Towards high efficiency solid emitters with aggregation-induced emission and electron-transport characteristics

Article abstract of DOI:10.1039/c1cc14122h

Combination of an aggregation-induced emission (AIE) moiety and a dimesitylboron group yields a new three-coordinate boron compound exhibiting a synergistic effect: the resultant TPEDMesB shows AIE feature with solid-state emission efficiency up to unity

Full text of DOI:10.1039/c1cc14122h

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COMMUNICATION
Towards high efficiency solid emitters with aggregation-induced emission
and electron-transport characteristicsw
Wang Zhang Yuan,ab Shuming Chen,c Jacky W. Y. Lam,a Chunmei Deng,a Ping Lu,d
Herman H-Y. Sung,a Ian D. Williams,a Hoi Sing Kwok,c Yongming Zhangb and
Ben Zhong Tang*ae
Received 9th July 2011, Accepted 1st September 2011
DOI: 10.1039/c1cc14122h
Combination of an aggregation-induced emission (AIE) moiety
and a dimesitylboron group yields a new three-coordinate boron
compound exhibiting a synergistic effect: the resultant TPED-
MesB shows AIE feature with solid-state emission efficiency up
to unity and good electron-transport property, and thus remark-
able electroluminescence (EL) performances.
formation.7 The mechanism of such AIE phenomenon has
been identified as the restriction of intramolecular rotation
(RIR).7b We wondered whether we can create new multifunc-
tional materials through rational simple coupling of different
functional moieties, namely AIE chromophores and charge-
transport segments. Encouragingly, we recently reported the
synthesis of triphenylamine-containing solid emitters with
good hole-transport property and solid-state emission
efficiency up to unity.8 Such a win–win strategy eliminated
the ACQ effect of triphenylamine without sacrificing its hole-
transport property, moreover, giving greatly enhanced emission
efficiencies.8
Design and synthesis of high efficiency solid-state organic
materials is of fundamental importance and technical implications
for various optoelectronic and photonic applications, such as
organic light-emitting diodes (OLEDs),1 organic lasers,2
organic light-emitting transistors (OLETs),3 and sensors.4
Particularly, for OLEDs, development of multifunctional
compounds which are capable of functioning as efficient
emitters as well as charge-transport materials is of crucial
importance to simplify the device configuration, improve the
device duration, enhance the efficiency, and reduce the cost.5
Therefore, intense endeavors have been made to generate
electron- and/or hole-transport emitters in the past decades.6
Whereas many molecules are highly emissive in dilute solutions,
they become weakly fluorescent or non-emissive when aggregated
in poor solvents or fabricated as thin films in the solid state.
This notorious effect of aggregation-caused quenching (ACQ)
has made the development of highly emissive multifunctional
organic luminogens a daunting task.
The success of obtaining hole-transport and highly luminescent
organic solids promoted us to create electron-transport/
emitting bifunctional materials. It is known that electron
mobility in organic materials is generally orders of magnitude
lower than the hole mobility. Therefore, the development of
efficient electron-transport materials plays an important role
in the improvement of the overall OLED device performance.9
So far, several types of electron-transport materials, such as
CQN double bond-containing heterocycles, metal-quinolinol
complexes, fluoro-substituted compounds, cyano-substituted
poly(p-phenylenevinylene)s, and three-coordinated organoborons
have been developed.9–11 Among these, the organoboron
compounds are particularly attractive and promising due to
their unique properties stemming from the pp–p* conjugation
between the vacant p orbital on the boron atom with the p*
orbital of the p-conjugated framework.5,10b,12 Many works
have shown the applications of three-coordinate organoboron
compounds in nonlinear optical organic material, light-emitting
diodes, fluorescent sensing, and so on.4,13–19 Here, we reported
our work on the preparation of electron-transport highly
fluorescent solid emitters based on a new approach through
combination of AIE-active units (TPE) and the dimesitylboron
group. The resulting compound TPEDMesB shows unique
AIE characteristics with a solid state efficiency of 100% as well
as good electron-transport ability. The OLED employing
TPEDMesB as an emitter without an additional electron-
transport layer showed bright bluish-green emission with a
In 2001, our group discovered a novel phenomenon of
aggregation-induced emission (AIE): some propeller-like molecules
such as siloles and tetraphenylethene (TPE) are non-emissive
in solutions but are induced to emit intensely by aggregate
a Department of Chemistry, The Hong Kong University of Science &
Technology, Clear Water Bay, Kowloon, Hong Kong, China.
