Synthesis of blue light emitting bis(triphenylethylene) derivatives: A case of aggregation-induced emission enhancement

Article abstract of DOI:10.1016/j.dyepig.2010.09.003

A series of new aggregation-induced emission enhancement derivatives from bis(triphenylethylene) with strong blue-light-emitting properties and high thermal stability have been synthesized. Their maximum fluorescence emission wavelengths range between 464

Full text of DOI:10.1016/j.dyepig.2010.09.003

Dyes and Pigments 89 (2011) 56e62
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of blue light emitting bis(triphenylethylene) derivatives: A case
of aggregation-induced emission enhancement
*
*
Xiqi Zhang, Zhenguo Chi , Bingjia Xu, Haiyin Li, Zhiyong Yang, Xiaofang Li, Siwei Liu, Yi Zhang, Jiarui Xu
PCFM Lab and DSAPM Lab, OFCM Institute, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering,
Sun Yat-sen University, Guangzhou 510275, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new aggregation-induced emission enhancement derivatives from bis(triphenylethylene)
with strong blue-light-emitting properties and high thermal stability have been synthesized. Their
maximum fluorescence emission wavelengths range between 464e468 nm, with fluorescence quantum
yields of 0.58e0.88. Their glass transition temperatures range from 125 ꢀC to 178 ꢀC. The obtained
experimental results demonstrate the different aggregation-induced emission enhancement phenomena
caused by the effect of solvent and formation of both H- and J- aggregation states. An emitting device was
fabricated using a bis(triphenylene) derivative in the emitting layer which exhibited a luminance effi-
ciency of up to 2 cd/A with a maximum brightness of 548 cd/m2.
Received 29 March 2010
Received in revised form
18 August 2010
Accepted 9 September 2010
Available online 18 September 2010
Keywords:
Aggregation-induced emission
enhancement
Ó 2010 Elsevier Ltd. All rights reserved.
High glass transition temperature
Blue light emission
Bis(triphenylethylene) derivatives
Device
Synthesis
1. Introduction
fabrication of electroluminescent devices with high efficiency and
stimuli-responsiveness for use in multifunctional switches [14,15].
In the past decade, organic luminophores have become of great
value to the fields of physiology, chemical engineering and display
technology due to their applications as biological probes, chemical
sensors, and organic light-emitting diodes [1e4]. However, prob-
lems related to the production of blue light-emitting materials with
strong brightness, long lifetimes and high glass transition
temperatures (Tg) have suppressed their application on an even
wider scale.
Most luminescent materials exhibit relatively weak emissions in
the solid state due to the aggregation of molecules leading to the
formation of excimers, which results in emission quenching [5,6].
Thus, considerable effort has been exerted in the search for
methods to either suppress the quenching of organic luminophores
or to produce significant enhancements in their light emission
upon aggregation. In line with this, two novel processes have been
identified: (1) aggregation-induced emission enhancement (AIEE)
and (2) aggregation-induced emission (AIE) [7e13]. AIEE (or AIE)
materials have been found to be promising emitters in the
The Tg temperature of an organic luminophore is one of the
important factors influencing the stability and lifetime of a device.
If a device is heated above the Tg of the organic luminophore,
irreversible failure can occur.
In aggregates, strong clustering of aromatic chromophores is
often observed with the spectral shift of a maximum absorption
band. In aromatic moieties, the blue shift is usually assigned to the
parallel head-to-head alignment of aromatic chromophores, also
referred to as H-aggregation. In contrast, a red shift is normally
observed when chromophores are aggregated in a head-to-tail
manner, which is referred to as J-aggregation. The aggregation state
of aromatic chromophores can greatly influence the morphologies
of the formed aggregates, as well as their absorption and fluores-
cence spectra [16e19].
We have already reported a new class of triphenylethylene
carbazole derivatives that display strong blue light emissions, high
Tg temperatures and AIE effects [20]. For comparison, in this work
a new class of bis(triphenylethylene) derivatives were synthesized.
These derivatives also show strong blue light emissions and high Tg
temperatures, but unlike their earlier counterparts, they are AIEE-
active. It is expected that these novel compounds can be used as
potential materials for blue-light emitters in luminescent devices.
* Corresponding authors. Tel.: þ86 20 84112712; fax: þ86 20 84112222.
E-mail addresses: chizhg@mail.sysu.edu.cn (Z. Chi), xjr@mail.sysu.edu.cn (J. Xu).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.09.003

X. Zhang et al. / Dyes and Pigments 89 (2011) 56e62
57
2. Experimental
thermal evaporation. Compound C4 (30 nm thick layer) was evap-
orated on the NPD layer and a 20 nm thick 8-hydroxyquinoline
aluminum (Alq3) layer was then deposited as the electron trans-
porting layer. Finally LiF (1 nm) and Al (100 nm) were deposited on
top of the organic layers by thermal evaporation. The fabricated
multilayer organic light emitting devices possessed the structure
of ITO/NPB (50 nm)/Sample(30 nm)/Alq3(20 nm)/LiF (1 nm)/Al
(100 nm). Each layer was deposited at >10ꢁ5 Pa with BOC Edwards
Auto 500 box chamber system. The thickness of layer was taken by
Dektak 6 M profilometer (Veeco). EL spectrum at 10 V electric
voltage was recorded using F-4500 fluorescence spectrometer
(Hitachi). The current-voltage-luminance characteristics were
determined in ambient atmosphere using a Matrix DC power
supply MPS-3002L-3 (Matrix Technology Inc, China) and a Minolta
LS-110 meter (Konica Minolta Sensing, Japan). The currents were
determined by UNI-T (UT71 series) intelligent digital multimeter
(Uni-Trend Group Limited, Hongkong).
