Piezofluorochromic and aggregation-induced-emission compounds containing triphenylethylene and tetraphenylethylene moieties

Article abstract of DOI:10.1002/asia.201100211

New fluorescent compounds containing triphenylethylene and tetraphenylethylene moieties were synthesized, and their piezofluorochromic and aggregation-induced emission behaviors were investigated. The results show that all compounds exhibit aggregation-induced emission characteristics and only the crystalline compound possesses piezofluorochromic properties. The color, emission spectra, and morphological structures of the one piezofluorochromic compound exhibit reversibility upon grinding and annealing (or fuming) treatments. The piezofluorochromic behaviors are caused by a change between different modes of solid state molecular packing under external pressure. The single crystal X-ray diffraction analysis reveals that the twisted conformation of the aggregation-induced emission compound leads to the formation of metastable crystal lattice with cavity which is readily destroyed under external pressure. A possible mechanism of piezofluorochromic phenomenon has been proposed. Copyright

Full text of DOI:10.1002/asia.201100211

FULL PAPERS
DOI: 10.1002/asia.201100211
Piezofluorochromic and Aggregation-Induced-Emission Compounds
Containing Triphenylethylene and Tetraphenylethylene Moieties
Bingjia Xu, Zhenguo Chi,* Jianyong Zhang, Xiqi Zhang, Haiyin Li, Xiaofang Li,
Siwei Liu, Yi Zhang, and Jiarui Xu*[a]
Abstract: New fluorescent compounds
containing triphenylethylene and tetra-
phenylethylene moieties were synthe-
sized, and their piezofluorochromic
and aggregation-induced emission be-
haviors were investigated. The results
show that all compounds exhibit aggre-
gation-induced emission characteristics
and only the crystalline compound pos-
sesses piezofluorochromic properties.
The color, emission spectra, and mor-
phological structures of the one piezo-
fluorochromic compound exhibit rever-
sibility upon grinding and annealing
(or fuming) treatments. The piezofluor-
ochromic behaviors are caused by a
change between different modes of
solid state molecular packing under ex-
ternal pressure. The single crystal X-
ray diffraction analysis reveals that the
twisted conformation of the aggrega-
tion-induced emission compound leads
to the formation of metastable crystal
lattice with cavity which is readily de-
stroyed under external pressure. A pos-
sible mechanism of piezofluorochromic
phenomenon has been proposed.
Keywords: emission · fluorescence ·
piezofluorochromism
·
stimuli-
responsive materials · X-ray diffrac-
tion
Introduction
solid state are incomplete and irreversible. Even worse,
most chemical reactions often lead to loss of fluorescence
ability. However, piezofluorochromic materials that are de-
pendent on a physical change remain extremely rare.[1]
Recently, we reported some aggregation-induced emission
Piezofluorochromic or piezochromic fluorescent materials
are “smart” or “intelligent” materials whose fluorescence
properties change in response to external pressure stimuli.
As such, piezofluorochromic materials are one class of me-
chanoresponsive materials and have various potential appli-
cations in optical recording and pressure-sensing systems.
