Piezochromic luminescent and electroluminescent materials comprised of tetraphenylethene plus spirobifluorene or 9,9-diphenylfluorene

Article abstract of DOI:10.1016/j.dyepig.2014.02.026

In this work, a series of luminogens comprised of tetraphenylethene plus spirobifluorene or 9,9-diphenylfluorene are synthesized and characterized. Whereas these luminogens are weakly fluorescent in solutions, they are highly emissive in the aggregated state, with high fluorescence quantum yields up to 99% in solid films, demonstrating aggregation-induced emission characteristics. Reversible piezochromic luminescence is observed from the solids of the luminogens. A notable emission color change from blue (445 nm) to green (503 nm) is readily realized by grinding the pristine powder of the luminogen. The blue emission is recovered by fuming the ground powder with dichloromethane vapor. The undoped electroluminescence devices using the luminogens as light-emitting layers are fabricated, affording high current efficiencies up to 7.2 cd/A.

Full text of DOI:10.1016/j.dyepig.2014.02.026

Dyes and Pigments 106 (2014) 87e93
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Piezochromic luminescent and electroluminescent materials
comprised of tetraphenylethene plus spirobifluorene or 9,
9-diphenylfluorene
Bairong He a, Zhengfeng Chang a, Yibin Jiang c, Xiaofei Xu a, Ping Lu e, Hoi Sing Kwok c,
,
a
a,
b,
b,d,
**
Jian Zhou a , Huayu Qiu , Zujin Zhao
*
**
, Ben Zhong Tang
a College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China
b State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China
c Center for Display Research, The Hong Kong University of Science & Technology (HKUST), Kowloon, Hong Kong, China
d Department of Chemistry, Division of Biomedical Engineering, Institute for Advanced Study and Institute of Molecular Functional Materials,
State Key Laboratory of Molecular Neuroscience, HKUST, Kowloon, Hong Kong, China
e State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, a series of luminogens comprised of tetraphenylethene plus spirobifluorene or 9,9-
diphenylfluorene are synthesized and characterized. Whereas these luminogens are weakly fluores-
cent in solutions, they are highly emissive in the aggregated state, with high fluorescence quantum yields
up to 99% in solid films, demonstrating aggregation-induced emission characteristics. Reversible pie-
zochromic luminescence is observed from the solids of the luminogens. A notable emission color change
from blue (445 nm) to green (503 nm) is readily realized by grinding the pristine powder of the lumi-
nogen. The blue emission is recovered by fuming the ground powder with dichloromethane vapor. The
undoped electroluminescence devices using the luminogens as light-emitting layers are fabricated,
affording high current efficiencies up to 7.2 cd/A.
Received 13 January 2014
Received in revised form
17 February 2014
Accepted 24 February 2014
Available online 11 March 2014
Keywords:
Piezochromism
Photoluminescence
Electroluminescence
Aggregation-induced emission
Tetraphenylethene
Fluorene
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
in mechano-sensors, optical data storage, security papers, etc [6].
The high emission efficiency of the solid is of particular importance
Organic solids with efficient photoluminescence (PL) are highly
desired because of their practical applications in organic light-
emitting diodes (OLEDs), organic lasers, non-linear optics, fluo-
rescent sensors, etc [1]. Recently, intense research activities are
attracted by the piezochromic luminescent organic solids that
respond to external stimulus, such as mechanical stress, organic
vapor, temperature, etc [2]. The piezochromic luminescence is
mainly caused by the morphology transition between crystalline
and amorphous states, or between different crystalline states,
without provoking structural damage of the molecule [3e5]. This
interesting emission property allows the organic solids to be used
to generate vivid emission color change under external stimulation.
However, many chromophores that show good emissions in solu-
tions suffer from emission-quenching in the solid state, which
limits their practical utility. To address this issue, creation of
luminogens by adopting building blocks that possess aggregation-
induced emission (AIE) [7] characteristics is a facile and effective
approach. The integration of AIE-active units with various con-
ventional chromophores or diverse functional groups can generate
efficient solid-state luminescent materials with specific property,
and thus, great potential in environmental, scientific and techno-
logical aspects [8].
Tetraphenylethene (TPE) is
a typical AIE-active molecule.
Currently, it becomes the subject of numerous investigations
because of not only its remarkable AIE attribute, but also its simple
molecular structure that allows facile preparation and chemical
modification [9]. The propeller-like conformation of TPE avoids
tight packing of the molecules in the aggregated state, which
* Corresponding author. Tel.: þ86 571 28867898; fax: þ86 571 28867899.