2.1. Materials and measurements
All reagents and chemicals were purchased from Alfa-Aesar Ltd.
and used as received. Analytical grade DMF was purified by distil-
lation under an inert nitrogen atmosphere. Tetrahydrofuran (THF)
was distilled from sodium/benzophenone. Ultra-pure water was
used in the experiments. All other solvents as analytical grade were
purchased from Guangzhou Dongzheng Company and used
without further purification. The intermediates 1 and 2 were
prepared according to the literature methods [20,21], and 3 was
obtained in a similar manner. 1,4-Bis(diethoxyphosphinylmethyl)
benzene was synthesized according to the literature method
starting from 4,40-bis(bromomethyl)biphenyl [22]. Aliquat 336
(tricaprylylmethylammonium chloride) used as phase transfer
agent in the Suzuki reaction is both harmful and an irritant, may
cause irreversible damage to the eyes and is also toxic to aquatic
organisms leading to potential long term adverse effects to the
aquatic environment. Be careful to use it.
2.2. Preparation of aggregates [24]
1H-NMR was measured on a Varian Mercury-Plus 300 spec-
trometer with chemical shifts reported as ppm (in CDCl3 or DMSO-
d6, TMS as internal standard). Mass spectra were measured on
a Thermo MAT95XP-HRMS spectrometer. Elemental analyses were
performed with an Elementar Vario EL Elemental Analyzer. UVeVis
absorption spectra (UV) were recorded on a Hitachi UVeVis spec-
trophotometer (U-3900). Fluorescence spectra (PL) were deter-
mined with a Shimadzu RF-5301PC spectrometer and the slit widths
were 1.5 nm for excitation and 3.0 nm for emission. To measure the
PL spectra in thin-layer chromatography (TLC) plates, aluminum TLC
plates (Merck, Silica 60 F254) were used. The samples in TLC plates
were prepared and determined according to the literature [9]. The
fluorescence quantum yields (FFL) were measured by the standard
optically diluted method in degassed dichloromethane solutions
Stock THF solutions of the synthesized compounds with
a concentration of 0.1 mM were prepared. 1 mL of this stock solu-
tion was transferred to a 10 mL volumetric flask. After adding an
appropriate amount of THF, Ultra-pure water was added dropwise
under ultrasound treatment to prepare a 10 mM solution in a THF/
H2O mixture with a specific water fraction. Absorption and emis-
sion spectra of the resulting solutions and aggregates were
measured immediately after the sample preparation.
2.3. Synthesis of intermediate 3
A solution of dibiphenyl-4-ylmethanone 3.3 g (10 mmol) and
diethyl 4-bromobenzylphosphonate 3.1 g (10 mmol) in anhydrous
tetrahydrofuran (40 mL) was stirred under a N2 atmosphere at 0 ꢀC.
Postassium tert-butoxide 1.1 g (10 mmol) was added quickly and
the mixture was stirred continuously for 2 h at room temperature.
The reaction mixture was precipitated into ethanol. The precipitate
obtained was filtered, and washed with ethanol three times to
afford 3.9 g product as a white powder (80% yield). 1H NMR
(ca. 10ꢁ5 M) using 9,10-diphenylanthracene
(
FFL ¼ 0.90, in
dichloromethane) as a reference standard [23]. Differential scan-
ning calorimetry (DSC) curves were obtained with a TA thermal
analyzer (Q10) at a heating rate of 10 ꢀC/min under N2 atmosphere.
Thermogravimetric analyses (TGA) were performed with a TA
thermal analyzer (A50) under N2 atmosphere with a heating rate of
20 ꢀC/min. The sizes of the particles of the compounds in THF/water
mixtures were determined using a ZetaPALS dynamic light scat-
tering system (Brookhaven, ZetaPALS Zeta Potential Analyzer).
Cyclic voltammetry (CV) measurements were carried out on
a Shanghai Chenhua electrochemical workstation (CHI660C) in
a three-electrode cell with a Pt disk working electrode, an Ag/AgCl
reference electrode, and a glassy carbon counter electrode. All CV
measurements were performed under an inert argon atmosphere
with supporting electrolyte of 0.1 M tetrabutylammonium
perchlorate (n-Bu4NClO4) in dichloromethane at scan rate of
100 mV/s using ferrocene as standard. The HOMO energy levels
were obtained using the onset oxidation potentials from the CV
curves. The lowest unoccupied molecular orbital/highest occupied
(300 MHz, DMSO-d6)
d: 7.70e7.59 (m, 6H), 7.59e7.55 (d, 2H),
7.51e7.46 (d, 2H), 7.46e7.40 (d, 4H), 7.40e7.34 (m, 2H),7.32e7.26
(m, 4H), 7.00e6.96 (d, 2H), 6 .95 (s, 1H); MS (EI), m/z: 486 ([M]þ,
calcd for C32H23Br, 486); Anal. calc. for C32H23Br: C, 78.85; H, 4.76;
Br, 16.39 Found: C, 78.79; H, 4.72.
