy to develop blue AIE
luminophores. Interestingly, there are no reports concerned
with this idea.
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
bState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, People’s Republic of China. E-mail: mdg1014@ciac.
jl.cn
† Electronic supplementary information (ESI) available: The EL peaks
and maximum current efficiencies of TPE and typical TPE-based
luminogens, CV plots and UV spectra of PhTPE, Ph2TPE and
Ph3TPE. See DOI: 10.1039/c2jm31855e
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In 2007, we successfully developed a group of benzene-cored
AIE-active molecules with phenyl ring peripheries as efficient
rotors11 (Chart 1), in which the substituted benzenes were very
crowded and liable to form a crystalline state. If TPE moieties
were used instead of the phenyl rings in these benzene-cored
molecules, the resultant dyes should be more crowded in space.
These molecules are therefore promising in achieving new blue-
emitting luminophores with AIE activity. Thus, to further
demonstrate our idea and develop new AIE systems with blue
emission, in this work we decorated the benzene core with TPE
moieties at the periphery (Chart 2). This design would improve
the EL performance of the corresponding LED devices by
extending the conjugation length of TPE similarly to the reported
cases.12 However, more importantly, the twisted conformation
between the TPE moiety and the benzene core could partially
disrupt the prolonged conjugation and give the molecules
a greater tendency to be crystalline, possibly resulting in blue
emission. In reality, when fabricated as the emissive layer in
OLEDs, these three dyes, PhTPE, Ph2TPE and Ph3TPE,
exhibited blue emissions from 457 to 488 nm and enhanced
efficiencies up to 5.0 cd Aꢀ1, which demonstrated our idea well.
Herein, we would like to present the synthesis, characterization,
photophysical properties, theoretical calculations and OLED
device performances of these new TPE-based luminophors
in detail.
Chart 2 Mono-, di-, and tritetraphenylethylene benzene derivatives
PhTPE, Ph2TPE and Ph3TPE studied in this work.
exhibited a high glass transition temperature (Tg) of 123 ꢂC,
ꢂ
compared to Ph2TPE with a Tg value of 77 C, indicating that
the rigid and bulky TPE moiety could improve the thermal
stability of the resultant materials. However, the Tg value of
PhTPE was not detected, possibly due to its low molecular
weight.
Optical properties
All three compounds are soluble in common solvents such as
tetrahydrofuran, chloroform and dichloromethane, but insoluble
in water. Fig. S1 (ESI†) shows the absorption spectra of PhTPE,
Ph2TPE and Ph3TPE in dilute THF solution. Their absorption
spectra are similar, with the maximum absorption wavelength at
329, 320 and 325 nm, respectively, indicating the extended
conjugation between the benzene core and TPE units in
comparison with that of TPE (299 nm). Also, the slight difference
of the wavelength reveals that PhTPE is more conjugated than
the other two compounds. To know more about this, we have
optimized the molecular structures of the three compounds and
calculated the dihedral angles between TPE moiety and benzene
core. It was found that PhTPE possesses the smallest dihedral
angle, in other words, the whole molecule of PhTPE is more
planar than Ph2TPE and Ph3TPE. Theoretically, Ph3TPE,
containing the most TPE units at its periphery, should exhibit the
longest maximum absorption wavelength. However, its peak
maximum is actually not very red-shifted owing to the twisted
conformation of the molecule, suggesting that the conjugation is
controllably prolonged to some degree.
Results and discussion
Synthesis
Scheme 1 illustrates the synthetic route to PhTPE, Ph2TPE and
Ph3TPE. As the key intermediate, TPE boronic ester was
obtained from diphenylmethane in three steps according to the
literature. The targeted molecules PhTPE, Ph2TPE and Ph3TPE
were prepared through the palladium-catalyzed Suzuki coupling
reaction of TPE boronic ester and compounds 2–4 in good yields.
The products were purified by column chromatography on silica
gel using petroleum ether (60–90 ꢂC)–dichloromethane as eluent
and fully characterized by 1H and 13C NMR, mass spectrometry,
and elemental analysis.
