End-group effects of piezofluorochromic aggregation-induced enhanced emission compounds containing distyrylanthracene

Article abstract of DOI:10.1039/c2jm33140c

Distyrylanthracene derivatives, AnP₃ and AnP₃P with triphenylethylene end-groups, and AnP₄ and AnP₄P with tetraphenylethylene end-groups, were synthesized and characterized using nuclear magnetic resonance, mass spectrometry, elemental analysis, photoluminescence, ultraviolet-visible absorption, wide-angle X-ray diffraction, differential scanning calorimetry and other techniques. The results show that the piezofluorochromic properties of these four aggregation-induced enhanced emission (AIEE) compounds were influenced significantly by the structures of the end-groups in the molecules. The AnP₄ and AnP₄P derivatives with tetraphenylethylene end-groups exhibited distinct piezofluorochromic properties, which might be switched reversibly upon pressing or annealing because of crystalline-amorphous phase transformation. Thus, we propose that those AIEE compounds that contain steric hindrance groups may exhibit evident piezofluorochromic activities.

Full text of DOI:10.1039/c2jm33140c

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 18505
www.rsc.org/materials
PAPER
End-group effects of piezofluorochromic aggregation-induced enhanced
emission compounds containing distyrylanthracene†
a
a
Xiqi Zhang, Zhenguo Chi, Bingjia Xu,a Chengjian Chen,a Xie Zhou,b Yi Zhang,a Siwei Liua and Jiarui Xua
*
Received 17th May 2012, Accepted 19th July 2012
DOI: 10.1039/c2jm33140c
Distyrylanthracene derivatives, AnP3 and AnP3P with triphenylethylene end-groups, and AnP4 and
AnP4P with tetraphenylethylene end-groups, were synthesized and characterized using nuclear
magnetic resonance, mass spectrometry, elemental analysis, photoluminescence, ultraviolet-visible
absorption, wide-angle X-ray diffraction, differential scanning calorimetry and other techniques. The
results show that the piezofluorochromic properties of these four aggregation-induced enhanced
emission (AIEE) compounds were influenced significantly by the structures of the end-groups in the
molecules. The AnP4 and AnP4P derivatives with tetraphenylethylene end-groups exhibited distinct
piezofluorochromic properties, which might be switched reversibly upon pressing or annealing because
of crystalline-amorphous phase transformation. Thus, we propose that those AIEE compounds that
contain steric hindrance groups may exhibit evident piezofluorochromic activities.
emission (PAIE) compound was first reported by Park et al. in
2010.8 Almost within the same period, a number of new PAIE
Introduction
Piezofluorochromic materials are a class of ‘‘smart’’ materials
compounds were also synthesized in our laboratory and by other
with fluorescent properties that change in response to external
groups.9 However, the piezofluorochromic mechanism of the
force stimuli.1 As the tuning of solid luminescent compound
PAIE compounds is still debated because the precise molecular
emissions by controlling the mode of molecular packing is more
packing structure of the pressed sample is usually hard to iden-
practical than altering the chemical structures, these materials
tify. Thus, the structure–property relationships of these
may be widely used in sensors,2 memory chips3 and security inks.4
compounds needs to be established to help the identification and
However, organic piezofluorochromic materials are still rare,
synthesis of more novel piezofluorochromic materials.
partly because the fluorescence efficiency of organic luminescent
materials often becomes very weak in the solid state as a result of
the aggregation-caused quenching (ACQ) effect which is disad-
Experimental section
vantageous to the observation of the piezofluorochromic
Materials and measurements
phenomenon.
In 2001, Tang et al.5 reported an important class of anti-ACQ
materials, namely aggregation-induced emission (AIE) materials,
which can emit more efficiently in the aggregated state. In 2002,
Park et al.6 reported on aggregation-induced enhanced emission
(AIEE) materials, which are similar to AIE materials. Since then,
AIE (or AIEE) materials have attracted considerable research
attention for their potential application in various fields, such as
organic light-emitting devices and chemosensors.7 On the other
9,10-Bis(chloromethyl)anthracene, triethyl phosphite, benzo-
phenone, diethyl 4-bromobenzylphosphonate, n-butyl lithium
(n-BuLi) in hexane (2.4 M), 1-formylpiperidine, 4-for-
mylphenylboronic acid, tetrabutyl ammonium bromide (TBAB),
tetrakis(triphenylphosphine) palladium(0) [Pd(PPh3)4], diphe-
nylmethane, 4-bromophenyl phenylmethanone, p-toluene-
sulfonic acid and potassium tert-butoxide (t-BuOK) were
purchased from Alfa Aesar and used as received. All other
reagents and solvents were purchased as analytical grade from
Guangzhou Dongzheng Company (China) and used without
further purification.