** Corresponding authors.
E-mail addresses: zhoujian@hznu.edu.cn (J. Zhou), zujinzhao@gmail.com
(Z. Zhao), tangbenz@ust.hk (B.Z. Tang).
http://dx.doi.org/10.1016/j.dyepig.2014.02.026
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

88
B. He et al. / Dyes and Pigments 106 (2014) 87e93
alleviates intermolecular
p
e
p
interaction and thus emission
temperature and poured into water. The organic layer was extrac-
ted with dichloromethane, and the combined organic layers were
washed with a saturated brine solution and water, and dried over
anhydrous magnesium sulfate. After filtration and solvent evapo-
ration, the residue was purified by silica-gel column chromatog-
raphy using hexane/dichloromethane as eluent. White solid of
(TPE)2PF was obtained in 56% yield. 1H NMR (400 MHz, CDCl3),
quenching. Therefore, solids of TPE derivatives can fluoresce
strongly, enabling them to serve as efficient host light emitters for
OLEDs [10,11]. In addition, the loose packing also offers opportunity
for the molecules to form metastable morphology, which induces
stimuli-responsive emission behavior [12]. To enrich efficient solid-
state emitters for piezochromic study as well as for the application
in OLEDs, in this work, a series of luminogens with AIE character-
istics are prepared based on TPE and fluorene derivatives. Highly
luminescent solids of the adducts of TPE and fluorene derivatives
are obtained. The piezochromic luminescent and electrolumines-
cent properties of the adducts are investigated and delineated.
d
(TMS, ppm): 7.67 (d, 2H, J ¼ 7.2 Hz), 7.49e7.45 (m, 4H), 7.22e6.95
(m, 48H). 13C NMR (100 MHz, CDCl3),
d
(TMS, ppm): 152.2, 145.9,
143.8, 143.7, 142.8, 141.1, 140.5, 140.1, 139.0, 138.8, 131.8, 131.5, 131.4,
128.3,127.8,127.7,126.7,126.6,126.5,126.2,124.5,120.5, 65.7. HRMS
(MALDI-TOF): m/z 978.4228 (Mþ, calcd 978.4226). Anal. Calcd for
C77H54: C, 94.44; H, 5.56. Found: C, 94.33 H, 5.52.
2. Experimental
2.3.2. 1,2-Bis(4-(9,9-diphenyl-9H-fluoren-2-yl)phenyl)-1,2-
diphenylethene (TPE(PF)2)
2.1. General
Pale green solid; yield 55%. 1H NMR (400 MHz, CDCl3),
d (TMS,
THF was distilled from sodium benzophenone ketyl under dry
nitrogen immediately prior to use. Compounds 2 and 4 were pre-
pared by the methods in the literature [13]. All other chemicals and
reagents were purchased from Aldrich or J&K Scientific Ltd. and
used as received without further purification. 1H and 13C NMR
spectra were measured on a Bruker AV 400 spectrometer in
ppm): 7.78e7.75 (m, 4H), 7.59e7.54 (m, 4H), 7.40e7.21 (m, 30H),
7.12e7.03 (m, 14H). 13C NMR (100 MHz, CDCl3),
d
(TMS, ppm): 151.8,
151.5,145.9,143.8,142.9,140.6,140.1,139.8,139.3,138.9,131.8,131.4,
128.3, 128.2, 127.9, 127.7, 127.6, 126.7, 126.6, 126.5, 126.2, 125.6,
124.5, 120.4, 120.2, 68.0, 65.6. HRMS (MALDI-TOF): m/z 964.5 (Mþ,
calcd 964.4069). Anal. Calcd for C76H52: C, 94.57; H, 5.43. Found: C,
94.26; H, 5.35.
deuterated chloroform using tetramethylsilane (TMS;
d
¼ 0) as
internal reference. High resolution mass spectra (HRMS) were
recorded on a GCT premier CAB048 mass spectrometer operating in
MALDI-TOF mode. Elementary analysis was performed on Vario EL
III. UV spectra were measured on a Shimadzu UV-2450 spectro-
photometer. PL spectra were recorded on a PerkineElmer LS 55
spectrofluorometer.
2.3.3. 2,7-Bis[4-(1,2,2-triphenylvinyl)phenyl]-9,90-spirobifluorene
((TPE)2SF)
Green solid; yield 41%. The NMR signals are too weak due to the
poor solubility of (TPE)2SF. HRMS (MALDI-TOF): m/z 976.4080 (Mþ,
calcd 976.4069). Anal. Calcd for C77H52: C, 94.64; H, 5.36. Found: C,
94.46; H, 5.28.