Thermal properties
The thermal properties of PhTPE, Ph2TPE and Ph3TPE were
investigated by thermal gravimetric analyses (TGA) and differ-
ential scanning calorimetry (DSC). As shown in Fig. 1, the new
compounds exhibited good thermal stability. Their thermal
decomposition temperatures (Td, corresponding to 5% weight
loss) were determined to be about 308, 403 and 422 ꢂC (Table 1),
respectively. Ph3TPE possessed the highest thermal stability and
The UV data is consistent with our idea of designing TPE-
based blue luminophors. The p-conjugation of the benzene ring
is small and the rotatable TPE unit peripheries are too crowded
to be in the same plane as the benzene core, so the whole mole-
cule is quite twisted. So on the one hand, we decorated the central
Chart 1 Previous benzene-based AIE luminophors developed in our lab.
12002 | J. Mater. Chem., 2012, 22, 12001–12007
Scheme 1 Synthetic routes to PhTPE, Ph2TPE and Ph3TPE.
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Fig. 1 (A) TGA and (B) DSC (second heating cycle) thermograms of
PhTPE, Ph2TPE and Ph3TPE recorded under N2 at a heating rate of (A)
10 and (B) 15 ꢂC minꢀ1
.
benzene core with TPE moiety peripheries for proper elongation
of p-conjugation to achieve better EL performance, while on the
other hand, the twisted structure could ensure the blue emission
to a large extent.
In order to investigate the AIE properties of PhTPE, Ph2TPE
and Ph3TPE, PL spectra were recorded, while tetrahydrofuran
and water were chosen as the solvent pair for their miscibility.
They are all non-emissive when readily dissolved in the solutions,
but show strong emission in the aggregate state. Fig. 2 demon-
strates the PL change and fluorescent images of the luminophors
in THF and THF–water mixtures. In dilute THF solution, the
PL spectra are virtually flat lines parallel to the abscissa,
revealing that they are weak emitters in the solution state.
However, when a large amount of water is added, intense emis-
sion is observed. As shown in Fig. 2, when the water fraction is
over 80%, the PL intensity increases swiftly, indicating the
formation of the aggregates. At a fw value of 95%, the PL spectra
peak at 502, 498 and 494 nm for PhTPE, Ph2TPE and Ph3TPE,
respectively. However, unlike PhTPE and Ph2TPE, the PL
spectra of Ph3TPE in THF–water mixtures reveal that the
emission maximum is blue shifted from 503 to 494 nm when the
water fraction is increased from 80% to 95%. This should be
ascribed to the morphological change of the aggregates from
amorphous to crystalline state, that is to say, Ph3TPE is more
easily crystallized than the other two compounds, owing to its
twisted conformation. Notably, the PL intensity of Ph3TPE is
over 450 times higher than that in pure THF, compared to the
values of 50-fold for PhTPE and 200-fold for Ph2TPE. The
structure difference of the three compounds is that Ph3TPE
possesses more rotatable TPE units, confirming that the restric-
tion of intramolecular rotation is the main reason for the AIE
effect. The quantitative enhancement of emission was evaluated
by the PL quantum yields (FF), using 9,10-diphenylanthracene as
Fig. 2 PL spectra of (A) PhTPE, (B) Ph2TPE and (C) Ph3TPE in THF–
water mixtures with different water fractions (fw). (D) Plots of fluores-
cence quantum yields determined in THF/H2O solutions using 9,10-
diphenylanthracene (F ¼ 90% in cyclohexane) as standard versus water
fractions. Concentration (mM): 12.6, 13.7 and 11.6 for PhTPE, Ph2TPE
and Ph3TPE, respectively. Inset in (D): photos of PhTPE, Ph2TPE and
Ph3TPE (from left to right) in THF–water mixtures (fw ¼ 0 and 95%)
taken under the illumination of a 365 nm UV lamp.
the standard. From pure solution in THF to aggregate state in
95% aqueous mixture, the FF values of Ph3TPE increased from
0.15% to 29%.
Further studies on the PL spectra of the three luminophors in
the solid state were carried out, because light-emitting materials
are used as thin films in OLED devices. Thus, we prepared
amorphous and crystal films of the three compounds by freezing
their melts with liquid nitrogen and by solvent vapor,7 but
unfortunately, the crystal films of Ph2TPE and Ph3TPE were not
obtained, probably due to their relatively high molecular
weights.
As shown in Fig. 3, the amorphous films of PhTPE, Ph2TPE
and Ph3TPE show strong sky blue emission with PL peaks at
about 490 nm, close to that of the molecules in the aggregate
state, indicating that when a large amount of water was added to
the pure THF solution, amorphous aggregates were formed.