hand,
a piezofluorochromic aggregation-induced enhanced
aPCFM Lab, DSAPM Lab, KLGHEI of Environment and Energy
Chemistry, State Key Laboratory of Optoelectronic Materials and
Technologies, School of Chemistry and Chemical Engineering, Sun
Yat-sen University, Guangzhou 510275, P. R. China. E-mail: chizhg@
mail.sysu.edu.cn; Fax: +86 20 84112222; Tel: +86 20 84112712
bSchool of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou
510275, China
Intermediates 1, P4Br and P3Br were synthesized according to
the literature methods.10 Intermediate P4A and compound AnP4
were synthesized according to procedures in the literature
previously published by us.11
Proton nuclear magnetic resonance (1H NMR) spectra were
measured on a Mercury-Plus 300 spectrometer [CDCl3, tetra-
methylsilane (TMS) as the internal standard]. Mass spectra (MS)
† Electronic supplementary information (ESI) is available: Fig. S1 and
S2. See DOI: 10.1039/c2jm33140c
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 18505–18513 | 18505

View Article Online
were measured on an EI-mass spectrometer or FAB-double
focussing mass spectrometer. Elemental analyses (EA) were
performed with an Elementar Vario EL elemental analyzer.
Photoluminescence spectra (PL) were measured on a Shimadzu
RF-5301pc spectrometer with a slit width of 1.5 nm for both
excitation and emission. Ultraviolet-visible absorption spectra
(UV) were recorded on a Hitachi ultraviolet-visible spectropho-
tometer (U-3900). The solid powder samples for PL and UV
measurements were loaded on a grooved quartz glass slide to be
determined by using a solid sample holder (for PL) and a 60 mm
integrating sphere (for UV) which were standard fittings of the
PL and UV spectrometers. Differential scanning calorimetry
(DSC) curves were obtained with a NETZSCH thermal analyzer
Synthesis of P3PA
P3Br (3.00 g, 9.0 mmol) and 4-formylphenylboronic acid (2.03 g,
13.5 mmol) were dissolved in a mixture of toluene (80 mL),
TBAB (0.32 g, 1.0 mmol) and 2 M potassium carbonate aqueous
solution (24 mL). The mixture was stirred at room temperature
for 0.5 h under Ar gas followed by adding Pd(PPh3)4 (0.010 g,
8.70 ꢂ 10ꢁ3 mmol) and then heating 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 pres-
sure, the residue was chromatographed on a silica gel column
with n-hexane/CH2Cl2 (5 : 1 by volume) as eluent to give P3PA
1
(2.50 g, 78% yield). H NMR (300 MHz, CDCl3) d (ppm): 7.01
ꢁ1
ꢀ
(DSC 204 F1) at a heating rate of 10 C min under N2 atmo-
sphere. Wide-angle X-ray diffraction (WAXD) measurements
were performed by using a Bruker X-ray diffractometer
(D8 ADVANCE, Germany) with an X-ray source of Cu Ka
(l ¼ 0.15406 nm) at 40 kV and 40 mA, at a scan rate of 4ꢀ (2q) per
1 min. Pressed samples were prepared by pressing in an IR pellet
press at 1500 psi for 5 min. Annealing experiments were done on
a hot-stage with an automatic temperature control system for 5
(s, 1H), 7.12 (d, 2H, J ¼ 8.1 Hz), 7.26–7.47 (m, 12H), 7.70 (d, 2H,
J ¼ 8.1 Hz), 7.91 (d, 2H J ¼ 8.1 Hz), 10.02 (s, 1H); MS (EI), m/z:
360, calcd for C27H20O, 360.
Synthesis of P4PA
P4Br (4.11 g, 10 mmol) and 4-formylphenylboronic acid (2.25 g,
15 mmol) were dissolved in a mixture of toluene (60 mL), TBAB
(0.32 g, 1.0 mmol) and 2 M potassium carbonate aqueous solu-
tion (18 mL). The mixture was stirred at room temperature
for 0.5 h under Ar gas followed by adding Pd(PPh3)4 (0.010 g,
8.70 ꢂ 10ꢁ3 mmol) and then heating 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 pres-
sure, the residue was chromatographed on a silica gel column
with n-hexane/CH2Cl2 (3 : 1 by volume) as eluent to give P4PA
(2.80 g, 64% yield). 1H NMR (300 MHz, CDCl3) d (ppm): 7.01–
7.17 (m, 17H), 7.40 (d, 2H, J ¼ 8.1 Hz), 7.70 (d, 2H, J ¼ 8.1 Hz),
7.90 (d, 2H J ¼ 8.1 Hz), 10.02 (s, 1H); MS (EI), m/z: 436, calcd
for C33H24O, 436.