2.2. Device fabrication and measurements
2.3.4. 1,2-Bis[4-(9,90-spirobifluorene-7-yl)phenyl]-1,2-
The devices were fabricated on 80 nm-ITO coated glass with a
sheet resistance of 25 U/,. Prior to loading into the pretreatment
chamber, the ITO-coated glass was soaked in ultrasonic detergent
for 30 min, followed by spraying with de-ionized water for 10 min,
soaking in ultrasonic de-ionized water for 30 min, and oven-baking
for 1 h. The cleaned samples were treated by perfluoromethane
plasma with a power of 100 W, gas flow of 50 sccm, and pressure of
0.2 Torr for 10 s in the pretreatment chamber. The samples were
transferred to the organic chamber with a base pressure of
7 ꢀ 10ꢁ7 Torr for the deposition of NPB, emitter, and TPBi, which
served as hole-transport, light-emitting, and electron-transport
layers, respectively. The samples were then transferred to the
metal chamber for cathode deposition which composed of lithium
fluoride (LiF) capped with aluminum (Al). The light-emitting area
was 4 mm2. The current density-voltage characteristics of the de-
vices were measured by a HP4145B semiconductor parameter
analyzer. The forward direction photons emitted from the devices
were detected by a calibrated UDT PIN-25D silicon photodiode. The
luminance and external quantum efficiencies of the devices were
inferred from the photocurrent of the photodiode. The electrolu-
minescence spectra were obtained by a PR650 spectrophotometer.
All measurements were carried out under air at room temperature
without device encapsulation.
diphenylethene (TPE(SF)2)
Green solid; yield 45%. 1H NMR (400 MHz, CDCl3),
d (TMS, ppm):
7.83 (d, 8H, J ¼ 7.6 Hz), 7.56e7.53 (m, 2H), 7.37e7.33 (m, 6H), 7.15e
7.02 (m, 16H), 6.96e6.88 (m, 10H), 6.74e6.69 (m, 6H). 13C NMR
(100 MHz, CDCl3),
d (TMS, ppm): 149.3, 149.2, 148.7, 143.8, 142.7,
141.8, 141.4, 141.0, 140.4, 140.3, 138.6, 131.7, 131.6, 131.4, 131.3, 127.9,
127.7, 127.6, 126.7, 126.5, 126.4, 126.2, 126.0, 124.1, 124.0, 122.3,
120.2, 120.0, 68.0, 66.0. HRMS (MALDI-TOF): m/z 960.3771 (Mþ,
calcd 960.3756). Anal. Calcd for C76H48: C, 94.97; H, 5.03. Found: C,
94.78; H, 5.01.
3. Results and discussion
3.1. Synthesis
Scheme 1 illustrates the synthetic routes to the luminogens
comprised of TPE plus spirobifluorene or 9,9-diphenylfluorene. The
detailed procedures and characterization data are given in Exper-
imental section. Suzuki couplings of commercially available fluo-
rene derivatives 5e8 with 2 or 4 generated target products in
moderate yields. The structures of the final products are charac-
terized by NMR, mass spectrometry, and elementary analysis. The
solubility of (TPE)2PF, TPE(PF)2 and TPE(SF)2 is good in common
organic solvents, such as THF, dichloromethane, chloroform, etc.,
but (TPE)2SF shows poor solubility in these solvents. All the lumi-
nogens are insoluble in water and methanol.
2.3. Synthesis
2.3.1. 9,9-Diphenyl-2,7-bis(4-(1,2,2-triphenylvinyl)phenyl)fluorene
((TPE)2PF)
3.2. Optical properties
A mixture of 2 (0.83 g, 2.2 mmol), 5 (0.48 g, 1 mmol), Pd(PPh3)4
(0.11 g, 0.1 mmol), and K2CO3 (1.1 g, 8.0 mmol) in toluene/ethanol/
water (100 mL, 8/1/1 v/v/v) was heated to reflux for 12 h under
nitrogen. Then the reaction mixture was cooled to room
Fig. 1 shows the absorption spectra of the adducts of TPE and
fluorene derivatives. The spectral profiles are varied as different
fluorene derivatives incorporated. (TPE)2PF and (TPE)2SF show

B. He et al. / Dyes and Pigments 106 (2014) 87e93
89
OH
1) n-BuLi, THF
PTSA
1) n-BuLi
B
Br
OH
2)
toluene
Br
2) B(OMe)3
3) HCl
O
2
1
O
OH
1) n-BuLi
Zn
Br
B
HO
OH
Br
B
TiCl4
2) B(OMe)3
3) HCl
HO
Br
4
3
Br
Br
2
Pd(PPh3)4
K2CO3
5
(TPE)2PF
2
Br
Br
Pd(PPh3)4
K2CO3
6
(TPE)2SF
Br
4
Pd(PPh3)4
K2CO3
7
TPE(PF)2
4
Br
Pd(PPh3)4
K2CO3
8
TPE(SF)2
Scheme 1. Synthetic routes to the adducts of TPE and fluorene derivatives.
close absorption maxima at 359 and 358 nm, respectively, which
are longer than those of TPE(PF)2 (349 nm) and TPE(SF)2 (347 nm).