However, PhTPE emits deep blue light when the thin film
transforms from amorphous state to crystalline state by solvent
vapor, demonstrating that the emission of PhTPE can be tuned
Table 1 The thermal, electrochemical and photophysical data of PhTPE, Ph2TPE and Ph3TPE
Tg (ꢂC)
Egb (eV)
EHOMOc (eV)
PL lmax (aggr)f (nm)
PL lmax (film)g (nm)
a
Td (ꢂC)
d
ELUMO (eV)
e
lmax,ab (nm)
PhTPE
Ph2TPE
Ph3TPE
338
403
422
—
77
123
3.30
3.34
3.31
5.47
5.56
5.53
2.17
2.22
2.22
329
320
325
502
498
494
492
494
496
a
5% weight loss temperature measured by TGA under N2. b Band gap estimated from optical absorption band edge of the solution. c Calculated from
the onset oxidation potentials of the compounds. d Estimated using empirical equations ELUMO ¼ EHOMO + Eg. e Observed from absorption spectra in
dilute THF solution. f Determined in THF : H2O ¼ 5 : 95 solution. Amorphous film.
g
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The HOMO energy levels were calculated to be 5.47, 5.56
and 5.53 eV according to the following equation: HOMO ¼
ꢀ(4.8 + Eox) eV, indicating that better hole-transporting prop-
erty is achieved as the number of TPE moiety peripheries
increases. Their LUMOs could be obtained from optical band
gap energies (estimated from the onset wavelength of their UV
absorptions) and HOMO values, which were 2.17, 2.22 and
2.22 eV, respectively.
Electroluminescence
The good thermal stability and efficient solid-state emissions of
PhTPE, Ph2TPE and Ph3TPE prompted us to investigate their
electroluminescence properties. As shown in Fig. 5, we fabricated
non-doped multilayer organic light-emitting diodes (OLEDs)
with configurations of ITO/MoO3 (10 nm)/NPB (80 nm)/EML
(20 or 30 nm)/TPBi (30 nm)/LiF (1 nm)/Al. In these OLED
devices, MoO3, NPB, and TPBi served as hole-injection, hole-
transporting, and hole-blocking layers respectively, and PhTPE,
Ph2TPE and Ph3TPE served as emitters.
Fig. 3 EL spectra of the devices and PL spectra of the amorphous or
crystalline films for (A) PhTPE, (B) Ph2TPE and (C) Ph3TPE.
by changing its morphology. Their electroluminescent spectra
will be discussed later.
Theoretical calculations
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
carried out to obtain orbital distributions of HOMO and LUMO
energy levels and optimized molecular structures. As shown in
Fig. 4, the LUMOs of PhTPE, Ph2TPE and Ph3TPE are mainly
located at the TPE moiety, and the electron clouds of the
HOMOs are distributed over the whole molecules, showing the
weak intramolecular charge transfer of the three compounds,
partially ensuring the blue emission of the devices. The calculated
dihedral angles between the TPE peripheries and the benzene
core are about 35ꢂ, 45ꢂ and 46ꢂ, respectively for PhTPE, Ph2TPE
and Ph3TPE, showing that the single TPE moiety is more
conjugated with benzene in PhTPE while for Ph2TPE and
Ph3TPE, the conjugation is controllably prolonged due to their
more twisted structure.
As shown in Fig. 6(D), both Ph2TPE and Ph3TPE exhibited
blue emission at 457 and 467 nm with Commission International
d’Eclairage (CIE) chromaticity coordinates of (0.16, 0.15) and
(0.17, 0.20), compared to PhTPE with sky blue emission at
488 nm (0.19, 0.30). From Fig. 3, we observed that the EL
spectrum of PhTPE peaked at 488 nm and nearly overlaps with
the PL spectrum of its amorphous film, showing that the emitting
species in the device is in the amorphous state. However, for
Ph2TPE and Ph3TPE, their EL spectra are 30–40 nm blue-
shifted in comparison with the PL spectra of the corresponding
amorphous films, indicating that in the EL devices the molecules
are mainly in the crystalline state, owing to the more twisted
conformations between the bulky TPE moiety and the central
benzene core in Ph2TPE and Ph3TPE. Thus, the EL spectra of
Ph2TPE and Ph3TPE are more blue-shifted than that of PhTPE
as the result of the different states of the packing order and the
partial disruption of the prolonged conjugation.