ꢀ
min, the annealing temperatures were 200, 250, 340 and 250 C
for AnP3, AnP3P, AnP4 and AnP4P, respectively. The THF–
water mixtures with different water fractions were prepared by
slowly adding distilled water into the THF solution of samples
under ultrasound at room temperature.
Time-resolved emission decay behaviors were measured on an
Edinburgh Instruments Ltd. spectrometer (FLSP920) by exciting
at 380 nm and the data was processed according to the literature
method.12 Decay in the fluorescence intensity (I) with time (t) was
fitted by a double-exponential function:
I ¼ A1eꢁt/s + A1eꢁt/s
1
2
where s1 and s2 are the lifetimes of the shorter- and longer-lived
species, respectively, and A1 and A2 are their respective ampli-
tudes. The weighted mean lifetime hsi was calculated by the
following equation:
Synthesis of AnP3
P3A (0.55 g, 1.9 mmol) and 1 (0.38 g, 0.80 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
recrystallized with THF/EtOH to give AnP3 (0.55 g, 94% yield).
1H NMR (300 MHz, CDCl3) d (ppm): 6.81 (s, 1H), 6.87 (s, 1H),
7.03 (s, 2H), 7.11 (d, 4H, J ¼ 8.4 Hz), 7.26–7.49 (m, 28H), 7.83 (s,
1H), 7.89 (s, 1H), 8.35 (q, 4H, J ¼ 3.3 Hz); MS (EI), m/z: 738,
calcd for C58H42, 738. Anal. calcd for C58H42: C 94.27, H 5.73;
found: C 94.22, H 5.70%.
A1s1 þ A2s2
\s . ¼
A1 þ A2
Synthesis of P3A
To a stirred solution of P3Br (3.35 g, 10 mmol) in anhydrous
THF (40 mL) was added n-butyl lithium solution in hexane
(2.4 M, 8.30 mL, 20 mmol) dropwise slowly at ꢁ78 ꢀC. The
mixture was stirred at ꢁ78 ꢀC under Ar gas for an additional 2 h.
1-Formylpiperidine (2.20 mL) was added quickly at ꢁ78 ꢀC and
the mixture was stirred overnight allowing the temperature to
rise gradually to room temperature. The product was extracted
into dichloromethane (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
Synthesis of AnP3P
P3PA (0.69 g, 1.9 mmol) and 1 (0.38 g, 0.80 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
recrystallized with THF/EtOH to give AnP3P (0.56 g, 79% yield).
1H NMR (300 MHz, CDCl3) d (ppm): 6.92 (d, 2H, J ¼ 9.3 Hz),
6.97–7.04 (m, 4H), 7.13 (d, 4H, J ¼ 8.1 Hz), 7.21 (s, 2H), 7.26–
7.40 (m, 18H), 7.43–7.51 (m, 6H), 7.59–7.75 (m, 8H), 7.94 (s, 1H),
8.00 (s, 1H), 8.40 (q, 4H, J ¼ 3.3 Hz); MS (EI), m/z: 890, calcd for
chromatographed on
a silica gel column with n-hexane/
dichloromethane (5 : 1 by volume) as eluent to give P3A (3.50 g,
1
62% yield). H NMR (300 MHz, CDCl3) d (ppm): 7.00 (s, 1H),
7.13–7.23 (m, 4H), 7.30–7.38 (m, 8H), 7.64 (d, 2H, J ¼ 8.4 Hz),
7.89 (s, 1H); MS (EI), m/z: 284, calcd for C21H16O, 284.
18506 | J. Mater. Chem., 2012, 22, 18505–18513
This journal is ª The Royal Society of Chemistry 2012

View Article Online
C70H50, 890. Anal. calcd for C70H50: C 94.34, H 5.66; found: C
94.24, H 5.70%.
Synthesis of AnP4P
P4PA (0.22 g, 0.50 mmol) and 1 (0.10 g, 0.21 mmol) were dis-
solved 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 recrystallized with THF/EtOH to give AnP4P (0.37 g, 85%
yield). 1H NMR (300 MHz, CDCl3) d (ppm): 6.93 (s, 2H), 6.99 (s,
2H), 7.02–7.20 (m, 32H), 7.40–7.52 (m, 8H), 7.62–7.75 (m, 8H),
7.94 (s, 1H), 7.99 (s, 1H), 8.41 (q, 4H, J ¼ 3.3 Hz); MS (FAB)
calcd. for C82H58 1042, found 1043 [M + H]+. Anal. calcd for
C82H58: C 94.40, H 5.60; found: C 94.46, H 5.55%.