2.4. Synthesis of C2B
To a stirred solution of 2 (6.0 g, 9.0 mmol) in anhydrous THF
(100 mL) was added n-butyllithium solution in hexane (2.2 M,
6 mL, 13.2 mmol) dropwise slowly at ꢁ78 ꢀC. The mixture was
stirred at -78 ꢀC under Ar gas for an additional 5 h. B(OCH3)3
(2.2 mL) was added quickly at ꢁ78 ꢀC and the mixture was stirred
overnight allowing the temperature to rise gradually to room
temperature. Water (40 mL) was added and then acidified with
concentrated HCl. The mixture was stirred for a further 2 h. The
product was extracted into ethyl acetate (3 ꢂ 50 mL). The organic
layer was separated and dried over anhydrous sodium sulfate. After
removing the solvent under reduced pressure, the residue was
chromatographed on a silica gel column with acetone/dichloro-
methane (1:20 by volume) as eluent to give C2B (2.1 g, 55% yield).
molecular orbital (LUMO/HOMO) energy gaps
DEg for the
compounds were estimated from the onset absorption wavelengths
of the UV absorption spectra.
2.1.1. Device fabrication and measurement
The glass substrate pre-coated with indium-tin-oxide (ITO) was
cleaned by an ultrasonic bath of acetone, ethanol and deionized
water, and then dried in an oven at 80 ꢀC. Surface treatment was
carried out by exposing ITO to UV-ozone plasma. The electrolu-
minescence (EL) device was fabricated as follows. The hole-trans-
porting layer, a 50 nm thick film of 4,40-bis(1-naphthylphenylamino)
biphenyl (NPD) was deposited on the ITO surface by high vacuum
1H NMR (300 MHz, DMSO-d6)
(m, 16 H), 7.62e7.78 (m, 8 H), 7.98 (s, 2 H, -B(OH)2), 8.20e8.30 (m, 4
H, carbazole-H); MS (EI), m/z: 586 ([M-(B(OH)2)]þ, calcd for C44H29
d: 7.14 (s, 1 H, >C]CHe), 7.25e7.58
,
585); Anal. calc. for C44H31BN2O2: C, 83.81; H, 4.96; N, 4.44 Found:
C, 83.64; H, 4.89; N, 4.48.

58
X. Zhang et al. / Dyes and Pigments 89 (2011) 56e62
2.5. Synthesis of C2B2
chromatographed on a silica gel column with n-hexane/dichloro-
methane (2:1 by volume) as eluent to give B4 (0.10 g, 28% yield). 1H
NMR (300 MHz, CDCl3) : 7.06 (s, 2 H,>C]CHe), 7.09-7.17 (d, 4 H,
J ¼ 8.4 Hz), 7.30e7.51 (m, 24 H), 7.54e7.69 (m, 16 H); MS (EI), m/z:
814 ([M]þ, calcd for C64H46, 814); Anal. calc. for C64H46: C, 94.31; H,
5.69 Found: C, 94.47; H, 5.63.
1-Bromo-4-(2,2-di(biphenyl)ethenyl)benzene (3) (0.20 g, 0.41
mmol) and C2B (0.31 g, 0.49 mmol) were dissolved in the mixture
of toluene (30 mL), tricaprylylmethylammonium chloride (Aliquat
336) (5 drops) and 2 M potassium carbonate aqueous solution
(8 mL). The mixture was stirred at room temperature for 0.5 h
under Ar gas followed adding Pd(PPh3)4 (0.010 g, 8.70 ꢂ 10ꢁ3
mmol) and then heated to 90 ꢀC for 24 h. After that the mixture was
poured into water and extracted three times with ethyl acetate. The
organic layer was dried over anhydrous sodium sulfate. After
removing the solvent under reduced pressure, the residue was
chromatographed on a silica gel column with n-hexane/dichloro-
methane (2:1 by volume) as eluent to give C2B2 (0.20 g, 49% yield).
d
3. Results and discussion
3.1. Synthesis
As illustrated in Scheme 1, ketone 1 was converted into the
triarylethene 2 using Wadsworth-Emmons chemistry as previously
described starting from carbazole [20,21]. The related bis(biphenyl)
bromoarylethene 3 was obtained in a similar manner starting from
dibiphenyl-4-ylmethanone. C2B2 was constructed using the palla-
dium (Pd)-catalyzed Suzuki coupling reaction of 3 with the inter-
mediate compound C2B, which was prepared from 2, n-BuLi and B
(OCH3)3.
1H NMR (300 MHz, CDCl3)
d: 7.08 (s, 1 H,>C]CHe), 7.16 (s, 1
H,>C]CHe), 7.18e7.24 (m, 4 H), 7.27e7.39 (m, 8 H), 7.40e7.71 (m,
34 H), 8.16 (d, 4 H, J ¼ 7.8 Hz, carbazole-H); MS (EI), m/z: 992 ([M]þ,
calcd for C76H52N2, 992); Anal. calc. for C88H58N4: C, 91.90; H, 5.28;
N, 2.82 Found: C, 91.73; H, 5.23; N, 2.90.