As mentioned in the introduction, we would like to develop
blue-emitting AIE luminophores with good performance. TPE
itself emits strong blue fluorescence in its solid films. However, its
corresponding light emitting diode (LED) devices exhibited low
efficiency (0.45 cd Aꢀ1).9 By introducing some aromatic rings,
Electrochemical properties
The electrochemical properties of PhTPE, Ph2TPE and Ph3TPE
(Fig. S2, ESI†) were investigated by cyclic voltammetry (CV).
Fig. 5 Energy level diagrams and device configurations of multilayer EL
devices of PhTPE, Ph2TPE and Ph3TPE. Abbreviations: NPB ¼ 4,4-
Fig. 4 Calculated molecular orbital amplitude plots of HOMO and
LUMO levels and optimized molecular structures of PhTPE, Ph2TPE
and Ph3TPE.
bis(1-naphthylphenylamino)biphenyl, TPBi
benzimidazol-2-yl)benzene.
¼
1,3,5-tris(N-phenyl-
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successful cases are few. And our examples will only stimulate
this new design strategy for AIE dyes with blue emission. To
further confirm this idea, more work is still needed.
Fig. 6 shows the current density–voltage–brightness (J–V–L)
characteristics, efficiency versus current density curves and EL
spectra of the devices. As listed in Table 2, better EL perfor-
mance is observed for PhTPE with a turn-on voltage (Von) of
4.1 V, a maximum luminance (Lmax) of 3966 cd mꢀ2, a maximum
current efficiency (hC, max) of 5.0 cd Aꢀ1 and a maximum power
efficiency (hP, max) of 3.87 lm Wꢀ1. However, the device emits sky
blue light and the EL spectrum peaks at 488 nm due to the
prolonged conjugation. The device of Ph3TPE shows a very
blue-shifted EL peak with Von, Lmax, hC, max and hP, max of 3.7 V,
2497 cd mꢀ2, 3.7 cd Aꢀ1 and 2.5 lm Wꢀ1, respectively. Ph2TPE
possesses both limited conjugation and twisted conformation
which leads to deep blue emission of the device with fair EL
performance (4.1 V for Von, 1261 cd mꢀ2 for Lmax, 2.3 cd Aꢀ1 for
hC, max and 1.7 lm Wꢀ1 for hP, max), confirming that AIE-active
blue-emitting luminophors could be obtained through proper
elongation of p-conjugation and twisted conformation. It is
believed that further improvement of the EL performance could
be achieved by optimizing device structures.
Fig. 6 (A) Current density–voltage–luminance characteristics of multi-
layer EL devices of PhTPE, Ph2TPE and Ph3TPE. Changes in (B)
current and (C) power efficiencies with the current density in multilayer
EL devices. (D) EL spectra of PhTPE, Ph2TPE and Ph3TPE. Device
configurations: ITO/MoO3 (10 nm)/NPB (80 nm)/EML (20 or 30 nm)/
TPBi (30 nm)/LiF (1 nm)/Al.
which could also be considered as the rotators, to the TPE
moieties, the resultant efficiencies could be improved to a large
extent (Chart S1†), but the maximum electroluminescent (EL)
emission wavelengths are very red-shifted, and the emission color
was no longer blue, due to the prolonged p-conjugation system.
Thus, on one side the introduction of the additional aromatic
rotators could improve the LED efficiency, while one the other
side the bonded aromatic moieties would extend the formed
p-conjugation system, leading to the bathochromic shift of the
emission, that is, not blue emission any longer. Thus, the balance
should be adjusted accurately by structural modification to
obtain blue-emitting TPE-based luminophores. We have
successfully prepared two blue luminophores, SFTPE and TPE-
2Cz, in which TPE moiety is linked to the twisted spirofluorene
by sharing either one phenyl ring or carbazole through the single
carbon–nitrogen bond, to control the extent of the conjugation.10
Earlier in 2005, Tang et al. reported that a crystalline AIE
molecule could demonstrate a more blue-shifted emission than
that in the amorphous state.7b,c Thus, the control of the state of
the luminophors in their thin films, amorphous or crystalline
state, should be another approach to obtain AIE dyes with blue
emission. Fortunately, the EL spectra of Ph2TPE and Ph3TPE
confirmed our idea. Also, their structures tell us that crowded
and twisted conformation will encourage the formation of the
crystalline state in the thin films in LED devices. However, the
Conclusions
In summary, three benzene-cored AIE-active molecules were
synthesized and served as blue emitters in OLED devices. It is
beneficial to simply decorate benzene with TPE moiety periph-
eries, both for increasing thermal stability and for enhancing
the EL performance. We have successfully partially controlled
the prolonged conjugation through the twisted structures of the
molecules and utilized the property of crystallization causing
blue-shift for ensuring blue emission. When fabricated as emis-
sive layers in OLEDs, these three luminophors exhibit blue
emissions from 457 to 488 nm with Lmax, hC, max and hP, max up to
3966 cd mꢀ2, 5.0 cd Aꢀ1 and 3.87 lm Wꢀ1, respectively. The
present results provide a new approach to design AIE blue
emitters and encourage us to develop more TPE-based emitters
with both good EL performance and blue emission.