Results and discussion
In this paper, structural units with AIEs, such as triphenyl-
ethylene or tetraphenylethylene groups, have been introduced
into the piezofluorochromic distyrylanthracene core to synthe-
size four novel AIE compounds. The target compounds were
synthesized according to the routes shown in Scheme 1. These
compounds were characterized using nuclear magnetic reso-
nance, mass spectrometry and elemental analysis.
To determine whether the compounds have AIEE properties,
the ultraviolet-visible (UV) absorption and photoluminescence
(PL) emission behaviors of their diluted mixtures were studied
using THF–water mixtures with different water fractions. The
compounds were insoluble in water, while soluble in THF, thus,
increasing the water fractions in the mixed solvents would change
the existing compound solutions in pure THF into aggregated
particles, thereby changing their UV and PL spectra.
As shown in Fig. 1, when the water fraction is less than 50%,
the PL intensity of AnP3 is very weak. However, with a water
fraction of 60%, the THF–water mixture’s luminescence signifi-
cantly increased and the PL intensity increased from 4.2 to 95,
which is approximately 23 times higher than that in the pure
THF. Thus, the increase in PL intensity can be attributed to the
AIEE effect caused by the formation of molecular aggregates
when water is added into the solution. This result indicates that
Scheme 1 Synthetic routes for AnP3, AnP3P, AnP4 and AnP4P.
the compound exhibits
a
strong AIEE activity. Similar
determination of the AIEE effect, but the reasons remain
unclear. There are two possible explanations for this phenom-
enon.13 First, after the aggregation, 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 in the aggregation state could
enhance light emission. The net output of these antagonistic
processes depends on which process plays a predominant role in
affecting the fluorescent behavior of the aggregated molecules.14
Second, when water is added, the solute molecules can aggregate
into various kinds of nanoparticle suspensions, typically, crystal
particles and amorphous particles.15 The former leads to an
enhancement in the PL intensity, while the latter leads to a
reduction in intensity. Thus, the measured overall PL intensity
data depends on the combined actions of the two kinds of
nanoparticles. However, it is difficult to control the formation of
nanoparticles in high water content. Thus, the measured PL
phenomena were observed for the other three compounds.
However, the starting points for the increase in PL intensity in
AnP3P, AnP4 and AnP4P differed from one another and were
found at 30%, 40% and 40% water fractions, respectively,
because of the varying solubility in mixed solvents. The smaller
compound AnP3 exhibited a higher solubility, and thus it was the
least easily aggregated and precipitated one from the mixed
solvents.
As can be seen from the insets in Fig. 1, the PL intensity for
each compound shows no trend of monotonic increasing, but
exhibits a zig-zag pattern after the formation of molecular
aggregates. After reaching a maximum intensity at 60% (for
AnP3 and AnP3P systems), 80% (for AnP4 system) or 40% (for
AnP4P system) water fraction the PL intensity of the four
compounds decreases with increasing water content. Then, a
further increase of the water fraction resulted in an increase of PL
intensity. This phenomenon was often observed in the
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 18505–18513 | 18507

View Article Online
structures, AnP3 should have the best crystallization capacity
among the compounds.
The colors of the images of these compounds in THF–water
mixtures with different water fractions significantly differed. This
can be attributed to the formation of various aggregation states
of the molecules in the mixed solvent, such as crystalline and
amorphous particles.
The absorption spectra of AnP3 in the THF–water mixtures
(10 mM) are shown in Fig. 2. The spectral profile was significantly
changed when 60% water was added into the THF solution.
Then, the entire absorption spectra started to increase, indicating
the formation of nanoscopic AnP3 aggregates. The light scat-
tering, or Mie effect, of the nanoaggregate suspensions in the
solvent mixtures effectively decreased the light transmission in
the mixture and caused the evident absorbance difference and
levelling-off tail in the visible region of the UV absorption
spectrum.16 Similar results were observed in AnP3P, AnP4 and
AnP4P. It also can be seen that the absorption spectra of the
compounds exhibit some differences in absorption band shapes
and maxima when the water fraction increases continuously after
the formation of nanoscopic aggregates. This also can be
attributed to the formation of various aggregated states of the
molecules.
To gain the lowest energy spatial conformation of the
compounds, we conducted quantum mechanical computations
by using the Gaussian 03 software.17 The HOMOs and LUMOs
of these compounds were obtained (Fig. S1, ESI†) according to
the density functional method at the B3LYP/6-31 G level after
the structural optimization. These HOMOs and LUMOs showed
similar electron cloud distributions. Moreover, these compounds
adopted a twisted spatial conformation, as shown by their
optimized structures. The dihedral angles of the adjacent phenyl
planes (defined in Fig. S1†) differed among the compounds
(Table 1). For example, the dihedral angles of the plane C–E were
measured as 64ꢀ both in compounds AnP3 and AnP3P and 80ꢀ in
AnP4 and AnP4P. The differences in the dihedral angles causes
the compounds to use different molecular packing modes and
affects the macroscopic properties of the materials.