E-mail: tangbenz@ust.hk
b School of Chemistry and Chemical engineering, Shanghai Jiao Tong
University, Shanghai 200240, China
c Center for Display Research, HKUST, Kowloon, Hong Kong, China
d State Key Laboratory of Supramolecular Structure and Materials,
Jilin University, Changchun 130023, China
e Department of Polymer Science and Engineering, Zhejiang
University, Hangzhou 310027, China
w Electronic supplementary information (ESI) available: Experimental
details, NMR, mass and UV-vis spectra, DSC, TGA curves, other EL
data and cyclic voltammogram. CCDC 833200. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
c1cc14122h
luminous efficiency of 7.13 cd AÀ1
.
The target compound TPEDMesB was obtained in good yield
(82.5%) by the lithiation of 1-(4-bromophenyl)-1,2,2-triphenylethene
c
11216 Chem. Commun., 2011, 47, 11216–11218
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Scheme 1 The synthetic route to TPEDMesB.
(1) at À78 1C followed by reaction with dimesitylboron
fluoride (2) (Scheme 1). Detailed synthetic procedure is
provided in ESIw. TPEDMesB was characterized spectro-
scopically, with satisfactory data obtained (Fig. S1–S3,
Table S1, ESIw).
TPEDMesB shows an absorption peak around 338 nm
(Fig. S4, ESIw). When illuminated under UV light, no emission
is seen from its solutions but intense fluorescence from solid
films was observed, indicating that aggregation has turned on
its light emission. Further photoluminescence (PL) measure-
ment of TPEDMesB in THF–water mixtures duly verified its
AIE-active nature. As shown in Fig. 1, a flat line is record for
its pure THF solution, and even when the water fraction (fw) is
as high as 70%, only a weak PL signal is recorded because the
molecules are still genuinely dissolved in the mixture. How-
ever, when fw 480%, addition of even a small amount of
water gives sharp promotion in the PL intensity. From the
molecular solution in THF to the aggregate suspension in the
90% aqueous mixture, the PL peak intensity of TPEDMesB
at 489 nm is increased by 505 times. Comparison of the
quantum yield (FF) values of TPEDMesB in the solution
and solid states further validates its AIE activity. While the
FF,s value in THF is merely 0.25%, that of its solid films is
boosted to 100%, giving an AIE factor (aAIE = FF,f/FF,s) as
high as 400, which is 6.6-fold higher than that of TPE.20
The single-crystal structure of TPEDMesB was attained to
illustrate the AIE effect. As depicted in Fig. 2A, the TPED-
MesB molecule is highly twisted so when it is dissolved in
solvents, multiple phenyls undergo active intramolecular rotations
which dissipate the exciton energy effectively, making it none-
missive in solution. However, when it is aggregated as nano-
suspensions or solid films, on one hand, the intramolecular
Fig. 2 (A) ORTEP drawing, (B) molecular packing in crystals, and
(C) B3LYP/6-31G (d) calculated molecular orbital amplitude plots of
HOMO and LUMO levels of TPEDMesB.
rotations are impeded, and on the other hand, the propeller-
like configuration prevents the formation of detrimental
excimers or exciplexes via p–p stackings (Fig. 2B), thus
endowing the aggregates with intense emission.
The B3LYP/6-31G (d) calculated HOMO and LUMO levels
of TPEDMesB are shown in Fig. 2C. The electron cloud in
HOMO level is mainly located on the TPE unit, whereas that
of the LUMO level is predominantly located on the boron
atom and the surrounding aromatic rings, revealing the electron
transfer in the molecules.
The thermal properties of TPEDMesB was examined by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements. The results reveal the
decomposition temperature for a 5% weight loss (Td) and
melting temperature (Tm) of TPEDMesB are 359 and 206 1C
(Fig. S6, ESIw), respectively, indicating it is thermally stable.
No glass-transition temperature, however, was detected.
Due to its high thermal stability and efficient film emission,
we then used TPEDMesB to fabricate EL devices. We first
constructed an organic light-emitting diode (device I) with the
configuration of ITO/NPB (60 nm)/TEPDMesB (20 nm)/TPBi
(40 nm)/LiF (1 nm)/Al (100 nm),21 where TPEDMesB serves
as emitter and NPB and TPBi work as hole-transport and
electron-transport layers, respectively. The EL spectrum of the
device peaks at 496 nm (Fig. 3), which is slightly red-shifted
from the PL peaks of the thin film (478 nm) and nanoaggre-
gates (489 nm) of TPEDMesB, but far from the crystal
emission (463 nm), indicating that the EL emission should
be from the amorphous TPEDMesB layer. The device perfor-
mance is good (Table 1 and Fig. S7, ESIw), with maximum
luminance (Lmax), maximum current efficiency (CEmax), power
efficiency (PEmax), and eternal quantum efficiency (EQE) being
5581 cd mÀ2, 5.78 cd AÀ1, 3.2 lm WÀ1 and 2.3%, respectively.