Compound C4 was synthesized via the WittigeHorner reaction
of intermediate 1 and 1,4-bis(diethoxyphosphinylmethyl)benzene.
Although C4 has appeared in two patents [25,26], to the best of our
knowledge, the properties of the compound have not been well
studied, especially the AIEE properties. Compound B4 was prepared
from 1,4-bis(diethoxyphosphinylmethyl) benzene and 4,40-diphe-
nylbenzophenone under the same reaction conditions (Scheme 2).
2.6. Synthesis of C4
1,4-Bis(diethoxyphosphinylmethyl)benzene (0.90 g, 2.0 mmol)
and 1 (2.6 g, 5.0 mmol) were dissolved in THF (60 mL), then t-BuOK
(0.56 g) was added under Ar gas. The solution was stirred at room
temperature overnight. After removing the solvent under reduced
pressure, the residue was recrystallized with THF/EtOH to give C4
(1.7 g, 75% yield). 1H NMR (300 MHz, CDCl3)
d: 7.24 (s, 2 H, >C]
3.2. Thermal properties
CHe), 7.25e7.35 (m, 12 H), 7.43e7.48 (m, 8 H), 7.52e7.55 (m, 12 H),
7.58e7.72 (m, 16 H), 8.18 (d, 8 H, J ¼ 8.0 Hz, carbazole-H); MS (FAB),
m/z: 1171 ([MþH]þ, calcd for C88H58N4, 1171); Anal. calc. for
C88H58N4: C, 90.23; H, 4.99; N, 4.78. Found: C, 90.04; H, 4.70; N, 5.01.
The thermal and photophysical properties of the synthesized
compounds C4, C2B2 and B4 are summarized in Table 1. The thermal
properties were investigated by TGA and DSC. As can be seen from
Table 1 and Fig. S1, relatively high Tg temperatures ranging from 125
to 178 ꢀC were observed. These Tg values were much higher than
those reported for 4,40-bis(2,2-diphenylvinyl)-1,10-biphenyl (DPVBi,
Scheme 2) (64 ꢀC) [27], which might be attributed to the combined
presence of the carbazolyl and phenyl substituent groups in the
synthesized derivatives. It was also observed that the Tg of the
compound containing four carbazolyl groups (C4) was higher than
that of the corresponding compound containing two carbazolyl
2.7. Synthesis of B4
1,4-Bis(diethoxyphosphinylmethyl)benzene (0.20 g, 0.44 mmol)
and 4,40-diphenylbenzophenone (0.37 g, 1.1 mmol) were dissolved
in THF (20 mL), then t-BuOK (0.20 g) was added under Ar gas. The
solution was stirred at room temperature overnight. After
removing the solvent under reduced pressure, the residue was
Scheme 1. Synthetic routes of C2B2.

X. Zhang et al. / Dyes and Pigments 89 (2011) 56e62
59
emission wavelengths in cyclohexane and dichloromethane solu-
tions appeared at ca. 463 nm and at ca.467 nm, respectively (Table
1, Fig. S4). Despite the difference of the substituents in compounds
C4, C2B2 and B4, the maximum absorption wavelengths and the
maximum emission wavelengths of these compounds were very
similar in solution. In comparison to the data recorded in dilute
solutions, the maximum emission wavelengths of the compounds
on TLC plates were very close to those in solutions. However, the
maximum emission wavelengths of compounds in powder states
were significantly red-shifted compared to those determined for
solutions and on TLC plates (Fig. S5).
As shown in Fig. 1, a decreasing trend can be observed in the
absorption of C4 to C2B2 to B4 at 344 nm. The molar absorption
coefficients (
3
) of C4, C2B2 and B4 at 344 nm are 1.426 ꢂ 105,
1.171 ꢂ105 and 0.817 ꢂ 105 L molꢁ1 cmꢁ1 in dichloromethane. The
disappearance of 344 nm absorption peak in the UV spectrum of
compound B4 indicates that the 344 nm wavelength corresponds to
the absorption of carbazolyl groups. As the number of carbazolyl
groups for C4, C2B2 and B4 decreased from four to two to zero, the
absorption at 344 nm gradually weakened.
Scheme 2. Synthetic routes of C4 and B4, and the structures of DPVBi, MPPS and HPS.
The fluorescence quantum yields (FFL) were determined using
9,10-diphenylanthracene as a standard. It was observed that the
compounds had rather high quantum yields, and the FFL values
ranged from 0.58 to 0.88. The FFL values increased with the
decrease of carbazolyl groups.
groups and two phenyl groups (C2B2), which in turn was higher
than that of the compound containing four phenyl groups (B4). It is
well-known that carbazole group possesses a bulky rigid planar
molecular structure, which helps to effectively decrease rotation
and increase thermal stability. Thus, the differences in Tg of these
compounds were attributed to the introduction of carbazolyl
groups.
It is interesting to note that the melting-point temperatures (Tm)
of compounds C4 and B4 could be obtained by DSC at 363 ꢀC and
298 ꢀC, respectively (Fig. S2). However, no melting point could be
observed for C2B2 using DSC. The reason is that the asymmetric
structure of C2B2 influences the molecular tight packing in the solid
state, thus forming an amorphous structure.