These luminogens are almost nonfluorescent in dilute THF solu-
tions with very weak PL signals when photoexcited. The fluores-
cence quantum yields (FF) are low (0.4e1.7%, Table 1), measured by
using 9,10-diphenylanthracene as standard. This is because the
active intramolecular rotation (IMR) process of TPE units in solu-
tions has nonradiatively deactivated the excited-state of the
molecules [7,8]. In solid films, these luminogens are highly fluo-
rescent. The PL emission peak of (TPE)2PF is located at 491 nm,
which is blue-shifted by 7 nm than that of TPE(PF)2 (498 nm). A
similar blue-shift is observed when compare the PL emissions of
(TPE)2SF and TPE(SF)2 (Fig. 2). High FF values of 96, 95 and 99% are
attained from the films of (TPE)2PF, TPE(PF)2 and TPE(SF)2,
respectively, measured by integrating sphere. The FF value of
(TPE)2SF is somewhat low (69%), possibly due to its poor solubility,

90
B. He et al. / Dyes and Pigments 106 (2014) 87e93
(TPE)2PF
(TPE)2PF
TPE(PF)2
(TPE)2SF
TPE(SF)2
TPE(PF)2
(TPE)2SF
TPE(SF)2
280
340
400
460
Wavelength (nm)
375
475
575
675
Wavelength (nm)
Fig. 1. Absorption spectra of the adducts of TPE and fluorene derivatives in THF so-
lutions (10 M).
m
Fig. 2. PL spectra of the adducts of TPE and fluorene derivatives in solid films.
and thus, poor quality of the film prepared by drop-casting method.
In comparison with the FF values in solutions, the FF values of the
films have increased drastically, indicating that these luminogens
are AIE-active.
dichloromethane vapor. However, the emission color is not rever-
ted by heating method. By regrinding the fumed powder, green
emission comes back again. The emission color change is so obvious
that it can be readily distinguished by eyes under the illumination
of a UV lamp (Fig. 5). After several cycles of the switching, the
chemical structure of (TPE)2PF is not degraded, and facile and
reversible emission modulation is achieved by grinding and fuming
process, demonstrating the great potential of the luminogens as
stimuli-responsive materials. Similar piezochromic luminescence is
also found from other three luminogens.
Since the light emission is closely related to the alignment of
molecules in the aggregated state, the morphology of the powder is
hence investigated to explain the emission switching. Fig. 6 shows
the powder X-ray diffraction (XRD) patterns of pristine powder,
fumed powder and reground powder. Intensive and sharp diffrac-
tion peaks are recorded from the pristine powder, revealing that it
is crystalline in nature. In contrary, the profile of the ground
powder lacks discernable peaks, which discloses that the ground
The emission behaviors of the luminogens are further investi-
gated in THF/water mixtures. Fig. 3 displays the PL spectra of
(TPE)2PF in the mixture as an example. When a small amount of
water, a nonsolvent of (TPE)2PF, is added into the THF solution, the
PL emission remains faint. When a large amount of water is added,
the emission intensity increases swiftly. Since the dissolving ca-
pacity of the mixture with high water content becomes poor, the
(TPE)2PF molecules must have aggregated. The IMR process is
restricted in the condensed phase, and the radiative decay of the
excited state is promoted, leading to enhancement of PL intensity.
Similar emission behaviors are also recorded in other luminogens.
These results demonstrate that the adducts of TPE and fluorene
derivatives possess AIE characteristics indeed.
3.3. Piezochromism
In addition to the efficient solid-state emissions, reversible
piezochromic luminescence is observed for the adducts of TPE and
fluorene derivatives. Fig. 4 displays the PL spectra of (TPE)2PF in
different solid states. The pristine powder of (TPE)2PF shows blue
emission located at 445 nm, while the ground powder emits green
light at 503 nm, representing a red-shift of 58 nm. The emission
color of the ground powder is recovered to blue after fuming with
fw (vol %)
90
80
70
60
0
0
90
90
Table 1
Optical properties of the adducts of TPE and fluorene derivatives.
labs (nm)
lem (nm)
FF (%)
Solna
Aggrb
Filmc
Solnd
Filme
(TPE)2PF
TPE(PF)2
(TPE)2SF
TPE(SF)2
359
349
358
347
498
502
496
503
491
498
495
502
0.4
1.1
0.5
1.7
96
95
69
99
a
375
475
575
675
In THF solution (10 mM).
Aggregates in THF/water mixture with 90% (vol%) water fraction.
Film drop-casted on quartz plate.
b
c
Wavelength(nm)
d
Fluorescence quantum yield
(
FF
)
determined in THF solution using 9,10-
Fig. 3. PL spectra of (TPE)2PF in THF/water mixtures with various water fractions (fw).