A
common structural feature of the reported AIEE
compounds is that multiple peripheral phenyl groups are linked
to an olefinic core via rotatable carbon–carbon single bonds to
form an AIEE moiety. The steric effect between the phenyl rings
forces the AIEE moieties or the molecules to twist. Additionally,
anthracene spacing with double bonds also causes a significant
twist of the conjugated backbone due to steric effects.18 The
twisted conformation may sterically hinder p–p stacking in the
solid state and the molecular packing become relatively loose
because of the twisted conformation and weak p–p interactions.
Thus, defects are formed. An AIEE compound may exhibit
piezochromic fluorescence if it exists in two different pressure-
dependent stable or metastable states.1a
Fig. 1 PL spectra of AnP3, AnP3P, AnP4 and AnP4P in THF–water
mixtures with different water fractions. The insets depict the changes in
PL peak intensity and emission images of the compound in different
water fraction mixtures under 365 nm UV illumination.
The images of the four compounds taken at room temperature
under natural and UV light after pressing and annealing are
shown in Fig. 3. Compounds AnP3 and AnP3P with the triphe-
nylethylene end-group did not exhibit distinct piezochromic or
piezofluorochromic properties, whereas the AnP4 and AnP4P
with tetraphenylethylene end-groups did. A strong green-emit-
ting light-yellow solid of AnP4P was converted into an orange
solid with strong yellow luminescence when the compound was
intensity often shows no regularity in high water content. The
compound AnP3 displays a burst for one particular solvent
composition (60% water fraction) and is relatively non fluores-
cent at all others. A possible reason for this strange phenomenon
is attributed to the formation of perfect nanocrystals in this
solvent composition. In comparison to their molecular
18508 | J. Mater. Chem., 2012, 22, 18505–18513
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Table 1 Selected dihedral angles of AnP3, AnP3P, AnP4 and AnP4P
A–C
A–B
C–D
C–E
C–F
D–E
AnP3
56ꢀ
—
58ꢀ
—
—
60ꢀ
60ꢀ
58ꢀ
58ꢀ
64ꢀ
64ꢀ
80ꢀ
80ꢀ
—
75ꢀ
75ꢀ
75ꢀ
75ꢀ
AnP3P
AnP4
AnP4P
56ꢀ
—
—
75ꢀ
75ꢀ
58ꢀ
Fig. 3 The images of AnP3 (a), AnP3P (b), AnP4 (c) and AnP4P (d) taken
at room temperature under (left column) natural light and (right column)
UV light after pressing and annealing.
different conditions remained almost unchanged. The wave-
lengths only changed by 1 nm and 3 nm, respectively. Thus, AnP3
and AnP3P had no piezofluorochromic properties. However,
significant red-shifts that reached up to 68 and 44 nm were
observed in AnP4 and AnP4P, respectively. After the pressed
samples were annealed, the initial PL wavelengths (similar to the
unpressed samples) were recovered, thereby demonstrating high
reversibility in the emission spectra. This reversible color change
feature makes AnP4 and AnP4P promising candidates for optical
recording and temperature- or pressure-sensing systems.
The destruction of the crystalline structure leads to the
planarization of the molecular conformation because of the
release of the twist strain. This phenomenon is one of the possible
reasons for the increase in molecular conjugation resulting in a
red-shift in the PL spectrum.1a Generally, the excimer formation
of the fluorescent molecules induces a large red shift. In this case,
the UV absorption spectra can be used to determine whether the
red-shift in the PL spectrum is caused by the formation of exci-
mers. If the increased molecular conjugation induced by the
planarization of molecular conformation causes such a signifi-
cant red shift in the emission spectrum, the absorption spectra
should exhibit a similar change but remain the same in the case of
excimer emission. The experimental results showed that the UV
spectra of the annealed and pressed AnP4 and AnP4P samples
were significantly different; however, those of the AnP3 and
AnP3P samples were almost the same (Fig. 6). Consequently, the
colors of the annealed and pressed AnP4 and AnP4P samples
significantly differed (Fig. 3, left). The UV absorption spectra of
Fig. 2 UV-vis absorption spectra of AnP3, AnP3P, AnP4 and AnP4P in
THF–water mixtures with different water fractions.
pressed (Fig. 3). After the pressed samples were annealed, the
color returned to its original appearance, thus demonstrating
good reversibility in both the absorption and emission spectra.
Similar phenomena were observed in AnP4.
The PL spectra of the samples obtained under different
conditions (pressing and annealing) are shown in Fig. 4 and 5.