The thermal decomposition temperatures (Td) of these
compounds (corresponding to 5% weight loss under N2 atmo-
sphere) were in the range of 468e535 ꢀC (Table 1 and Fig. S3). The
Td values of the compounds were higher than those of the reported
AIEE-active compounds, 1-methyl-1,2,3,4,5-pentaphenylsilole
(MPPS, see Scheme 2) (309 ꢀC) and 1,1,2,3,4,5-hexaphenylsilole
(HPS, see Scheme 2) (351 ꢀC) [28]. The results indicate that the
compounds have very high levels of thermal stability. The thermal
stability of organic compounds is critical to the stability and life-
time of photoelectric devices. Thus, the high level of thermal
stability would be beneficial if the compounds were to be used in
photoelectric materials.
Cyclic voltammetry (CV) analyses were carried out to measure
the HOMO values of the synthesized compounds. According to the
CV data, the HOMO levels of compounds C4, C2B2 and B4 were
5.21 eV, 5.31 eV and 5.60 eV, respectively. These results indicate
that the increase in the number of carbazolyl groups decreases the
HOMO level of the molecule and lowers the barrier to hole injection
from the most widely used anode material, indium tin oxide (ITO),
which has a work function of 5.0 eV. The energy band gaps of the
compounds were estimated by analyzing absorption edge with
a plot of UV curve. The band gaps of C4, C2B2 and B4 were found to
be 2.98 eV, 2.96 eV and 2.93 eV, respectively. The LUMO levels of C4,
C2B2 and B4 calculated by HOMO- Eg were 2.23, 2.35 and 2.67 eV,
respectively.
3.4. AIEE properties
To determine whether these three new compounds are AIEE
active, the fluorescence behaviour of their diluted mixtures was
studied in a mixture of water/THF under different water fractions.
3.3. Photophysical and electrochemical properties
2.0
1.5
1.0
0.5
0.0
1
2
3
C
4
C B
2
2
344nm
The maximum UV absorption wavelengths of the compounds
were ca. 367 nm in dichloromethane and their maximum PL
B
4
1
2
Table 1
3
Thermal and photophysical properties of the compounds.
abs
em
l
a
e
Compd Tg (ꢀC) Tm (ꢀC) Td (ꢀC)
l
ðnmÞ
fFL
max
max
ðnmÞ
MCa CHb TLCc Powderd
C4
C2B2
B4
178
149
125
363
NA
298
535
493
468
368
367
368
462 466 464 479
464 467 465 490
464 468 468 496
0.58
0.75
0.88
a
Measured in dichloromethane solution.
Measured in cyclohexane solution.
Measured in TLC plate.
Measured in powder state.
Fluorescence quantum yields, measured in cyclohexane solution using 9,10-
b
c
300
400
500
600
d
e
Wavelength / nm
diphenylanthracene (fFLðrefÞ ¼ 0.90, in dichloromethane) as standard, excited at
365 nm.
Fig. 1. UV spectra of C4, C2B2, and B4 in dichloromethane solutions.

60
X. Zhang et al. / Dyes and Pigments 89 (2011) 56e62
Since the compounds were insoluble in water, increasing the water
fraction in the mixed solvent could change their existing forms
from a solution or well-dispersed state in THF to the aggregated
particles in the aq. THF with a high water content.
dynamic light scattering correlate with the water/THF ratio (Fig. 3,
Table S1). When the water fraction was up to 60%, the particles with
dimensions in the micro- or nano-scale could be detected. With the
further increasing proportion of water, the effective diameter of the
particles decreased (For example, for compound C4, water/THF
60:40, 1131 nm; 70:30, 362 nm; 80:20, 356 nm; 90:10, 240 nm). It
is also important to note, from the dynamic light scattering
measurements, that the nanoparticles could not form when the
water/THF ratio is lower than 60:40. This ratio is close to the ratio
that the PL emission intensity abruptly enhanced.
Changes in the PL peak intensities versus water fraction of the
mixture for compound C4 were plotted, as shown in Fig. 2 as an
example. When the water fraction was increased from 0% to 80%,
the fluorescence intensity of C4 was correspondingly enhanced 4.7-
fold. Similar enhancement was observed in the behaviour of the
remaining two compounds (Figs. S6 and S7). When the water
fraction was increased from 0% to 80%, the fluorescence intensities
of C2B2 and B4 were enhanced 4.2-fold and 2.3-fold, respectively.
This increase in fluorescence intensity was considered to be a result
of the AIEE effect. As aggregates formed the restriction of inter-
molecular rotation increased which led to increased fluorescence
emission. The results indicate that the AIEE effects of the
compounds increased along with increases in the number of car-
bazolyl groups that were present in the molecules.
As can be seen, after reaching a maximum intensity at 60% (for
C4 and C2B2 systems) or 80% (for B4 system) H2O content the PL
intensity of the three compounds decreases with increasing water
content. This phenomenon was often observed in some compounds
with AIEE properties, but the reasons remain unclear. There are two
possible explanations for this phenomenon: 1) after the aggrega-
tion, only the molecules on the surface of the nanoparticles emitted
light and contributed to the fluorescent intensity upon excitation,
leading to a decrease in fluorescent intensity. However, the
restriction of intramolecular rotations of the aromatic rings around
the carbon-carbon single bonds in the aggregation state could
enhance light emission. The net outcome of these antagonistic
processes depends on which process plays a predominant role in
affecting the fluorescent behaviour of the aggregated molecules
[29]; 2) when water is added, the solute molecules can aggregate
into two kinds of nanoparticle suspensions: crystal particles and
amorphous particles. The former leads to an enhancement in the PL
intensity, while the latter leads to a reduction in intensity [20].