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 micro-
analyzer. MALDI-TOF (matrix-assisted laser-desorption/ioni-
zation time-of-flight) mass spectra were performed on a Bruker
Table 2 EL performances of PhTPE, Ph2TPE and Ph3TPEa
lEL
(nm)
Von
(V)
Lmax
(cd mꢀ2
hP, max
(lm wꢀ1
hC, max
(cd Aꢀ1
CIE
(x, y)
)
)
)
PhTPE
Ph2TPE
Ph3TPE
488
457
467
4.1
4.1
3.7
3966
1261
2497
3.87
1.70
2.50
5.0
2.3
3.7
0.19, 0.30
0.16, 0.15
0.17, 0.20
a
Abbreviations: Von ¼ turn-on voltage at 1 cd mꢀ2, Lmax ¼ maximum luminance, hP, max, and hC, max ¼ maximum power and current efficiencies,
respectively. CIE ¼ Commission International de l’Eclairage coordinates.
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BIFLEX III TOF mass spectrometer. UV-vis absorption spectra
were recorded on a Shimadzu UV-2500 recording spectropho-
tometer. Photoluminescence spectra were recorded on a Hitachi
F-4500 fluorescence spectrophotometer. Differential scanning
calorimetry (DSC) was performed on a NETZSCH DSC 200 PC
(4.12 g, 10.0 mmol) in anhydrous tetrahydrofuran (60 mL) at
ꢀ78 ꢂC 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. The reaction was terminated and brine was added
after stirring overnight. The mixture was extracted with
dichloromethane and the organic layer was dried with anhydrous
sodium sulfate. After filtration and solvent evaporation, the
crude product was purified by silica gel column chromatography
using petroleum ether–dichloromethane (v/v 5/1) as eluent. A
unitꢂat a heating rate of 15 C minꢀ1 from room temperature to
ꢂ
300 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 determꢂined by measuring their weight loss while
1
light yellow powder of 2 was obtained in 60% yield (2.30 g). H
heating at a rate of 10 C minꢀ1 from 25 to 600 C. Cyclic vol-
NMR (300 MHz, CDCl3) d (ppm): 7.55–7.52 (m, 2H), 7.08–7.01
ꢂ
tammetry (CV) was carried out on a CHI voltammetric 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 mVꢀ1 sꢀ1 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 internal ferrocenium/
ferrocene (Fc+/Fc) standard.
(m, 17H), 1.31 (s, 12H).
General procedure for the synthesis of compounds PhTPE,
Ph2TPE and Ph3TPE. PhTPE. A mixture of 1 (460 mg, 1 mmol),
1-bromo-4-(hexyloxy)benzene (310 mg, 1.2 mmol), Pd(PPh3)4
(44 mg, 0.04 mmol) and sodium carbonate (530 mg, 5 mmol) in
10 mL of THF and 3 mL of distilled water in a 50 mL Schlenk
tube was refluxed for 2 days 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 chro-
matography on silica gel using dichloromethane–petroleum ether
(v/v 1/5) as eluent to afford the product as a white powder in the
Computational details
The geometric and electronic properties were optimized at
B3LYP/6-31g(d) level using the Gaussian 09 program. The
molecular orbitals were obtained at the same level of theory.
1
yield of 65.1% (332 mg). H NMR (300 MHz, CDCl3) d (ppm):
7.48–7.45 (d, J ¼ 9 Hz, 2H), 7.31–7.28 (d, J ¼ 9 Hz, 3H), 7.10–
7.03 (m, 16H), 6.93–6.90 (d, J ¼ 9 Hz, 2H), 3.99–3.95 (t, J ¼ 6 Hz,
2H), 1.48–1.44 (m, 2H), 1.35–1.34 (m, 4H), 0.93–0.89 (t, J ¼ 6
Hz, 3H). 13C NMR (75 MHz, CDCl3) d (ppm): 158.3, 143.4,
141.6, 140.5, 140.3, 138.2, 132.6, 131.4, 131.0, 127.4, 127.3, 126.1,
125.3, 114.4, 67.7, 31.2, 28.9, 25.4, 22.2, 13.7. MALDI-TOF:
508.17 ([M+], calcd for C38H36O, 508.69). Anal. calcd for
C38H36O: C, 89.72; H, 7.13%. Found: C, 90.22; H, 6.83%.