Piezofluorochromic behavior can be achieved by changing
the molecular chemical structures or altering the mode of
solid-state molecular packing. It is believed that the latter
change is easier than the former for affording dynamic con-
trol of the solid-state fluorescence with high efficiency and
reversibility because the latter is implemented by a physical
change and the former is implemented by a chemical reac-
tion. It is well known that most chemical reactions in the
(AIE) compounds containing
a phenylvinylanthracene
moiety.[2] Interestingly, four of these compounds are piezo-
fluorochromic compounds. We realized that there exists a
relationship between AIE and piezofluorochromic phenom-
ena, and called them piezofluorochromic aggregation-in-
duced emission (PAIE) materials as they possess both piezo-
fluorochromism and aggregation-induced emission proper-
ties. AIE materials are an important class of luminescent
materials, first reported by Tang and co-workers,[3] and ex-
hibit many special properties, such as strong solid emission,
excellent device performance and highly stimuli-sensitive
fluorescence.[4] We believe that PAIE materials have the ad-
vantages of both piezofluorochromic materials and aggrega-
tion-induced emission materials and can be used more
widely. To the best of our knowledge, so far there have only
been five PAIE compounds (the molecular structures are
shown in the Supporting Information, Scheme S1) reported
in the literature. One is DBDCS reported in 2010 by Parkꢀs
group,[5] and the others have been reported recently by our-
selves.[2] The former contains two butoxy groups and two
cyano groups in its molecular structure, which means it
should not have good thermal stability properties, such as
[a] Dr. B. Xu, Prof. Z. Chi, Prof. J. Zhang, Dr. X. Zhang, Dr. H. Li,
Dr. X. Li, Dr. S. Liu, Prof. Y. Zhang, Prof. J. Xu
PCFM Lab and DSAPM Lab, FCM Institute
State Key Laboratory of Optoelectronic Materials and Technologies
School of Chemistry and Chemical Engineering
Sun Yat-sen University
Guangzhou 510275 (China)
Fax : (+86)20-84112222
E-mail: chizhg@mail.sysu.edu.cn
xjr@mail.sysu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100211.
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high glass-transition temperature and high decomposition
temperature. However, the latter structures, which contain
phenylvinylanthracene, are wholly aromatic structures with
high thermal stabilities. For the former, the two-color lumi-
nescence-switching behavior was explained as an inter-
change between the metastable green-emitting G-phase and
the thermodynamically stable blue-emitting B-phase with
different modes of local dipole coupling (antiparallel and
head-to-tail arrangements, respectively); the origin of this
interchange was the two-directional shear-sliding capability
of molecular sheets formed via intermolecular multiple
triphenylethylene bromides with tetraphenylethylene boron-
ic acid in moderate yields (54%–61%; Scheme 1). The tri-
phenylethylene bromide, VP3-Br2, was synthesized through a
Wittig–Horner reaction of the ylide reagent diethyl benzyl-
phosphonate with 4,4’-dibromobenzophenone, whilst VP3-Br
and VP3-Br3 were synthesized under the same conditions
from the reactions of diethyl 4-bromobenzylphosphonate
with benzophenone and 4,4’-dibromobenzophenone, respec-
tively. 4-(1,2,2-Triphenylvinyl)phenylboronic acid (TPE-B)
was synthesized from diphenylmethane in three steps.[6] The
hydroxy intermediate, TPE(OH)-Br, was not purified fur-
ther and was used directly in the next step. After the dehy-
dration reaction of TPE(OH)-Br in the presence of para-tol-
uenesulfonic acid, the bromide intermediate TPE-Br was
obtained. TPE-B was then prepared from TPE-Br by lithia-
tion with n-butyllithium and boronation with trimethylbo-
rate. The products were purified by column chromatography
on silica gel using dichloromethane/n-hexane as the eluent.
Their molecular structures were confirmed by 1H and
13C NMR spectroscopy, high-resolution mass spectrometry
(HRMS), Fourier-transform infrared spectroscopy (FT-IR),
and elemental analysis (EA).
To determine whether or not these compounds are AIE-
active, the UV/Vis absorption and PL emission behaviors of
their diluted mixtures were studied in a mixture of water/
tetrahydrofuran with different water fractions. Because the
compounds were insoluble in water, increasing the water
fraction in the mixed solvent could thus change their exist-
ing forms from a solution state in the pure tetrahydrofuran
to the aggregated particles in the mixtures with high water
content, which results in changes in the UV and PL spectra.