Inset: photos of (TPE)2PF in THF/water mixtures (fw ¼ 0 and 90%) taken under the
illumination of a UV lamp (365 nm).
diphenylanthracene (FF ¼ 90% in cyclohexane) as standard.
e
Fluorescence quantum yield of solid film measured by integrating sphere.

B. He et al. / Dyes and Pigments 106 (2014) 87e93
91
pristine
ground
fumed
pristine
ground
fumed
reground
5
10 15 20 25 30 35 40 45 50
375
475
575
675
2θ (degree)
Wavelength(nm)
Fig. 6. XRD diffractograms of (TPE)2PF: pristine powder, ground powder and
dichloromethane-fumed powder.
Fig. 4. PL spectra of (TPE)2PF: pristine powder, ground powder, dichloromethane-
fumed powder and reground powder.
Table 2
powder is in amorphous state. The profile of fumed powder re-
sembles that of the pristine powder, although the intensity of
diffraction peaks are somewhat weak, demonstrating that the
amorphous ground powder has been recovered to crystalline ag-
gregates. These results clearly evidence that the blue to green
emission color change is caused by the morphology transition from
crystalline state to amorphous state, which is in good agreement
with those found in other TPE derivatives [12,14,15]. In the crys-
talline state, the molecules adopt a twisted conformation to reduce
the steric repulsion, while the external mechanical stress makes the
molecules less twisted and breaks regular packing as well. The
molecules in the resulting amorphous state are more planar and
experience stronger intermolecular interactions, which give rise to
red-shift in PL spectrum.
EL performances of OLEDs based on the adducts of TPE and fluorene derivatives.a
EL
Von
Lmax
hP,max
hC,max
hext,max
(nm)
(V)
(cd/m2)
(lm/W)
(cd/A)
(%)
(TPE)2PF
TPE(PF)2
(TPE)2SF
TPE(SF)2
518
510
496
532
4.5
4.6
4.2
5.6
10,500
14,500
14,900
11,100
4.0
4.7
2.2
2.6
5.5
7.2
5.2
6.0
2.3
2.6
2.0
1.9
a
Abbreviations: Von ¼ turn-on voltage at 1 cd/m2, Lmax ¼ maximum luminance,
hP,max
,
hC,max, and hext,max ¼ maximum power, current, and external quantum effi-
ciencies, respectively.
performances are summarized in Table 2, and the characteristic
curves are shown in Fig. 7. The devices of (TPE)2PF, TPE(PF)2,
(TPE)2SF and TPE(SF)2 show EL emission maxima in the range of
496e532 nm. The red-shifts observed from the EL emissions of
(TPE)2PF, TPE(PF)2 and TPE(SF)2 in comparison with their PL
emissions in solid films are probably due to the microcavity effect.
The device of TPE(PF)2 shows the highest EL efficiencies. It is turned
on at 4.6 V and affords high luminance up to 14,500 cd/m2. The
maximum current efficiency, maximum power efficiency and
maximum external quantum efficiency attained by the device are
7.2 cd/A, 4.7 lm/W, and 2.6%, respectively. The devices of other
luminogens also show good performances, with maximum lumi-
nance in the range of 10,500e14,500 cd/m2 and peak current effi-
ciencies in the range of 5.2e6.0 cd/A. These results indicate that the
3.4. Electroluminescence
The application of these new luminescent materials as light
emitters for electroluminescence (EL) devices is further evaluated.
Multilayer undoped OLEDs with
a configuration of ITO/NPB
(60 nm)/emitter (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) are
fabricated. In these EL devices, the adducts of TPE and fluorene
derivatives work as light-emitting layers, N,N-bis(1-naphthyl)-N,N-
diphenylbenzidine (NPB) functions as a hole-transporting layer,
and 2,20,200-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole)
(TPBi) serves as electron-transporting layer. The device
Fig. 5. Photographs of (TPE)2PF: pristine powder, ground powder, and dichloromethane-fumed powder, taken under the illumination of a UV lamp (365 nm).

92
B. He et al. / Dyes and Pigments 106 (2014) 87e93
Fig. 7. (A) EL spectra, (B) changes in luminance and current density with the applied voltage and (C) current efficiency versus current density curves in multilayer devices of adducts
of TPE and fluorene derivatives [ITO/NPB (60 nm)/emitter (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)].