OLED device fabrication and measurement
The hole-transporting material NPB (1,4-bis(1-naph-
thylphenylamino)-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 10ꢀ6 Torr onto glass precoated with a layer of indium
tin oxide (ITO) with a sheet resistance of 25 U per square. Before
deposition of an organic layer, the clear ITO substrates were
treated with oxygen plasma for 2 min. The deposition rate of
1
Ph2TPE. White powder. Yield: 57%. H NMR (300 MHz,
CDCl3) d (ppm): 7.33–7.30 (m, 6H), 7.10–7.08 (m, 34H), 3.10–
3.09 (t, J ¼ 3 Hz, 2H), 2.33 (s, 3H), 1.25–1.23 (m, 2H), 1.13–1.03
(m, 4H), 0.91–0.86 (t, J ¼ 6 Hz, 3H). 13C NMR (75 MHz, CDCl3)
d (ppm): 144.1, 142.6, 141.2, 137.2, 135.6, 133.4, 131.6, 131.2,
130.7, 129.0, 127.9, 126.7, 73.3, 31.9, 30.0, 25.8, 22.8, 21.0, 14.4.
MALDI-TOF: 852.66 ([M+], calcd for C65H56O, 853.14). Anal.
calcd for C65H56O: C, 91.51; H, 6.62%. Found: C, 91.43; H,
6.16%.
Ph3TPE. Light-green powder. Yield: 62%. 1H NMR (300
MHz, CDCl3) d (ppm): 7.42 (s, 2H), 7.36–7.33 (m, 6H), 7.08–7.05
(m, 51H), 3.15–3.14 (t, J ¼ 3 Hz, 2H), 1.25–1.04 (m, 8H), 0.91–
0.86 (t, J ¼ 3 Hz, 3H). 13C NMR (75 MHz, CDCl3) d (ppm):
144.0, 142.8, 141.3, 141.1, 137.1, 136.5, 136.2, 132.0, 131.6, 131.3,
129.1, 128.6, 127.9, 126.7, 126.3, 73.4, 31.8, 30.0, 25.8, 22.8, 14.3.
MALDI-TOF: 1168.35 ([M+], calcd for C90H72O, 1168.53).
Anal. calcd for C90H72O: C, 92.43; H, 6.21%. Found: C, 91.92;
H, 5.86%.
organic compounds was 1–2 A sꢀ1. Finally, a cathode composed
ꢀ
of lithium fluoride (1 nm) and aluminium (100 nm) was
sequentially deposited onto the substrate in the vacuum of 10ꢀ6
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 a JY SPEX CCD3000 spectrometer. All
measurements were carried out at room temperature under
ambient conditions.
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. The procedure for the synthesis of PhTPE
is given below as an example.
Acknowledgements
We are grateful to the National Science Foundation of China
(no. 21161160556), the National Fundamental Key Research
Program (2011CB932702), and the Opening Project of Key
Synthesis of compound 1. A 2.3 M solution of n-butyllithium in
hexane (15.0 mmol, 6.5 mL) was added to a solution of TPE–Br
12006 | J. Mater. Chem., 2012, 22, 12001–12007
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P. Dong, J. W. Y. Lam, Y. Ren, H. H. Y. Sun, K. Wong, P. Gao,
I. D. Williams, H. S. Kwok and B. Z. Tang, J. Phys. Chem. B,
2005, 109, 10061; (c) Y. Ren, J. W. Y. Lam, Y. Dong, B. Z. Tang
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Z. Zhao, S. Chen, C. Deng, J. W. Y. Lam, C. Y. K. Chan, P. Lu,
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C. Y. K. Chan, S. Chen, J. Liu, P. Lu, M. Rodriguez,
J. L. Maldonado, G. Ramos-Ortiz, H. H. Y. Sung, I. D. Williams,
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Laboratory of Biomedical Polymers of Ministry of Education
(Wuhan University) (20090401) for financial support.
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