Due to the electron-deficient nature of boron atom, three-
coordinated boron compounds can be used as efficient electron-
transport materials.12 To check whether TPEDMesB shows
good electron-transport property, we fabricated device II, in
which the TPBi layer was eliminated. The EL performances are
shown in Fig. 3 and summarized in Table 1. Compared to
device I, device II is turned on by the same voltage (Von = 6.3 V)
with similar EL spectrum profile and comparable intensity
Fig. 1 (A) PL spectra of TPEDMesB in THF and THF–water
mixtures. (B) Plot of PL peak intensity at 489 nm vs. water fraction
(fw) of the aqueous mixture. Luminogen concentration: 20 mm;
excitation wavelength: 360 nm. Inset: solution of TPEDMesB in
THF (fw = 0%) and its suspension in a THF–water mixture with
fw = 90%; photographs taken under UV illumination.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11216–11218 11217

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compounds with AIE activities and high solid-state efficiencies
is under way in our lab.
We thank the support from the Research Grants Council of
Hong Kong (603509, HKUST2/CRF/10, and 604711).
Notes and references
1 (a) S. R. Forrest, Org. Electron., 2003, 4, 45; (b) M. Shimizu and
T. Hiyama, Chem.–Asian J., 2010, 5, 1516.
2 (a) M. D. McGehee and A. J. Heeger, Adv. Mater., 2000, 12, 1655;
(b) I. D. W. Samuel and G. A. Turnbull, Chem. Rev., 2007,
107, 1272.
3 (a) J. Zaumseil and H. Sirringhaus, Chem. Rev., 2007, 107, 1296;
(b) F. Cicoira and C. Santato, Adv. Funct. Mater., 2007, 17, 3421.
4 X. Y. Liu, D. R. Bai and S. Wang, Angew. Chem., 2006, 118, 5601.
5 S.-L. Lin, L.-H. Chan, R.-H. Lee, M.-Y. Yen, W.-J. Kuo,
C.-T. Chen and R.-J. Jeng, Adv. Mater., 2008, 20, 3947.
6 (a) Y. Shirota, M. Kinoshita, T. Noda, K. Okumoto and T. Ohara,
J. Am. Chem. Soc., 2000, 122, 11021; (b) R.-Y. Wang, W.-L. Jia,
H. Aziz, G. Vamvounis, S. Wang, N.-X. Hu, Z. D. Popovic and
J. A. Coggan, Adv. Funct. Mater., 2005, 15, 1483; (c) H. Zhang,
C. Huo, J. Zhang, P. Zhang, W. Tian and Y. Wang, Chem.
Commun., 2006, 281; (d) M. Kinoshita, H. Kita and Y. Shirota,
Adv. Funct. Mater., 2002, 12, 780.
Fig. 3 (A) PL and EL spectra of TPEDMesB, (B) current efficien-
cy–luminance–voltage plots of its multilayer light-emitting diodes with
a general device configuration of ITO/NPB/X/LiF/Al [X = TPED-
MesB/TPBi (device I), X = TPEDMesB (device II)], and the energy
diagrams of (C) device I and (D) device II.
7 (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; (c) Q. Zeng, Z. Li, Y. Dong, C. Di,
A. Qin, Y. Hong, L. Ji, Z. Zhu, C. K. W. Jim, G. Yu, Q. Li, Z. Li,
Y. Liu, J. Qin and B. Z. Tang, Chem. Commun., 2007, 70;
(d) S. Dong, Z. Li and J. Qin, J. Phys. Chem. B, 2009, 113, 434;
(e) Z. Li, Y. Q. Dong, J. W. Y. Lam, J. Sun, A. Qin, M. Haußler,
Y. P. Dong, H. H. Y. Sung, I. D. Williams, H. S. Kwok and
B. Z. Tang, Adv. Funct. Mater., 2009, 19, 905.
8 W. Z. Yuan, P. Lu, S. Chen, J. W. Y. Lam, Z. Wang, Y. Liu,
H. S. Kwok, Y. Ma and B. Z. Tang, Adv. Mater., 2010, 22, 2159.
9 A. P. Kulkarni, C. J. Tonzola, A. Babel and S. A. Jenekhe, Chem.
Mater., 2004, 16, 4556, and references therein.
