Bipolar AIE-active luminogens comprised of an oxadiazole core and terminal TPE moieties as a new type of host for doped electroluminescence

Article abstract of DOI:10.1039/c2cc34966c

For the first time, two bipolar AIE-active luminogens (Oxa-pTPE and Oxa-mTPE) constructed from tetraphenylethene and oxadiazole were utilized as fluorescence host materials in sky-blue doped OLEDs and exhibited high efficiencies with L_max, η_C,max, η_P,max and η_ext,max of 10070 cd m^-2, 9.79 cd A^-1, 9.92 Im W^-1 and 5.0%, respectively, broadening the scope for the utilization of AIE materials in the optoelectronic field.

Full text of DOI:10.1039/c2cc34966c

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Bipolar AIE-active luminogens comprised of an oxadiazole core and
terminal TPE moieties as a new type of host for doped
electroluminescencew
Jing Huang,a Xiao Yang,b Xianjie Li,c Pengyu Chen,a Runli Tang,a Feng Li,c Ping Lu,c
Yuguang Ma,c Lei Wang,*b Jingui Qin,a Qianqian Lia and Zhen Li*a
Received 11th July 2012, Accepted 6th August 2012
DOI: 10.1039/c2cc34966c
For the first time, two bipolar AIE-active luminogens (Oxa-pTPE
and Oxa-mTPE) constructed from tetraphenylethene and oxadiazole
were utilized as fluorescence host materials in sky-blue doped OLEDs
and exhibited high efficiencies with Lmax, gC,max, gP,max and gext,max
of 10 070 cd mÀ2, 9.79 cd AÀ1, 9.92 Im WÀ1 and 5.0%, respectively,
broadening the scope for the utilization of AIE materials in the
optoelectronic field.
energy and charge from the AIE host to the dopant successfully
occurs, the resultant OLED devices should exhibit good
performance as expected.
As the first attempt, we still would like to obtain efficient blue
light emitting OLEDs. Considering all the AIE molecules in our
hands and reported by other scientists, TPE, with a band gap as
large as 3.76 eV, was selected to realize our idea. Unfortunately, our
experimental results disclosed that TPE is hole-dominated, since its
hole mobility (B10À4 cm2 VÀ1
s
À1) is one order-of-magnitude
À1). This
The intriguing phenomenon of aggregation-induced emission
(AIE),1 first reported by Tang et al. in 2001, has been demonstrated
as an effective way to tackle the notorious aggregation-caused
quenching (ACQ) effect2 and has aroused a new research topic of
AIE materials, with tetraphenylethene (TPE) as a prototype AIE
molecule for its facile synthesis and thermal stability.3 Recently, lots
of research work on TPE-based luminogens was reported with
excellent electroluminescence characteristics (Chart S1, ESIw).4
However, good blue light-emitting OLEDs utilizing AIE materials
are still very scarce, though the lack of highly efficient blue light
emitting materials caused by their intrinsic wide-band-gap nature is
a critical drawback to realize full color displays.5 The key feature of
AIE luminophores is that the aggregation could block the non-
radiative path, thus contributing to the efficient emission.1b,6
This feature, really, is almost the same as one of the basic
requirements for a good host in the guest–host systems, which is
another effective approach to achieve high performance of LED
devices.7 Thus, is it possible to utilize the AIE materials as
hosts, instead of their general role as emission layers, to achieve
good OLEDs with blue emission? Once the efficient transfer of
higher than its electron mobility (B10À5 cm2 VÀ1
s
indicated that the direct usage of TPE as the host could not achieve
the balance of the hole and electron injection. Accordingly,
oxadiazole is chosen as the co-block to build new AIE hosts, due
to its good electron-transporting property (Chart S2, ESIw).8
Thus, two new AIE luminogens, Oxa-pTPE and Oxa-mTPE,
were designed, as the potential good hosts (Chart 1). Also,
BUBD-1, a frequently used sky blue fluorescent dopant, was
utilized as the guest in this research.9
The synthetic routes to Oxa-pTPE and Oxa-mTPE are
illustrated in Scheme S1 (ESIw) and the detailed procedures are
presented in the ESI.w The thermal properties of Oxa-pTPE and
Oxa-mTPE were investigated by thermal gravimetric analyses
(TGA) and differential scanning calorimetry (DSC) (Fig. S1,
ESIw). Their thermal-decomposition temperatures (Td, corres-
ponding to 5% weight loss) were determined to be about 414 and
400 1C, respectively. Owing to the more planar conformation,
Oxa-pTPE possessed higher thermal stability and higher glass
transition temperature (Tg) of 113 1C, compared to Oxa-mTPE
with a Tg value of 103 1C.