The maximum emission wavelengths of AnP3 and AnP3P under
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 18505–18513 | 18509

View Article Online
Fig. 5 The normalized PL curves of AnP4 and AnP4P: (a) annealed
sample; (b) pressed sample; (c) annealed sample (b); (d) pressed sample
(The inset depicts the reversible changes of PL wavelength under the
above different conditions).
Fig. 4 The normalized PL spectra of AnP3 and AnP3P: (a) pressed
sample; (b) annealed sample; (c) pressed sample; (d) annealed sample.
the compounds before and after pressing treatment confirm that
the piezochromism is caused by the planarization of the molec-
ular conformation not by the formation of excimer.
Reproducible heating DSC results (Fig. 8) showed that the
pressed AnP4 and AnP4P samples had significant cold-crystalli-
zation peaks at approximately 336.5 ꢀC (9.1 kJ molꢁ1 enthalpy)
and 179.0 ꢀC (10.3 kJ molꢁ1 enthalpy), respectively. These peaks
indicated the presence of a metastable-state aggregation in the
pressed sample. This state could transform into a more stable
state through annealing. However, the annealed samples did not
have such peaks. These findings indicate that the pressing and
annealing treatments cause reversible solid-state morphological
changes. However, no such cold-crystallization peaks could be
found in AnP3 and AnP3P.
To further determine the piezofluorochromism mechanism,
wide-angle X-ray diffraction (WAXD) and differential scanning
calorimetry (DSC) measurements were conducted.
The WAXD curves of AnP4 and AnP4P (Fig. 7) exhibited
different molecular aggregation structures before and after
pressing. The diffraction curves of the annealed samples revealed
numerous sharp and intense reflections that indicated a crystal-
line order. The diffraction curves of the pressed samples showed
a weak, broad and diffused peak that indicates an amorphous
structure. Moreover, the WAXD curves of AnP3 and AnP3P
showed almost the same molecular aggregation structures before
and after pressing, which also indicates a crystalline order. These
results show that the structural changes in molecular aggregation
from the crystalline to amorphous state can induce significant
piezofluorochromic properties. The different piezofluorochromic
properties of the compounds may be due to the steric effect of the
end-groups, i.e., as the steric effect of the tetraphenylethylene
group was larger than that of the triphenylethylene group and the
degree of order of molecular aggregation decreased after
pressing, it was hard for the aggregates to revert into the
unpressed state because of the steric effect. The lower degree of
order was then maintained, resulting in the red-shift of the PL
wavelength. The red-shifted pressed state can be changed to the
higher degree of order state via annealing along with the blue-
shift of the PL wavelength.
The time-resolved emission-decay behaviors of the compounds
after annealing and pressing were studied. The time-resolved
fluorescence curves and the lifetime data are illustrated in Fig. S2
(ESI†) and Table 2, respectively. Table 2 showed the two relax-
ation pathways for the fluorescence decay, indicating that the
time-resolved PL spectra of the compound included independent
emissions from the segments with different p-conjugation
lengths because multiple lifetimes have been detected. The
weighted mean lifetimes hsi of the annealed and pressed AnP3 or
AnP3P samples were similar, and the time-resolved fluorescence
curves almost coincided with each other. In contrast, the hsi
values of the annealed and pressed AnP4 or AnP4P samples were
significantly different: 0.93 and 0.50 ns for the annealed and
pressed AnP4 samples, respectively, and 0.63 and 1.37 ns for the
two AnP4P samples, respectively. These results show that the
18510 | J. Mater. Chem., 2012, 22, 18505–18513
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Fig. 6 Normalized solid UV-vis absorption spectra of the annealed and
Fig. 7 WAXD curves of AnP3, AnP3P, AnP4 and AnP4P: (a) pressed
pressed of AnP3, AnP3P, AnP4 and AnP4P samples.
sample; (b) annealed sample; (c) pressed sample; (d) annealed sample.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 18505–18513 | 18511

View Article Online
Table 2 Solid-state fluorescence lifetime data of AnP3, AnP3P, AnP4
and AnP4P samples after being annealed and pressed
s1 (ns)a
s2 (ns)a
A1
A2
hsi (ns)c
b
b
Sample
AnP3
Annealed
Pressed
Annealed
Pressed
Annealed
Pressed
Annealed
Pressed
0.43
0.37
0.52
0.51
0.48
0.35
0.50
0.79
0.88
0.83
1.01
1.09
1.54
1.28
1.24
2.03
0.39
0.42
0.63
0.52
0.58
0.84
0.82
0.53
0.61
0.58
0.37
0.48
0.42
0.16
0.18
0.47
0.70
0.64
0.70
0.79
0.93
0.50
0.63
1.37
AnP3P
AnP4
AnP4P
a
b
c
Fluorescence lifetime. Fractional contribution. Weighted mean
lifetime.