[3] (a) Bu L, Sun M, Zhang D, Liu W, Wang Y, Zheng M, et al. Solid-state fluorescence
adducts of TPE and fluorene derivatives are good host emitters for
OLEDs.
properties and reversible piezochromic luminescence of aggregation-induced
emission-active 9,10-bis[(9,9-dialkylfluorene-2-yl)vinyl]anthracenes. J Mater
Chem C 2013;1(10):2028e35;
(b) Wang Y, Liu W, Bu L, Li J, Zheng M, Zhang D, et al. Reversible piezo-
chromic luminescence of 9,10-bis[(N-alkylcarbazol-3-yl)vinyl]anthracenes
and the dependence on N-alkyl chain length. J Mater Chem C 2013;1(4):
4. Conclusions
In summary, a series of luminescent materials comprised of TPE
plus spirobifluorene or 9,9-diphenylfluorene are synthesized and
characterized. These adducts are almost nonfluorescent in solu-
tions but can emit strongly in aggregates or solid films, with high
solid-state FF values up to 99%. The piezochromic luminescent
properties are studied on these adducts. Notable reversible piezo-
chromic luminescence is observed from (TPE)2PF, which shows
blue light (445 nm) in crystalline state and green light (503 nm) in
amorphous state. Multilayer EL devices using the adducts as host
emitters are fabricated, which afford high EL efficiencies up to
7.2 cd/A. These results demonstrate that these adducts combine the
merits of high solid-state emission efficiency, stimuli-responsive
luminescence and efficient electroluminescence, which enable
them to find an array of high-tech applications in mechano-sensors,
optoelectronic devices, etc.
856e62;
(c) Dou C, Han L, Zhao S, Zhang H, Wang Y. Multi-stimuli-responsive
fluorescence switching of a donoreacceptor
Chem Lett 2011;2(6):666e70;
(d) Nagura K, Saito S, Yusa H, Yamawaki H, Fujihisa H, Sato H, et al. Distinct
responses to mechanical grinding and hydrostatic pressure in luminescent
chromism of tetrathiazolylthiophene. J Am Chem Soc 2013;135(28):10322e5;
(e) Sagara Y, Kato T. Brightly tricolored mechanochromic luminescence from a
single luminophore liquid crystal: reversible writing and erasing of images.
Angew Chem Int Ed Engl 2011;50(39):9128e32.
p-conjugated compound. J Phys
[4] (a) Luo XL, Li JN, Li CH, Heng LP, Dong YQ, Liu ZP, et al. Reversible switching of the
emission of diphenyldibenzofulvenes by thermal and mechanical stimuli. Adv
Mater 2011;23(29):3261e5;
(b) Wei R, Song P, Tong A. Reversible thermochromism of aggregation-induced
emission-active benzophenone azine based on polymorph-dependent excited-
state intramolecular proton transfer fluorescence. J Phys Chem C 2013;117(7):
3467e74;
(c) Han T, Zhang Y, Feng X, Lin Z, Tong B, Shi J, et al. Reversible and hydrogen
bonding-assisted piezochromic luminescence for solid-state tetraaryl-buta-1,3-
diene. Chem Commun 2013;49(63):7049e51;
(d) Han T, FengX, ShiJ, Tong B, Dong Y, LamJWY, et al. DMF-inducedemissionof an
aryl-substituted pyrrole derivative: a solid thermo-responsive material to detect
temperature in a specific range. J Mater Chem C 2013;1(45):7534e9;
(e) Mei J, Wang J, Qin A, Zhao H, Yuan W, Zhao Z, et al. Construction of soft porous
crystal with silole derivative: strategy of framework design, multiple structural
transformability and mechanofluorochromism. J Mater Chem 2012;22(10):
4290e8.
Acknowledgments
We acknowledge the financial support from the National Nat-
ural Science Foundation of China (21344997, 51273053, 21104012
and 21284034), the Program for Changjiang Scholars and Innova-
tive Research Teams in Chinese Universities (IRT 1231), the
Fundamental Research Funds for the Central Universities
(2013ZZ0002), and the Guangdong Innovative Research Team
Program of China (20110C0105067115).
[5] (a) Teng MJ, Jia XR, Yang S, Chen XF, Wei Y. Reversible tuning luminescent
color and emission intensity: a dipeptide-based light-emitting material. Adv
Mater 2012;24(9):1255e61;
(b) Wang Y, Li M, Zhang Y, Yang J, Zhu S, Sheng L, et al. Stress acidulated
amphoteric molecules and mechanochromism via reversible intermolecular
proton transfer. Chem Commun 2013;49(59):6587e9;
(c) Zhang J, Chen J, Xu B, Wang L, Ma S, Dong Y, et al. Remarkable fluorescence
change based on the protonationedeprotonation control in organic crystals.
Chem Commun 2013;49(37):3878e80;
References
(d) Dong Y, Xu B, Zhang J, Tan X, Wang L, Chen J, et al. Piezochromic lumi-
nescence based on the molecular aggregation of 9,10-bis((E)-2-(pyrid-2-yl)
vinyl) anthracene. Angew Chem Int Ed Engl 2012;51(43):10782e5.