Thus, the measured overall PL intensity data depends on the
combined actions of the two kinds of nanoparticles. However, it is
hard to control the formation of nanoparticles in high water
content. Thus, the measured PL intensity often shows no regularity
in high water content.
The absorption spectra of the compounds in the water/THF
mixtures (10 mM) are shown in Fig. 4, Figs. S8 and S9. The spectral
profile was significantly changed when >50% water was added to
the THF solution. The entire spectrum started to increase in
absorption, indicating the formation of nanoscopic aggregates of
the compounds. The light scattering, or Mie effect, of the nano-
aggregates suspensions in the solvent mixtures effectively
decreased light transmission in the mixture and caused the
apparent high absorbance and level-off tail in the visible region of
the UV absorption spectrum [30,31].
With increasing water content, the maximum absorption bands
of B4 showed blue shifts, indicating the formation of H-aggregates
[16]. However, under the same condition, the maximum absorption
bands of C4 and C2B2 showed red shifts, suggesting the formation of
J-aggregates. When the water fraction in the solvent increased from
0% to 90%, the UV absorption wavelength of B4 blue-shifted from
370 nm to 359 nm (an 11-nm blue shift), while C4 and C2B2 showed
red shifts from 370 nm to 402 nm (a 32-nm red shift) and from
368 nm to 376 nm (an 8-nm red shift), respectively. The H-aggre-
gates of B4 and J-aggregates of C4 and C2B2 may have been derived
from the molecular architecture of the above-mentioned
compounds, which could influence the aggregation state in two
aspects: (1) The distance of the adjacent molecules decreased with
the increase of phenyl groups and decrease of carbazolyl groups in
the compounds, respectively; (2) The twisted structure of C4 and
C2B2 prevented face-to-face
pep stacking interactions along the
long molecular axis by the bulky carbazolyl groups, which tended
to favor J-aggregation instead of H-aggregation in the solid state
(Fig. 5). Due to the difference of number of carbazolyl groups in C4
and C2B2, the steric hindrance effects would result in differences in
molecular spatial structures, then affect their aggregation states.
It could be seen from the PL spectra of C4, C2B2 and B4 in THF
with varying amounts of water when the water fraction was
It is noteworthy that the diameters of the particles of the
compounds formed in the water/THF mixtures measured by
350
C
4
C4
C2B2
B4
300
300
1600
250
200
150
100
50
Water fraction / %
250
200
150
100
50
0
10
20
30
50
60
70
80
90
1200
800
0
0
20
40
60
80
100
Water fraction / vol%
400
0
350
400
450
500
550
600
650
700
60
70
80
90
Wavelength / nm
Water fraction / vol %
Fig. 2. PL spectra of C4 (10
volume) (The inset depicts the changes of PL peak intensiy with water fraction).
m
M) in THF with varying amounts of water (% fraction of
Fig. 3. Effective diameter (ED) determined by dynamic light scattering of the
compounds (10 M) in water/THF mixtures as a function of water fraction.
m

X. Zhang et al. / Dyes and Pigments 89 (2011) 56e62
61
Fig. 6. OLED device configuration (left) and the image of the device taken at 10 V
voltage (right).
3.5. Device fabrication and electroluminescence properties
The strong blue light emission and high thermal stability of the
compounds prompted us to use them to construct electrolumi-
nescence (EL) devices. As an example, a multilayer electrolumi-
nescence device, ITO/NPB (50 nm)/Sample(30 nm)/Alq3(20 nm)/
LiF (1 nm)/Al(100 nm), was fabricated using vapor deposition
processes. 4,40-Bis(1-naphthylphenylamino)biphenyl (NPB) was
used as the hole-transporting layer, and sample C4 as the emitting
layer, 8-hydroxyquinoline aluminum (Alq3) as the electron-trans-
porting layer. LiF was used between the electron-transporting layer
and cathode Al to enhance electron injection, and indium-tin oxide
(ITO) coated glass was used as the substrate and anode. The
configuration and the image taken at 10 V voltage of the OLED
device is shown in Fig. 6.
The currentevoltage-luminance characteristics of the unopti-
mized device is presented in Fig. 7. The turn-on voltage, which is
defined as the voltage when the luminance is 1 cd/m2, was 6.0 V.
The device exhibited a good light-emitting property in which its
luminance efficiency can reach up to 2 cd/A with a maximum
brightness of 548 cd/m2.
The EL spectrum of the device is shown in Fig. S10. Comparison
of the EL spectrum with PL emission spectra of the compound in
powder state and in dichloromethane solution showed that the
profiles of the EL band and the PL bands were similar with about
8 nm red-shift and 5 nm blue-shift for the EL peak (474 nm)
compared with those of the PL peaks in dichloromethane solution
(466 nm) and in powder state (479 nm). Extended investigation
and further optimization on the electroluminescence devices based
on this new series of AIE compounds are being carried out in our
laboratory and will be reported in due course.
