Tuning the electronic nature of aggregation-induced emission chromophores with enhanced electron-transporting properties

Article abstract of DOI:10.1039/c2jm16308j

In organic light-emitting devices, materials with efficient electron-transporting properties, are essential. In this report, oxadiazole-containing tetraphenylethene TPE-Oxa is synthesized and its optical physics and electronic properties are investigated. The dye is almost nonluminescent when molecularly dissolved in solutions, but becomes highly emissive when aggregated in poor solvents or fabricated as thin films in the solid state. A quantum yield of unity has been achieved in its solid thin film. Inherited from the oxadiazole component, the dye molecule enjoys low-lying electronic band energies. Benefiting from the good electron-transporting and hole-blocking properties of the dye, the two-layer OLED devices using TPE-Oxa as both light-emitting and electron-transporting materials show superior performance, i.e., lower turn-on voltage, higher brightness and efficiencies, to the devices of typical configuration with a dedicated electron-transporting layer.

Full text of DOI:10.1039/c2jm16308j

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PAPER
Tuning the electronic nature of aggregation-induced emission chromophores
with enhanced electron-transporting properties†
c
a
a
c
ad
Yang Liu,ab Shuming Chen, Jacky W. Y. Lam, Faisal Mahtab, Hoi Sing Kwok and Ben Zhong Tang
*
Received 2nd December 2011, Accepted 10th January 2012
DOI: 10.1039/c2jm16308j
In organic light-emitting devices, materials with efficient electron-transporting properties, are essential.
In this report, oxadiazole-containing tetraphenylethene TPE-Oxa is synthesized and its optical physics
and electronic properties are investigated. The dye is almost nonluminescent when molecularly
dissolved in solutions, but becomes highly emissive when aggregated in poor solvents or fabricated as
thin films in the solid state. A quantum yield of unity has been achieved in its solid thin film. Inherited
from the oxadiazole component, the dye molecule enjoys low-lying electronic band energies. Benefiting
from the good electron-transporting and hole-blocking properties of the dye, the two-layer OLED
devices using TPE-Oxa as both light-emitting and electron-transporting materials show superior
performance, i.e., lower turn-on voltage, higher brightness and efficiencies, to the devices of typical
configuration with a dedicated electron-transporting layer.
both efficient solid-state emissions and good charge-transporting
properties because it will simplify the device fabrication proce-
dure and lower the construction cost.6 In our previous work, AIE
luminogens with good hole-transporting ability were demon-
strated. By introducing triphenylamine (TPA) functionalities
with hole-transporting properties into TPE derivatives, efficient
solid emitters with good hole-transporting properties were ach-
ieved (Scheme 1).7 OLEDs using them as both emitting and hole-
transporting layer showed superior performances to those with
a dedicated hole-transporting layer.7 In comparison, electron
Introduction
Organic materials with enhanced solid-state emissions have been
well established in organic light-emitting diodes (OLEDs) in
recent years.1 Most of the organic conjugate chromophores emit
strongly in their dilute solutions but become weak fluorophores
when fabricated into solid films or aggregated in their poor
solvents due to strong intermolecular interactions.2 This problem
must be solved because luminogenic molecules are commonly
used as thin films in real-world applications.3 We have recently
discovered an ‘‘abnormal’’ process of aggregation-induced
emission (AIE). A series of propeller-shaped nonemissive dyes
such as siloles and tetraphenylethenes (TPEs) can be induced to
emit efficiently by aggregate formation.4 Because they are free of
the common fluorescence ‘‘aggregation-caused quenching’’
effect, OLEDs utilizing AIE molecules exhibit outstanding
performances.5
To enhance the device performance, OLEDs with multilayers
are generally adopted to balance the charge injection and
transportation. In reality, it would be nice to have emitters with
aDepartment of Chemistry, Institute of Molecular Functional Materials,
The Hong Kong University of Science & Technology (HKUST), Clear
Water Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk
bState Key Laboratory of Crystal Materials, Shandong University, Jinan,
250100, PR China
cCenter for Display Research, HKUST, Kowloon, Hong Kong, China
dDepartment of Polymer Science and Engineering, Key Laboratory of
Macromolecular synthesis and Functionalization of the Ministry of
Education of China, Zhejiang University, Hangzhou, 310027, China
† Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for the synthesis of TPE-Oxa,
UV absorption spectrum and CV spectrum. See DOI:
10.1039/c2jm16308j
Scheme 1 Chemical structures of TPA (triphenylamine)-functionalized
AIE-active tetraphenylethene derivatives with enhanced hole-trans-
porting properties.