Oxa-pTPE and Oxa-mTPE show absorption maxima at
a Department of Chemistry, Hubei Key Lab on Organic and Polymeric
Opto-Electronic Materials, Wuhan University, Wuhan 430072,
China. E-mail: lizhen@whu.edu.cn; Fax: +86-27-68755363;
Tel: +86-27-68755363
320 and 302 nm, respectively (Fig. S2, ESIw), indicating that
b Wuhan National Laboratory for Optoelectronics, School of
Optoelectronic Science and Engineering, Huazhong University of
Science and Technology, Wuhan 430074, China.
E-mail: lei96wang@163.com
c State Key Laboratory of Supramolecular Structure and Materials,
Department of Chemistry, Jilin University, Jilin 130022, China
w Electronic supplementary information (ESI) available: Experimental
section and characterization data for Oxa-pTPE and Oxa-mTPE.
CCDC 883924. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc34966c
Chart 1 Chemical structures of Oxa-mTPE and Oxa-pTPE.
c
9586 Chem. Commun., 2012, 48, 9586–9588
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Fig. 1 (A) PL spectra of Oxa-pTPE in THF–H2O mixtures with
different water fractions (fw). Concentration (mM): 10.7; excitation
wavelength (nm): 320. (B) Plots of fluorescence quantum yields
determined in THF–H2O solutions using 9,10-diphenylanthracene
(F = 90% in cyclohexane) as standard versus water fractions. Inset
in (B): photos of Oxa-pTPE in THF–water mixtures (fw = 0 and 99%)
taken under the illumination of a 365 nm UV lamp.
Fig. 2 Electron and hole mobilities versus E1/2 for TPE, Oxa-pTPE
and Oxa-mTPE.
mobility of the new fluorophores (1.2–1.6 Â 10À4 cm2 VÀ1 sÀ1
)
is one order-of-magnitude higher than that of TPE
(B10À5 cm2 VÀ1 sÀ1) by introducing the widely used oxadiazole
unit, while the hole mobilities are of the same order of
magnitude (B10À4 cm2 VÀ1 À1) as TPE, which demonstrate
s
the former is more conjugated than the latter, as a result of the
meta- or para-linkage of TPE and oxadiazole unit. To investigate
the possible AIE characteristics of these two dyes, their fluorescent
behaviors were studied. Considering that the good and poor
solvents should be well miscible, tetrahydrofuran and water were
chosen as the solvent pair. Fig. 1 demonstrates the PL change and
fluorescent image of Oxa-pTPE in THF and THF–water
mixtures. It is easily seen that for dilute THF solution, the PL
curve is practically a flat line parallel to the abscissa, confirming that
it is nearly nonemissive in the solution state. However, when a large
amount of water is added, intense emission is observed. As shown
in Fig. 1, 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 spectrum peak is at 479 nm for Oxa-pTPE,
and the PL intensity is 120-fold higher than that in pure THF.
Similar behavior was observed for Oxa-mTPE (Fig. S3, ESIw). The
quantitative enhancement of emission was evaluated by the PL
quantum yields (FF), using 9,10-diphenylanthracene as the
standard. From pure solution in THF to aggregate states in
95% or 99% aqueous mixtures, the FF values for Oxa-pTPE
and Oxa-mTPE increase from 0.0052 and 0.0044 to 0.27 and
0.346. Evidently, these two dyes are AIE-active.
their ambipolar property. This is beneficial for the dual charge
transport and recombination in the diode structure, and also
well realized our molecular design.
The good thermal property, efficient solid-state emission
and bipolar characteristics encouraged us to investigate the EL
performance of Oxa-pTPE and Oxa-mTPE as host materials.
As shown in Fig. 3, the spectral overlap between the PL
spectra of the thin solid films of Oxa-pTPE and Oxa-mTPE
and the BUBD-1 absorption spectrum indicates the efficient
Forster energy transfer between them. Thus, we fabricated doped
¨
multilayer organic light-emitting diodes (OLEDs) with configura-
tions of ITO–NPB (10 nm)/Oxa-pTPE or Oxa-mTPE: x%
BUBD-1 (40 nm)/TPBi (10 nm)/Alq3 (20 nm)/Al (100 nm). In
these OLED devices, 1,4-bis(1-naphthylphenylamino)-biphenyl
(NPB), 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI)
and tris(8-hydroxyquinolinato)aluminium (Alq3) were used
as hole-transporting, hole-blocking and electron-transporting
layers, respectively, Oxa-pTPE and Oxa-mTPE served as host
materials and BUBD-1 was the dopant. The energy gap of
BUBD-1 with a lowest unoccupied molecular orbital/highest
occupied molecular orbital level of 2.6/5.1 eV is well confined
To further understand the structure–property relationship at the
molecular level, Density Functional Theory (DFT) calculations
(B3LYP/6-31g*) were carried out. As shown in Fig. S4 (ESIw), the
LUMOs of both molecules are located on the electron-accepting
oxadiazole unit and the HOMO orbitals are mainly distributed on
the TPE moiety, showing the almost separation of HOMO and
LUMO orbitals. Fortunately, we have obtained the single crystal
of Oxa-mTPE. As shown in Fig. S4 (ESIw), Oxa-mTPE has a
twisted conformation and no intermolecular interactions are
found in the packing arrangement, which is beneficial to prevent
the formation of species detrimental to emission. This should be
one advantage of these AIE molecules when used as hosts.