Fig. 4. UV spectra of C4 (10 mM) in THF with varying amounts of water (% fraction of
volume).
increased from 0% to 90%, the PL emission wavelength of C4 was
maintained; meanwhile, C2B2 and B4 exhibited 2-nm red shift and
a 13-nm red shift changes, respectively. This phenomenon may
have also been derived from two aspects: (1) the aggregation state
of the compounds and (2) the influence of the solvent effect. As the
water fraction increases in the mixed solvents, C4 and C2B2 may
form J-aggregates, while B4 may form H-aggregates. As we know,
the formation of J-aggregates could cause the fluorescence wave-
length to blue-shift, while the formation of H-aggregates could
cause the same to red-shift [32]. The fluorescence wavelength
could also be red-shifted by increases in solvent polarity, such as
those caused by increasing water fractions. For C4, the influences of
aggregation and the solvent effect may counteract each other such
that the fluorescence wavelength is maintained and unchanged.
The solvent effect appears to have dominated in the compounds
C2B2 and B4, and thus, their fluorescence wavelengths have red-
shifted. It was also observed that when the water fraction in C2B2
was increased to 80%, a new PL band appeared in the region of
shorter wavelength, indicating the formation of J-aggregation
(Fig. S6).
3
75
10
Luminance
Current
60
2
10
45
30
1
10
15
0
0
10
0
2
4
6
8
10
12
14
16
Voltage (V)
Fig. 5. Molecular models of the energy minimized structures of C4, C2B2, and B4
(simulated by Chem 3D Ultra 8.0).
Fig. 7. Diagram of luminance and current vs. voltage of the OLED device.

62
X. Zhang et al. / Dyes and Pigments 89 (2011) 56e62
2,3,4,5-tetraphenylsiloles. Journal of the American Chemical Society
4. Conclusions
2005;127:6335e46.
[9] Chen J, Law C, Lam J, Dong Y, Lo S, Williams I, et al. Synthesis, light emission,
nanoaggregation, and restricted intramolecular rotation of 1,1-substituted
2,3,4,5-tetraphenylsiloles. Chemistry of Materials 2003;15:1535e46.
[10] Bhongale CJ, Hsu CS. Emission enhancement by formation of aggregates in
hybrid chromophoric surfactant amphiphile/silica nanocomposites. Ange-
wandte Chemie International Edition 2006;45:1404e8.
In this work, we have developed further examples of AIEE-active
compounds in the form of bis(triphenylethylene) derivatives. These
new derivatives showed high levels of thermal stability, with Tg
values ranging from 125e178 ꢀC. The derivatives were all catego-
rized as blue-light emitters and their maximum emission wave-
lengths were in the range of 464e468 nm. The compounds
exhibited high fluorescent efficiencies and their fluorescence
quantum yields were in the range of 0.58e0.88. The resultant
compounds with high Tg values and high fluorescence quantum
yield could enable the development and processing of new high-
performance organic electronic devices. The HOMO levels of C4 and
C2B2 allows their use as materials in OLED devices to lower the
barrier of hole injection. The observed AIEE properties demonstrate
that the composition of a mixture of THF and water influences the
formation of different aggregation states and that the solvent effect
contributes to the variation of UV and PL spectra. The device using
C4 as emitting layer exhibited a good light-emitting property in
which its luminance efficiency can reach up to 2 cd/A with
a maximum brightness of 548 cd/m2.
[11] Tong H, Hong YN, Dong YQ, Ren Y, Häussler M, Lam JWY, et al. Color-tunable,
aggregation-induced emission of
a butterfly-shaped molecule comprising
a pyran skeleton and two cholesteryl wings. The Journal of Physical Chemistry
B 2007;111:2000e7.
[12] Dong Y, Lam JWY, Li Z, Tong H, Dong Y, Feng X, et al. Vapochromism of
hexaphenylsilole. Journal of Inorganic and Organometallic Polymers and
Materials 2005;15:287e91.
[13] Dong YQ, Lam JWY, Qin AJ, Sun JX, Liu JZ, Li Z, et al. Aggregation-induced and
crystallization-enhanced
emissions
of
1,2-diphenyl-3,4-bis(diphenyl-
methylene)-1-cyclobutene. Chemical Communications; 2007:3255e7.
[14] Liu J, Lam J, Tang B. Aggregation-induced emission of silole molecules and
polymers: fundamental and applications. Journal of Inorganic and Orgnome-
tallic Polymers and Materials 2009;19:249e85.
[15] Hong Y, Lam J, Tang B. Aggregation-induced emission: phenomenon, mech-
anism and applications. Chemical Communications 2009;29:4332e53.
[16] Zhang W, Xie J, Yang Z, Shi W. Aggregation behaviors and photoresponsive
properties of azobenzene constructed phosphate dendrimers. Polymer
2007;48:4466e81.
[17] Yao H, Morita Y, Kimura K. Effect of organic solvents on J aggregation of
pseudoisocyanine dye at mica/water interfaces: morphological transition
from three-dimension to two-dimension. Journal of Colloid and Interface
Science 2008;318:116e23.
[18] Zhang Y, Xiang J, Tang Y, Xu G, Yan W. Aggregation behaviour of two
thiacarbocyanine dyes in aqueous solution. Dyes and Pigments 2008;76:
88e93.