Conclusions
In summary, distyrylanthracene derivatives capped with triphe-
nylethylene and tetraphenylethylene groups were successfully
synthesized. All four compounds exhibit strong AIEE activities.
However, only the derivatives capped with tetraphenylethylene
groups showed significant piezofluorochromic properties and the
ability to revert into their original forms upon annealing due to
the crystalline-amorphous phase transformation. These excellent
properties make the compounds promising stimuli-responsive,
smart, luminescent materials for information-recording, pres-
sure-sensors and light-emitting device applications. Thus, AIEE
compounds that contain steric hindrance groups can potentially
exhibit distinct piezofluorochromic activities. The general struc-
ture–property relationships established in this study should be
helpful in guiding researchers to identify and synthesize more
piezofluorochromic materials.
Acknowledgements
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (51173210,
51073177), 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 Guangdong
Province (Grant numbers: 20081203), the Fundamental
Research Funds for the Central Universities and the Natural
Science Foundation of Guangdong (S2011020001190).
Notes and references
1 (a) Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu and
J. Xu, Chem. Soc. Rev., 2012, 41, 3878; (b) Y. Sagara and T. Kato,
Nat. Chem., 2009, 1, 605; (c) A. Pucci and G. Ruggeri, J. Mater.
Chem., 2011, 21, 8282; (d) M. M. Caruso, D. A. Davis, Q. Shen,
S. A. Odom, N. R. Sottos, S. R. White and J. S. Moore, Chem.
Rev., 2009, 109, 5755; (e) K. Ariga, T. Mori and J. P. Hill, Adv.
Mater., 2012, 24, 158.
Fig. 8 DSC curves of AnP3, AnP3P, AnP4 and AnP4P samples obtained
from: (a) pressed sample; (b) annealed sample.
2 (a) Z. Ning, Z. Chen, Q. Zhang, Y. Yan, S. Qian, Y. Cao and H. Tian,
Adv. Funct. Mater., 2007, 17, 3799; (b) B. R. Crenshaw,
M. Burnworth, D. Khariwala, A. Hiltner, P. T. Mather, R. Simha
and C. Weder, Macromolecules, 2007, 40, 2400; (c) M. Kinami,
B. R. Crenshaw and C. Weder, Chem. Mater., 2006, 18, 946; (d)
S. J. Toal, K. A. Jones, D. Magde and W. C. Trogler, J. Am. Chem.
Soc., 2005, 127, 11661.
time-resolved emission-decay behaviors of the annealed and
pressed compounds exhibit distinct changes if the materials show
significant piezofluorochromic properties.
18512 | J. Mater. Chem., 2012, 22, 18505–18513
This journal is ª The Royal Society of Chemistry 2012

View Article Online
3 (a) S. Hirata and T. Watanabe, Adv. Mater., 2006, 18, 2725; (b)
S. J. Lim, B. K. An, S. D. Jung, M. A. Chung and S. Y. Park,
Angew. Chem., Int. Ed., 2004, 43, 6346; (c) C. E. Olson,
M. J. R. Previte and J. T. Fourkas, Nat. Mater., 2002, 1, 225; (d)
M. Irie, T. Fukaminato, T. Sasaki, N. Tamai and T. Kawai,
Nature, 2002, 420, 759.
4 A. Kishimura, T. Yamashita, K. Yamaguchi and T. Aida, Nat.
Mater., 2005, 4, 546.
5 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.
6 B. K. An, S. K. Kwon, S. D. Jung and S. Y. Park, J. Am. Chem. Soc.,
2002, 124, 14410.
S. Liu and J. Xu, J. Fluoresc., 2012, 22, 565; (h) Z. Chi, K. He,
H. Li, X. Zhang, B. Xu, S. Liu, Y. Zhang and J. Xu, Chem. J.
Chin. Univ., 2012, 33, 725; (i) C. Dou, L. Han, S. Zhao, H. Zhang
and Y. Wang, J. Phys. Chem. Lett., 2011, 2, 666; (j) X. Luo, J. Li,
C. Li, L. Heng, Y. Q. Dong, Z. Liu, Z. Bo and B. Z. Tang, Adv.
Mater., 2011, 23, 3261; (k) S. J. Yoon and S. Y. Park, J. Mater.
Chem., 2011, 21, 8338.