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mobilities in organic materials can be several orders of magni-
tude less than hole mobilities.8 There are a number of commer-
cially available hole transport substances but just few electron
transport materials with suitable parameters.8,9 Electron trans-
port materials help transport electrons from the cathode and into
the emitting layer. Charge injection is the limiting factors for
device efficiency. In order to lower the barrier height, a suitable
energy level relative to the adjacent layer is essential.8 Integrating
the electron transport and emitting layers into one can also
simplify the transport process and eliminate one of the barriers.10
Among the mostly used electron transport materials, besides
some metal chelates such as the well-known Alq3 (tris(8-
hydroxyquinoline)aluminum)11 and some N]C (imine)12 con-
taining heterocycle compounds, the 2,5-diaryl-1,3,4-oxadiazole
(Oxa) building block is an attractive unit. Because it features
electron deficient aromatic groups necessary for electron trans-
port13 along with good stability and little negative effect on
luminescence,14 the introduction of an oxadiazole ring into
molecular structures is one of the promising approaches to the
attainment of high charge-carrier mobility.15 Our purpose in this
report is to combine the highly efficient solid-state emission of
AIE-active luminogens and the good electron-transport property
of the Oxa unit, leading to a new molecule with both enhanced
electron transport and excellent optical properties. Next we
present the synthesis of Oxa-containing tetraphenylethene TPE-
Oxa and its photophysical properties. An efficient OLED using it
as both emitter and electron-transporting layers was constructed.
It showed superior performance to one with a dedicated electron-
transporting layer.
elemental analysis, and high-resolution mass spectra (HRMS).
see ESI†).
Optical properties
TPE-Oxa is soluble in common organic solvents such as
chloroform and tetrahydrofuran (THF) but insoluble in water,
while it is almost nonluminescent when molecularly dissolved in
its solvents. As shown in Fig. 1, the photoluminescence (PL)
spectra of TPE-Oxa in THF are basically flat lines parallel to the
abscissa. The fluorescence quantum yield (FF) in THF is merely
0.23%. When a large amount of water is added into its THF
solution, intense emission at ꢀ480 nm is observed under identical
measurement conditions. As seen from Fig. 1B, at a water
fraction of 90%, its fluorescence intensity (I) is boosted by more
than 550 times. Since water is a non-solvent for the dye mole-
cules, the molecules must be aggregated in the solvent mixtures
with high water fractions. Apparently, the emission of TPE-Oxa
is induced by aggregation formation; in other words, it is AIE-
active. The molecule in THF/water mixtures with high water
fractions are, however, macroscopically homogenous with no
precipitates, suggesting that the aggregates are of nano-
dimensions, as proved by the TEM image shown in Fig. 2 and the
level-off tails in the long wavelength region of their absorption
spectra in the aqueous mixtures with high water contents due to
the Mie effect of the dye nanoparticles.18 The photograph shown
in the inset of Fig. 1 clearly manifests the nonluminescent and
emissive natures of the molecular isolated species and aggregate
particles. Like its aggregates suspended in aqueous media,
TPE-Oxa is highly emissive in the solid state. Upon photoexci-
tation, its film emits at 466 nm (Fig. 6A). The FF values measured
by the integrating sphere method are much higher than those in
the aqueous mixtures and reached 100%.
Results and discussion
Synthesis
Closer inspection of the PL spectra of TPE-Oxa in the aqueous
mixtures reveals that the emission maximum is blue-shifted
slightly accompanying an increase in intensity when the water
fraction is increased from 80 to 95% (from 487 to 480 nm). The
electron diffraction microscopy analysis of the aggregates
formed in the solvent mixtures with 90% water fraction displays
many clear diffraction spots (Fig. 2 right), proving the crystalline
nature of the aggregates. This blue-shift can be further developed
when the sample is crystallized. As shown in Fig. 3, the crystal-
line clusters of TPE-Oxa showed intense deep blue fluorescence,
with a maximum at 455 nm. One may notice that its film emits at
466 nm, a value between those of the nano-aggregates and the
crystallites, which is probably due to its semi-crystallinity. In our
previous study, we have observed similar phenomena on several
AIE-active luminogens. The change in the packing order of the
aggregates from an amorphous state to a crystalline one is
expected to be the origin.19 That is, for these AIE-active lumi-
nogens, crystallization will enhance and blue-shift the emission.
The Oxa chromophore is easy to build as part of a p-conjugated
system. The synthetic route to the Oxa-containing TPE is
depicted in Scheme 2.15d,16 The detailed procedures for the
syntheses of the intermediates and final products are described in
the ESI.† Briefly, the key reactant 1-(phenyl-4-boronic acid)-
1,2,2-triphenylethene was prepared in high yield by lithiation
of 1-(4-bromophenyl)-1,2,2-triphenylethene,17 followed by
treatment with trimethyl borate and hydrolysis catalyzed by acid.