[19] Jiang S, Zhang L, Liu M. Photo-triggered J-aggregation and chiral symmetry
breaking of an anionic porphyrin (TPPS) in mixed organic solvents. Chemical
Communications; 2009:6252e4.
[20] Yang Z, Chi Z, Yu T, Zhang X, Chen M, Xu B, et al. Triphenylethylene carbazole
derivatives as a new class of AIE materials with strong blue light emission and
high glass transition temperature. Journal of Materials Chemistry
2009;19:5541e6.
[21] Zhang X, Yang Z, Chi Z, Chen M, Xu B, Wang C, et al. A multi-sensing fluo-
rescent compound derived from cyanoacrylic acid. Journal of Materials
Chemistry 2010;20:292e8.
[22] Mongin O, Porrès L, Moreaux L, Mertz J, Blanchard-Desce M. Synthesis and
photophysical properties of new conjugated fluorophores designed for two-
photon-excited fluorescence. Organic Letters 2002;4:719e22.
Acknowledgements
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (Grant numbers:
50773096, 50473020), the Start-up Fund for Recruiting Profes-
sionals from “985 Project” of SYSU, the Science and Technology
Planning Project of Guangdong Province, China (Grant numbers:
2007A010500001-2, 2008B090500196), Construction Project for
University-Industry cooperation platform for Flat Panel Display
from The Commission of Economy and Informatization of Guang-
dong Province (Grant numbers: 20081203) and the Open Research
Fund of State Key Laboratory of Optoelectronic Materials and
Technologies.
[23] Danel K, Huang T, Lin JT, Tao Y, Chuen C. Blue-emitting anthracenes with end-
capping diarylamines. Chemistry of Materials 2002;14:3860e5.
[24] Hu R, Lager E, Aguilar A, Liu J, Lam J, Sung H, et al. Twisted intramolecular
charge transfer and aggregation-induced emission of BODIPY derivatives. The
Journal of Physical Chemistry C 2009;113:15845e53.
[25] Chi Z, Yang Z, Zhang X, et al. Synthesis method and application of organic
luminescent material containing carbazolyl stilbene derivative structure.
Chinese Patent 2009, CN101343539.
Appendix. Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.dyepig.2010.09.003.
References
[26] Hajime N, Chihaya A. Diphenyl ethane derivative and organic solid-state laser
material using the same. Japanese Patent 2008, JP2008163190.
[27] Wang S, Oldham Jr WJ, Hudack Jr RA, Bazan GC. Synthesis, morphology, and
optical properties of tetrahedral oligo(phenylenevinylene) materials. Journal
of the American Chemical Society 2000;122:5695e709.
[1] Wakamiya A, Mori K, Yamaguchi S. 3-Boryl-2,20-bithiophene as a versatile
core skeleton for full-color highly emissive organic solids. Angewandte
Chemie International Edition 2007;46:4273e6.
[28] Ning ZJ, Chen Z, Zhang Q, Yan YL, Qian SX, Cao Y, et al. Aggregation-induced
emission (AIE)-active starburst triarylamine fluorophores as potential non-
doped red emitters for organic light-emitting diodes and Cl2 gas chemo-
dosimeter. Advanced Functional Materials 2007;17:3799e807.
[29] Dong S, Li Z, Qin J. New carbazole-based fluorophores: synthesis, character-
ization, and aggregation-induced emission enhancement. Journal of Physical
Chemistry B 2009;113:434e41.
[30] Tang B, Geng Y, Lam JWY, Li B, Jing X, Wang X, et al. Processible nano-
structured materials with electrical conductivity and magnetic susceptibility:
preparation and properties of maghemite/polyaniline nanocomposite films.
Chemistry of Materials 1999;11:1581e9.
[2] Citterio D, Takeda J, Kosugi M, Hisamoto H, Sasaki S, Komatsu H, et al. pH-
independent pluorescent chemosensor for highly selective lithium ion
sensing. Analytical Chemistry 2007;79:1237e42.
[3] McDonagh C, Burke CS, MacCraith BD. Optical chemical sensors. Chemical
Reviews 2008;108:400e22.
[4] Ligler FS, Sapsford KW, Golden JP, Shriver-Lake LC, Taitt CR, Dyer MA, et al. The
array biosensor: portable, automated systems. Analytical Sciences
2007;23:5e10.
[5] Jenekhe SA, Osaheni JA. Excimers and exciplexes of conjugated polymer.
Science 1994;265:765e8.
[6] Friend RH, Gymer RW, Holmes AB, Burroughes JH, Marks RN, Taliani C, et al.
Electroluminescence in conjugated polymers. Nature 1999;397:121e8.
[7] Luo JD, Xie ZL, Lam JWY, Cheng L, Chen HY, Qiu CF, et al. Aggregation-induced
emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chemical Communications;
2001:1740e1.
[31] Lechner M. Influence of Mie scattering on nanoparticles with different particle
sizes and shapes: photometry and analytical ultracentrifugation with
absorption optics. Journal of the Serbian Chemical Society 2005;70:361e9.
[32] Sun C, Zhou S, Chen P. Effects of different Hz sources on formation of
J
aggregation of cyanine dye. The Imaging Science Journal 2006;54:
[8] Yu G, Yin S, Liu Y, Chen J, Xu X, Sun X, et al. Structures, electronic states,
photoluminescence, and carrier transport properties of 1,1-disubstituted
142e6.