10 (a) H. Zhang, E. Guo, Y. Zhang, P. Ren and W. Yang, Chem. Mater.,
2009, 21, 5125; (b) Z. Zhao, S. Chen, J. W. Y. Lam, P. Lu, Y. Zhong,
K. S. Wong, H. S. Kwok and B. Z. Tang, Chem. Commun., 2010, 46,
2221; (c) S. Wang, W. J. Oldham, Jr, R. A. Hudack Jr and
G. C. Bazan, J. Am. Chem. Soc., 2000, 122, 5695.
7 (a) B. K. An, D. S. Lee, J. S. Lee, Y. S. Park, H. S. Song and
S. Y. Park, J. Am. Chem. Soc., 2004, 126, 10232; (b) Y. Hong,
J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2011, 40, 5361; (c)
X. Zhang, Z. Chi, H. Li, B. Xu, X. Li, S. Liu, Y. Zhang and J. Xu,
J. Mater. Chem., 2011, 21, 1788; (d) B. Xu, Z. Chi, Z. Yang,
J. Chen, S. Deng, H. Li, X. Li, Y. Zhang, N. Xu and J. Xu, J.
Mater. Chem., 2010, 20, 4135; (e) Z. Yang, Z. Chi, T. Yu,
X. Zhang, M. Chen, B. Xu, S. Liu, Y. Zhang and J. Xu, J. Mater.
Chem., 2009, 19, 5541.
8 S. J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M. G. Choi,
D. Kim and S. Y. Park, J. Am. Chem. Soc., 2010, 132, 13675.
9 (a) B. Xu, Z. Chi, J. Zhang, X. Zhang, H. Li, X. Li, S. Liu, Y. Zhang
and J. Xu, Chem.–Asian J., 2011, 6, 1470; (b) H. Li, X. Zhang,
Z. Chi, B. Xu, W. Zhou, S. Liu, Y. Zhang and J. Xu, Org. Lett.,
2011, 13, 556; (c) H. Li, Z. Chi, B. Xu, X. Zhang, X. Li, S. Liu,
Y. Zhang and J. Xu, J. Mater. Chem., 2011, 21, 3760; (d)
X. Zhang, Z. Chi, J. Zhang, H. Li, B. Xu, X. Li, S. Liu, Y. Zhang
and J. Xu, J. Phys. Chem. B, 2011, 115, 7606; (e) X. Zhou, H. Li,
Z. Chi, X. Zhang, J. Zhang, B. Xu, Y. Zhang, S. Liu and J. Xu,
New J. Chem., 2012, 36, 685; (f) B. Xu, Z. Chi, X. Zhang, H. Li,
C. Chen, S. Liu, Y. Zhang and J. Xu, Chem. Commun., 2011, 47,
11080; (g) X. Zhou, H. Li, Z. Chi, B. Xu, X. Zhang, Y. Zhang,
11 X. Zhang, Z. Chi, H. Li, B. Xu, X. Li, W. Zhou, S. Liu, Y. Zhang and
J. Xu, Chem.–Asian J., 2011, 6, 808.
12 A. Qin, C. K. Jim, Y. Tang, J. W. Y. Lam, J. Liu, F. Mahtab, P. Gao
and B. Z. Tang, J. Phys. Chem. B, 2008, 112, 9281.
13 (a) Z. Yang, Z. Chi, B. Xu, H. Li, X. Zhang, X. Li, S. Liu, Y. Zhang
and J. Xu, J. Mater. Chem., 2010, 20, 7352; (b) H. Li, Z. Chi, B. Xu,
X. Zhang, Z. Yang, X. Li, S. Liu, Y. Zhang and J. Xu, J. Mater.
Chem., 2010, 20, 6103; (c) X. Zhang, Z. Chi, B. Xu, H. Li, Z. Yang,
X. Li, S. Liu, Y. Zhang and J. Xu, Dyes Pigm., 2011, 89, 56; (d)
B. Xu, Z. Chi, H. Li, X. Zhang, X. Li, S. Liu, Y. Zhang and J. Xu,
J. Phys. Chem. C, 2011, 115, 17574.
14 S. Dong, Z. Li and J. Qin, J. Phys. Chem. B, 2009, 113, 434.
15 X. Zhang, Z. Yang, Z. Chi, M. Chen, B. Xu, C. Wang, S. Liu,
Y. Zhang and J. Xu, J. Mater. Chem., 2010, 20, 292.
16 B. Z. Tang, Y. Geng, J. W. Y. Lam, B. Li, X. Jing, X. Wang, F. Wang,
A. B. Pakhomov and X. Zhang, Chem. Mater., 1999, 11, 1581.
17 M. J. Frisch, G. W. Trucks and H. B. Schlegel, et al., Gaussian 03.
Revision D.01, Gaussian, Inc., Wallingford CT, 2004.
18 (a) F. Silvestri, A. Marrocchi, M. Seri, C. Kim, T. J. Marks,
A. Facchetti and A. Taticchi, J. Am. Chem. Soc., 2010, 132, 6108;
(b) A. Marrocchi, F. Silvestri, M. Seri, A. Facchetti, A. Taticchia
and T. J. Marks, Chem. Commun., 2009, 1380.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 18505–18513 | 18513