[6] (a) Irie M, Fukaminato T, Sasaki T, Tamai N, Kawai T. Organic chemistry: a digital
fluorescent molecular photoswitch. Nature 2002;420(6917):759e60;
(b) Crenshaw BR, Weder C. Deformation-induced color changes in melt-
processed photoluminescent polymer blends. Chem Mater 2003;15(25):
4717e24;
[1] (a) Saragi TPI, Spehr T, Siebert A, Fuhrmann-LiekerT, Salbeck J. Spiro compounds
for organic optoelectronics. Chem Rev 2007;107(4):1011e65;
(b) Burn PL, Lo SC, Samuel IDW. The development of light-emitting dendrimers
for displays. Adv Mater 2007;19(13):1675e88;
(c) Perepichka IF, Perepichka DF, Meng H, Wudl F. Light-emitting poly-
thiophenes. Adv Mater 2005;17(19):2281e305;
(d) AkcelrudL. Electroluminescentpolymers. ProgPolymSci2003;28(6):875e962;
(e) Shimizu M, Hiyama T. Organic fluorophores exhibiting highly efficient photo-
luminescence in the solid state. Chem Asian J 2010;5(7):1516e31.
[2] (a) Sagara Y, Kato T. Mechanically induced luminescence changes in molecular
assemblies. Nat Chem 2009;1(8):605e10;
(c) Kishimura A, Yamashita T, Yamaguchi K, Aida T. Rewritable phosphorescent
paper by the control of competing kinetic and thermodynamic self-assembling
events. Nat Mater 2005;4(7):546e9.
[7] (a) Luo J, Xie Z, Lam JWY, Cheng L, Chen H, Qiu C, et al. Aggregation-induced
emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chem Commun 2001;(18):
1740e1;
(b) Weder C. Mechanoresponsive materials. J Mater Chem 2011;21(23):8235e6;
(c) Chi Z, Zhang X, Xu B, Zhou X, Ma C, Zhang Y, et al. Recent advances in organic
mechanofluorochromic materials. Chem Soc Rev 2012;41(10):3878e96;
(d) Zhang X, Chi Z, Zhang Y, Liu S, Xu J. Recent advances in mechanochromic
luminescent metal complexes. J Mater Chem C 2013;1(21):3376e90.
(b) An BK, Kwon SK, Jung SD, Park SY. Enhanced emission and its switching in
fluorescent organic nanoparticles. J Am Chem Soc 2002;124(48):14410e5;

B. He et al. / Dyes and Pigments 106 (2014) 87e93
93
(c) Hong Y, Lam JWY, Tang BZ. Aggregation-induced emission. Chem Soc Rev
2011;40(11):5361e88.
[8] (a) Hong Y, Lam JWY, Tang BZ. Aggregation-induced emission: phenomenon,
mechanism and applications. Chem Commun 2009;(29):4332e53;
(b) Ding D, Li K, Liu B, Tang BZ. Bioprobes based on AIE fluorogens. Acc Chem
Res 2013;46(11):2441e53;
tetraphenylethene-based efficient blue luminophors: aggregation induced
emission and partially controllable emitting color. J Mater Chem 2012;22(6):
2478e84.
[12] (a) Zhao N, Yang Z, Lam JWY, Sung HHY, Xie N, Chen S, et al. Benzothiazolium-
functionalized tetraphenylethene: an AIE luminogen with tunable solid-state
emission. Chem Commun 2012;48(69):8637e9;
(c) Zhao Z, Lam JWY, Tang BZ. Tetraphenylethene: a versatile AIE building
block for the construction of efficient luminescent materials for organic light-
emitting diodes. J Mater Chem 2012;22(45):23726e40;
(b) Qi Q, Zhang J, Xu B, Li B, Zhang SX-A, Tian W. Mechanochromism and
polymorphism-dependent emission of tetra(4-dimethylaminophenyl)
ethylene. J Phys Chem C 2013;117(47):24997e5003;
(d) Wang M, Zhang G, Zhang D, Zhu D, Tang BZ. Fluorescent bio/chemosensors
based on silole and tetraphenylethene luminogens with aggregation-induced
emission feature. J Mater Chem 2010;20(10):1858e67;
(c) Yuan WZ, Tan Y, Gong Y, Lu P, Lam JWY, Shen XY, et al. Synergy between
twisted conformation and effective intermolecular interactions: strategy for
efficient mechanochromic luminogens with high contrast. Adv Mater
2013;25(20):2837e43;
(e) Zhao Z, Lam JWY, Tang BZ. Self-assembly of organic luminophores with
gelation-enhanced emission characteristics. Soft Matter 2013;9(18):4564e79.