In order to obtain the carriers’ mobilities of TPE, Oxa-pTPE
and Oxa-mTPE, we used the time-of-flight (TOF) transient
photocurrent technique. The carrier transit times tT were evaluated
from the intersection point of two asymptotes in the double
logarithmic representations of the TOF transits (Fig. S5 and S6,
inset (ESIw)). The carrier mobilities of Oxa-pTPE, Oxa-mTPE
and TPE versus E1/2 are shown in Fig. 2. Obviously, the electron
Fig. 3 Left: energy level diagrams and configurations of sky blue-doped
EL devices of Oxa-pTPE and Oxa-mTPE. Abbreviations: NPB =
4,4-bis(1-naphthylphenylamino)biphenyl, TPBi = 1,3,5-tris(N-phenyl-
benzimidazol-2-yl)benzene and Alq3 = tris(8-hydroxyquinolinato)-
aluminium. Right: UV spectrum of BUBD-1 in dilute solution and
PL spectra of solid thin films of Oxa-pTPE and Oxa-mTPE.
c
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Chem. Commun., 2012, 48, 9586–9588 9587

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the oxadiazole unit effectively suppress intermolecular inter-
actions and alleviate self-quenching of the dopant.
In summary, bipolar AIE-active luminogens Oxa-pTPE and
Oxa-mTPE were successfully synthesized and served as
fluorescence host materials in sky-blue doped OLED devices
with the aim of improving the EL performance. Thanks to the
bipolar characteristic and specific AIE feature, efficient charge
and energy transfer from Oxa-pTPE and Oxa-mTPE to
dopant BUBD-1 occurred via radiative decay in the emissive
layer. The doped devices exhibit high efficiencies with Lmax
,
,
Z
C,max, ZP,max and Zext,max of 10 070 cd mÀ2, 9.79 cd AÀ1
9.92 Im WÀ1 and 5.0%, which demonstrate that AIE mole-
cules are promising host materials for blue-emitting OLED
applications. Thus, our preliminary results might open up a new
avenue for the utilization of AIE materials in the photonic and
electronic research field.
This work was supported by the National Science Foundation
of China (no. 21161160556). Z. Li thanks the support of Open
Project of State Key Laboratory of Supramolecular Structure
and Materials (sklssm201219).
Fig. 4 Changes in (a) current density and luminance with the applied
voltage and (b) current efficiency with the current density in BUBD-1
doped multilayer EL devices of Oxa-pTPE. Inset in panel (b): EL spectrum
of the device. Device configuration: ITO/NPB (10 nm)/Oxa-pTPE: 6%
BUBD-1 (40 nm)/TPBi (10 nm)/Alq3 (20 nm)/Al (100 nm).
Notes and references
1 (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,
Chem. Commun., 2009, 4332.
2 A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz and
A. B. Holmes, Chem. Rev., 2009, 109, 897.
in Oxa-pTPE or Oxa-mTPE from the energy level diagram
(Fig. 3). In addition, we observed that the LUMO of BUBD-1
is close to the energy level of cathode Al, enough to cause electron
trapping, and the HOMOs of Oxa-pTPE and Oxa-mTPE
are close to the hole transport material NPB, beneficial for
transporting holes to the host. The detailed EL performances
of the devices are summarized in Table S2 (ESIw).