2-(4-Bromophenyl)-5-phenyl-1,3,4-oxadiazole was synthesized
according to the reported literature method. Suzuki coupling
reaction between the two reactants was catalyzed by Pd(PPh3)4,
giving 1-[40-(5-phenyl-2-1,3,4-oxadiazolyl)biphenyl-4-yl]-1,2,2-
triphenylethene (TPE-Oxa) in good yield. The dye molecule was
characterized by spectroscopic methods (1H and 13C NMR,
Thermal properties
The thermal properties of TPE-Oxa are investigated by thermog-
ravimetric (TGA) and differential scanning calorimetric (DSC)
analyses. As shown in Fig. 4, TPE-Oxa enjoys good thermal
stability with a Td value of 377 ꢁC. The sample melted at 240 ꢁC to
give an isotropic liquid in the first run of the DSC measurement.
Scheme
2
Synthetic route to 1-[40-(5-phenyl-2-1,3,4-oxadiazolyl)-
biphenyl-4-yl]-1,2,2-triphenylethene (TPE-Oxa).
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Fig. 1 (A) Photoluminescence spectra of TPE-Oxa in THF/water mixtures with different water fractions. Concentration: 10 mM; excitation: 355 nm. (B)
Plots of (I/I0) values versus compositions of the THF/water mixtures. Inset: fluorescent images of TPE-Oxa in THF/water mixtures with different water
fractions.
Fig. 2 TEM image of the nanoaggregates of TPE-Oxa formed in a THF/
water mixture with 90% water fraction and the electron diffraction (ED)
pattern.
Fig. 4 TGA and DSC thermograms of TPE-Oxa (recorded under N2 at
a heating rate of 20 (TGA) and 10 (DSC) ꢁC minꢂ1.)
Upon cooling, the isotropic liquid changed into a glassy state. As
the glassy sample was reheated for the second run, a glass transi-
tion was observed at 93 ꢁC, which is defined as the glass transition
temperature (Tg)of TPE-Oxa. Uponfurtherheating beyondTg, an
exothermal crystallization was observed at 140 ꢁC. Due to its low
Fig.
3 Fluorescence microscopic image of crystalline clusters of
TPE-Oxa.
molecular weight, the Tg of TPE-Oxa is not high.
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Electrochemical properties
As the introduction of the oxadiazole unit into our AIE system is
to tune its electronic nature, measuring its energy level to validate
the effect is essential. The electrochemical behaviour of TPE-Oxa
was studied by cyclic voltammetry. Its electrochemical potential
was calibrated with the ferrocene/ferrocenium (Fc/Fc+) standard
(4.8 eV below the vacuum level) to determine the HOMO energy
level (Fig. S5†) From the onset of the oxidation, its HOMO
energy level is 6.1 eV. The LUMO energy level deduced from the
difference between the HOMO level and the optical band gap is
3.1 eV (Fig. 5). Just like other traditional oxadiazole-containing
materials, TPE-Oxa shows low-lying electronic band energies.15
Comparing with the most famous electron-transporting material
TPBi, they possess a similar low-lying HOMO level, which will
benefit hole-blocking. On the other hand, its LUMO lies lower
than that of TPBi (3.1 vs. 2.7 eV) and is almost equal to the work
function of the electron injecting cathode (LiF/Al). This may
lower the charge injection barrier and the device operating
voltages. These electronic properties indicate that besides light-
emitting, TPE-Oxa can also plays a role as electron-transporting
hole-blocking material. Thus according to the energy level
diagrams, two different device architectures are adopted to
validate its optoelectronic behaviour. One is the typical device
structure with both hole-transporting and electron-transporting
layers and the other one only has a hole-transporting layer, as
shown in Fig. 5.
Fig. 6 (A) PL and EL spectra of solid thin film of TPE-Oxa. (B) Current
density and luminance–voltage characteristics of devices based on
TPE-Oxa.
much lower than the latter one (ꢀ2 V). Subsequently, the turn-on
voltage of the simple device (3.2 V) is lower than that of the
multilayer one (4.4 V). Profiting from the better electronic
properties, the simple OLED radiates more brightly with
the maximum luminance (Lmax) value reaching 7000 cd mꢂ2 at
15 V, which is much higher than the with ETL counterpart of
2800 cd mꢂ2. And also, the simple device exhibits high perfor-
mance in efficiency. Fig. 7 shows the current efficiency, power
efficiency and external quantum efficiency versus voltage char-
acteristics of the two kinds of devices based on TPE-Oxa. The
simple device worked at almost twice the efficiency of the
multilayer one in the whole bias range, e.g., 2.4 vs. 1.5 cd Aꢂ1 of
the maximum current efficiency (hC), and 2.2 vs. 1.1 lm Wꢂ1 of
the power efficiency (hP), for details see Table 1. These results
verified the enhanced electron-transporting and hole-blocking
behaviour of TPE-Oxa, clearly demonstrating that the EL
properties of AIE luminogens can be readily tuned by molecular
engineering endeavors. Combination of electronic tuning groups
into AIE-luminogens has led to their use as both light-emitting
and electron-transporting hole-blocking materials in OLEDs.