[9] Zhao Z, Lam JWY, Tang BZ. Aggregation-induced emission of tetraarylethene
luminogens. Curr Org Chem 2010;14(18):2109e32.
[10] (a) Zhao Z, Chen S, Shen X, Mahtab F, Yu Y, Lu P, et al. Aggregation-induced
emission, self-assembly, and electroluminescence of 4,40-bis(1,2,2-
triphenylvinyl)biphenyl. Chem Commun 2010;46(5):686e8;
(d) Luo X, Zhao W, Shi J, Li C, Liu Z, Bo Z, et al. Reversible switching emissions
of tetraphenylethene derivatives among multiple colors with solvent vapor,
mechanical, and thermal stimuli. J Phys Chem C 2012;116(41):21967e72;
(e) Chan CYK, Lam JWY, Zhao Z, Deng C, Chen S, Lu P, et al. A facile approach to
highly efficient and thermally stable solid-state emitters: knitting up AIE-
active TPE luminogens by aryl linkers. ChemPlusChem 2012;77(10):949e58;
(f) Wang J, Mei J, Hu R, Sun JZ, Qin A, Tang BZ. Click synthesis, aggregation-
induced emission, E/Z isomerization, self-organization, and multiple chrom-
isms of pure stereoisomers of a tetraphenylethene-cored luminogen. J Am
Chem Soc 2012;134(24):9956e66.
(b) Zhao Z, Chen S, Lam JWY, Lu P, Zhong Y, Wong KS, et al. Creation of highly
efficient solid emitter by decorating pyrene core with AIE-active tetraphe-
nylethene peripheries. Chem Commun 2010;46(13):2221e3;
(c) Zhao Z, Lam JWY, Chan CYK, Chen S, Liu J, Lu P, et al. Stereoselective
synthesis, efficient light emission, and high bipolar charge mobility of chias-
matic luminogens. Adv Mater 2011;23(45):5430e5;
[13] Liu Y, Deng C, Tang L, Qin A, Hu R, Sun JZ, et al. Specific detection of D-glucose
by
a tetraphenylethene-based fluorescent sensor. J Am Chem Soc
(d) Yuan WZ, Lu P, Chen S, Lam JWY, Wang Z, Liu Y, et al. Changing the
behavior of chromophores from aggregation-caused quenching to
aggregation-induced emission: development of highly efficient light emitters
in the solid state. Adv Mater 2010;22(19):2159e63;
(e) Zhao Z, Chen S, Chan CYK, Lam JWY, Jim CKW, Lu P, et al. A facile and
versatile approach to efficient luminescent materials for applications in
organic light-emitting diodes. Chem Asian J 2012;7(3):484e8.
2011;133(4):660e3.
[14] (a) Xu B, Chi Z, Zhang J, Zhang X, Li H, Li X, et al. Piezofluorochromic and
aggregation-induced emission compounds containing triphenylethylene and
tetraphenylethylene moieties. Chem Asian J 2011;6(6):1470e8;
(b) Zhang X, Chi Z, Xu B, Chen C, Zhou X, Zhang Y, et al. End-group effects of
piezofluorochromic aggregation-induced enhanced emission compounds
containing distyrylanthracene. J Mater Chem 2012;22(35):18505e13;
(c) Zhang X, Chi Z, Li H, Xu B, Li X, Zhou W, et al. Piezofluorochromism of an
aggregation-induced emission compound derived from tetraphenylethylene.
Chem Asian J 2011;6(3):808e11.
[11] (a) Huang J, Sun N, Yang J, Tang R, Li Q, Ma D, et al. Benzene-cored fluorophors
with TPE peripheries: facile synthesis, crystallization-induced blue-shifted
emission, and efficient blue luminogens for non-doped OLEDs. J Mater Chem
2012;22(24):12001e7;
[15] (a) Qi Q, Liu Y, Fang X, Zhang Y, Chen P, Wang Y, et al. AIE (AIEE) and
mechanofluorochromic performances of TPE-methoxylates: effects of single
molecular conformations. RSC Adv 2013;3(21):7996e8002;
(b) Huang J, Sun N, Dong Y, Tang R, Lu P, Cai P, et al. Similar or totally different:
the control of conjugation degree through minor structural modifications, and
deep-blue aggregation-induced emission luminogens for non-doped OLEDs.
Adv Funct Mater 2013;23(18):2329e37;
(b) Qi Q, Fang X, Liu Y, Zhou P, Zhang Y, Yang B, et al. A TPE-oxazoline mo-
lecular switch with tunable multi-emission in both solution and solid state.
RSC Adv 2013;3(38):16986e9.
(c) Huang J, Yang X, Wang J, Zhong C, Wang L, Qin J, et al. New