3 (a) Y. Dong, J. W. Y. Lam, A. Qin, J. Liu, Z. Li, B. Z. Tang, J. Sun and
H. S. Kwok, Appl. Phys. Lett., 2007, 91, 011111; (b) J. Huang, X. Yang,
J. Wang, C. Zhong, L. Wang, J. Qin and Z. Li, J. Mater. Chem., 2012,
22, 2478; (c) J. Huang, N. Sun, J. Yang, R. Tang, Q. Li, D. Ma, J. Qin
and Z. Li, J. Mater. Chem., 2012, 22, 12001; (d) Y. Yang, X. Su,
C. N. Carroll and I. Aprahamian, Chem. Sci., 2012, 3, 610;
(e) N. W. Tesng, J. Liu, J. C. K. Ng, J. W. Y. Lam, H. H. Y. Sung,
I. D. Williams and B. Z. Tang, Chem. Sci., 2012, 3, 493; (f) S. Chen, J. Liu,
Y. Liu, H. Su, Y. Hong, C. K. W. Jim, R. T. K. Kwok, N. Zhao, W. Qin,
J. W. Y. Lam, K. S. Wong and B. Z. Tang, Chem. Sci., 2012, 3, 1804.
4 (a) W. Z. Yuan, P. Lu, S. Chen, J. W. Y. Lam, Z. Wang, Y. Liu,
H. S. Kowk, Y. Ma and B. Z. Tang, Adv. Mater., 2010, 22, 2159;
(b) Z. Zhao, S. Chen, X. Shen, F. Mahtab, Y. Yu, P. Lu,
J. W. Y. Lam, H. S. Kwok and B. Z. Tang, Chem. Commun.,
2010, 46, 686; (c) Z. Zhao, J. W. Y. Lam, 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, H. Su, K. S. Wong, Y. Ma, H. S. Kwok,
H. Qiu and B. Z. Tang, Adv. Mater., 2011, 23, 5430; (d) C. Y. K. Chan,
Z. Zhao, J. W. Y. Lam, J. Liu, S. Chen, P. Lu, F. Mahtab, X. Chen,
H. H. Y. Sung, H. S. Kwok, Y. Ma, I. D. Williams, K. S. Wong and
B. Z. Tang, Adv. Funct. Mater., 2012, 22, 378.
5 (a) R. J. Tseng, R. C. Chiechi, F. Wudl and Y. Yang, Appl. Phys.
Lett., 2010, 88, 093512; (b) K. T. Kamtekar, A. P. Monkman and
M. R. Bryce, Adv. Mater., 2010, 22, 572.
6 (a) J. Chen, B. Xu, X. Ouyang, B. Z. Tang and Y. Cao, J. Phys.
Chem. A, 2004, 108, 7522; (b) Z. Li, Y. Dong, B. Mi, Y. Tang,
H. Tong, 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
and K. S. Wong, J. Phys. Chem. B, 2005, 109, 1135.
The optimal BUBD-1-doped concentration is 6% for Oxa-pTPE
and 3% for Oxa-mTPE. The EL spectra are shown in Fig. 4 (b,
inset) and Fig. S7 and S8 (b, inset, ESIw). The doped devices exhibit
sky blue emission due to the dopant BUBD-1 fluorescence and the
CIEx,y coordinates are (0.15, 0.34) and (0.15, 0.33), respectively. As
shown in Fig. 4 and Fig. S7 (ESIw), the devices turned on at a
voltage of 5.1 V for Oxa-pTPE and 6.25 V for Oxa-mTPE.
Accompanying the increase in the voltage, the luminance increased
rapidly. The doped device based on Oxa-mTPE exhibits a
maximum luminance (Lmax) of 7734 cd mÀ2, a maximum
current efficiency (ZC,max) of 9.82 cd AÀ1 and a maximum power
efficiency (ZP,max) of 7.96 Im WÀ1. Better EL performance
is observed for Oxa-pTPE with Lmax, ZC,max and ZP,max of
10 070 cd mÀ2, 9.79 cd AÀ1 and 9.92 Im WÀ1, respectively.
Noteworthily, the highest external quantum efficiency reaches
5.0%, which illustrates the excellent carrier recombination as
well as the balance of holes and electrons in the emissive layer.
The satisfactory EL data could be explained as follows: on the
one hand, when fabricated as thin films, the intramolecular
rotation of the two AIE-active hosts is restricted, which blocks
the non-radiative path and efficiently transfers charge and
energy from the host to the dopant via radiative decay; on
the other hand, the bulky nonplanar TPE moieties attached to
7 J. Shi and C. W. Tang, Appl. Phys. Lett., 2002, 80, 3201.
8 (a) G. Hughes and M. R. Bryce, J. Mater. Chem., 2005, 15, 94;
(b) W. Y. Wong, Z. He, S. H. So, K. L. Tong and Z. Lin,
Organometallics, 2005, 24, 4079; (c) Z. He, W. Y. Wong, X. Yu,
H. S. Kwok and Z. Lin, Inorg. Chem., 2006, 45, 10922.
9 M. Lin, L. Wang and W. K. Wong, Appl. Phys. Lett., 2006,
89, 121913.
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This journal is The Royal Society of Chemistry 2012