10 (a) D. Izuhara and T. M. Swager, J. Mater. Chem., 2011, 21, 3579;
(b) Z. M. Hudson and S. Wang, Dalton Trans., 2011, 40, 7805, and
references therein.
Table 1 EL performances of TPEDMesB based devicesa
lmax
Device nm
/
Von
V
/
Lmax/cd
mÀ2
PEmax/lm
WÀ1
CEmax/cd
AÀ1
EQE
(%)
I
496,
512
496,
512
6.3 5581
6.3 5170
3.4
3.2
5.78
7.13
2.3
2.7
II
a
Device configuration: ITO/NPB (60 nm)/X/LiF (1 nm)/Al (100 nm);
for device I: X = TPEDMesB (20 nm)/TPBi (40 nm); for device II:
X = TPEDMesB (60 nm); abbreviations: lmax = EL peak, Von
=
turn on voltage, Lmax = maximum luminance, PEmax = maximum
power efficiency, CEmax = maximum current efficiency, and EQE =
maximum external quantum efficiency.
11 Y. Shirota and H. Kageyama, Chem. Rev., 2007, 107, 953.
12 (a) P. Chen, R. A. Lalancette and F. Jakle, J. Am. Chem. Soc.,
2011, 133, 8802; (b) C. D. Entwistle and T. B. Marder, Angew.
Chem., Int. Ed., 2002, 41, 2927; (c) C. D. Entwistle and
T. B. Marder, Chem. Mater., 2004, 16, 4574.
(Lmax = 5170 cd cmÀ2); Moreover, it emits even more
efficiently (CEmax = 7.13 cd AÀ1) due to the lower contact
resistance. Evidently, TPEDMesB is serving as emissive com-
ponent as well as electron-transport material in the EL device,
which should be ascribed to an optimal energy match and
charge-balance of the device, as can be seen from the energy
alignment (Fig. 3C). The bifunctional property of TPED-
MesB helps to simplify device structure, shorten fabrication
process and lower production cost, and moreover enhance the
device performance.
13 H. Doi, M. Kinoshita, K. Okumoto and Y. Shirota, Chem. Mater.,
2003, 15, 1080.
14 (a) A. Wakamiya, K. Mori and S. Yamaguchi, Angew. Chem., Int.
Ed., 2007, 46, 4273; (b) S. Yamaguchi, T. Shirasaka and K. Tamao,
Org. Lett., 2000, 2, 4129; (c) C.-H. Zhao, A. Wakamiya, Y. Inukai
and S. Yamaguchi, J. Am. Chem. Soc., 2006, 128, 15934;
(d) M. Elbing and G. C. Bazan, Angew. Chem., Int. Ed., 2008,
47, 834.
15 A. Lorbach, M. Bolte, H. Li, H.-W. Lerner, M. C. Holthausen,
F. Jakle and M. Wagner, Angew. Chem., Int. Ed., 2009, 48, 4584.
16 N. Matsumi, K. Naka and Y. Chujo, J. Am. Chem. Soc., 1998,
120, 5112.
17 Y. Zhou, J. W. Kim, R. Nandhakumar, M. J. Kim, E. Cho,
Y. S. Kim, Y. H. Jang, C. Lee, S. Han, K. M. Kim, J.-J. Kim
and J. Yoon, Chem. Commun., 2010, 46, 6512.
18 Z. Yuan, J. C. Collings, N. J. Taylor, T. B. Marder, C. Jardin and
J. F. Halet, J. Solid State Chem., 2000, 154, 5.
19 A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891.
20 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.
21 ITO = indium tin oxide; NPB = 1,4-bis[(1-naphthylphenyl)-
amino]biphenyl (hole-transport layer); TPBi = 1,3,5-tris(N-phenyl-
benzimidazol-2-yl)benzene (electron-transport layer).
In summary, through combination of a typical AIE luminogen
TPE and dimesitylboron moiety, a new bluish-green emitter
TPEDMesB with AIE activity and excellent electron-transport
property has been obtained. This molecule has been demon-
strated to be a promising bifunctional material in the fabrication
of a non-doped EL device, giving maximum current efficiency
of 7.13 cd AÀ1. Moreover, the work presented herein shows a
new strategy on creating novel electron-transport high effi-
ciency solid organics. The expansion of applicability of such
strategy to create further multifunctional materials, such as
oxadiazole- and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
(BODIPY)-containing organics, and the study of ambipolar
c
11218 Chem. Commun., 2011, 47, 11216–11218
This journal is The Royal Society of Chemistry 2011