Electroluminescence properties
Under the typical multilayer device structure: ITO/NPB(60 nm)/
TPE-Oxa(20 nm)/TPBi(10 nm)/Alq3(30 nm)/LiF/Al(200 nm),
the device emits deep blue light with the EL maximum (lEL) of
466 nm, which is quite close to that of the PL in film. Fig. 6A
depicts the EL spectra and the PL spectrum of the thin film.
Under the two layer device structure: ITO/NPB(60 nm)/TPE-
Oxa(60 nm)/LiF/Al(200 nm), the device emits a little longer
wavelength than the former one with the EL maximum (lEL) of
476 nm. Both the EL show spectral stability with hardly any
changes along with the operating bias increasing from 8 V to
15 V. Fig. 6B shows the current density and luminance–voltage
characteristics of the two kinds of devices based on TPE-Oxa.
We can see with the simple device structure (no dedicated elec-
tron-transporting layer (ETL)), it shows better electronic char-
acter. The charge transfer in the simple device is more efficient
than that in the device with ETL, i.e., to reach the same current
density/brightness, the applied bias needed for the former one is
Conclusions
In this paper, oxadiazole-containing tetraphenylethene TPE-Oxa
is synthesized and its optical physics and electronic properties are
investigated. The dye possesses typical AIE characteristics; it
becomes highly emissive when aggregated in poor solvents or
fabricated as thin films in the solid state. A quantum yield of
unity has been achieved in its solid thin film. Inherited from the
oxadiazole component, the dye molecule enjoys low-lying elec-
tronic band energies. The two-layer OLED devices using
Fig. 7 (A) Current efficiency and power efficiency versus voltage char-
acteristics of device based on TPE-Oxa. (E) External quantum efficiency
versus voltage characteristics of device based on TPE-Oxa.
Fig. 5 Energy level diagrams of multilayer EL devices of TPE-Oxa with
and without electron-transporting layers (ETL).
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Table 1 Electroluminescence performances of TPE-Oxaa
ETL
lEL (nm)
Von (V)
Lmax (cd mꢂ2
)
CEmax (cd Aꢂ1
)
PEmax (lm Wꢂ1
)
EQEmax (%)
3
7
466
476
4.4
3.2
2800
7000
1.5
2.4
1.1
2.2
0.7
1.0
a
Device configurations, with ETL: ITO/NPB(60 nm)/TPE-Oxa(20 nm)/TPBi(10 nm)/Alq3(30 nm)/LiF/Al(200 nm); without ETL: ITO/NPB(60 nm)/
TPE-Oxa(60 nm)/LiF/Al(200 nm). Abbreviations: lEL ¼ EL maximum, Von ¼ turn-on voltage at 1 cd mꢂ2, Lmax ¼ maximum luminance, CEmax
¼
maximum current efficiency, PEmax ¼ maximum power efficiency, EQEmax ¼ maximum external quantum efficiency.
4 (a) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740; (b) Y. Hong, J. W. Y. Lam and B. Z. Tang,
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TPE-Oxa as both light-emitting and electron-transporting
materials show superior performances to the devices of typical
configuration with a dedicated electron-transporting layer.
Benefiting from the good electron-transporting and hole-block-
ing properties of the dye, the device with simple structure shows
much lower turn-on voltage, higher brightness and efficiency.
The carrier mobility of the material in other kinds of devices, e.g.,
organic field-effect transistors (OFETs) or measured by the time
of flight (TOF) method will be studied in future works, to see
whether the material has potential in organic light-emitting
transistors (OLETs). Using this combination strategy developed
in this work, it is anticipated that more AIE luminogens with
versatile functionalities will be generated.
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Acknowledgements
This work was partially supported by the Research Project
Competition of HKUST (RPC11SC09 and RPC10SC13), the
Research Grants Council of Hong Kong (604711, 603509, and
HKUST2/CRF/10), the National Natural Science Foundation of
China (20634020 and 20974028), and the University Grants
Committee of Hong Kong (AoE/P-03/08). B.Z.T. acknowledges
support from the Cao Guangbiao Foundation of Zhejiang
University.
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This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 5184–5189 | 5189