The absorption spectra of VP3-(TPE)1 in the water/tetra-
hydrofuran mixtures are shown in Figure 1. The UV/Vis
spectra for the other compounds are provided in the Sup-
porting Information, Figure S1. The spectral profile was vir-
tually unchanged, even when a water fraction of up to circa
60% was added to the tetrahydrofuran solution. When the
water fraction was increased further, the entire spectrum
started to rise. The increase in absorbance in the entire spec-
tral region was caused by the Mie effect[7] of the nanoaggre-
gate suspensions in the solvent mixtures. The result showed
ꢀ
ꢀ
C HgN and C HgO hydrogen bonds. However, for
the latter compounds, there were no such hydrogen bonds in
their structures. Thus, we proposed a mechanism to explain
the PAIE phenomenon: destruction of the crystalline struc-
ture leads to the planarization of the molecular conforma-
tion, resulting in a red shift of photoluminescence (PL) spec-
trum. In our previous papers, although the change in pack-
ing structures before and after pressing was observed using
wide-angle X-ray diffraction (WAXD) and differential scan-
ning calorimetry (DSC), single-crystal X-ray diffraction
analysis for direct confirmation of the proposed mechanism
was not provided because of the failure to grow single crys-
tals of the compounds owing to their bulky molecular struc-
tures.
Herein, we reported a new series of AIE compounds con-
taining triphenylethylene and tetraphenylethylene moieties.
The compound, VP3-(TPE)1, exhibited PAIE properties and
its structure was determined by single-crystal X-ray diffrac-
tion analysis. We anticipate that this work could offer us the
opportunity to elucidate the origin of the unique piezofluor-
ochromic characteristics of AIE materials.
Results and Discussion
The target compounds, triphenylethene combined with dif-
ferent numbers of tetraphenylethene groups, were synthe-
sized by palladium-catalyzed Suzuki-coupling reactions of
Abstract in Chinese:
Figure 1. UV absorption spectra of VP3-(TPE)1 in water/THF mixtures
with different volume fractions of water. THF=tetrahydrofuran.
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Scheme 1. Synthetic routes for the compounds.
that the VP3-(TPE)1 molecules started to markedly aggre-
gate when the mixture contained circa 70% of water.
of the other compounds are shown in the Supporting Infor-
mation, Figure S2). The emission from the tetrahydrofuran
solution of VP3-(TPE)1 was so weak that almost no PL
signal was recorded. However, a dramatic enhancement in
luminescence was observed for the approximately 70:30 (v/
v) water/tetrahydrofuran mixture. Whilst the PL intensity in
tetrahydrofuran was only 4, it was boosted to circa 334 in
95:5 (v/v) water/tetrahydrofuran, enhanced about 50 times.
Similar effects were observed for the other compounds. A
comparison of the UV/Vis absorption and PL emission spec-
tra showed that the abrupt broad change in the shape of the
absorbance for the mixture with approximately 70% water
agreed well with the sudden jump in the PL intensity; this
result confirmed that the emission enhancements were in-
duced by aggregation of the molecules. In other words, the
compounds are AIE active.
Almost no PL signals could be detected from the dilute
solutions of the compounds in tetrahydrofuran. The corre-
sponding emission spectra of compound VP3-(TPE)1 in
aqueous tetrahydrofuran with different water/tetrahydrofur-
an ratios are shown in Figure 2 as an example (PL spectra
A photograph showing the emission of the compounds in
pure tetrahydrofuran and 95:5 (v/v) water/tetrahydrofuran
under UV light (365 nm) at room temperature is shown in
Figure 3. Clearly, tetrahydrofuran solutions of the com-
pounds showed extremely weak fluorescence. However, the
compounds in high water content water/tetrahydrofuran
mixtures exhibited very strong fluorescence, thus indicating
that the compounds had a strong AIE effect.
Figure 2. PL spectra of VP3-(TPE)1 in water/THF mixtures. The inset de-
picts the changes of PL peak intensity.
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Piezofluorochromism and Aggregation-Induced Emission Compounds
fractions were much higher (>50 folds) than those in the
tetrahydrofuran solutions.
A decrease in photoluminescence intensity and photolu-
minescence FFL values during the addition of water was ob-
served. This phenomenon is often observed in some com-
pounds with AIE properties[8] and the reasons remain un-
clear.
The as-synthesized sample of VP3-(TPE)1, as well as the
annealed material and the fumed product with a good sol-
vent vapor such as dichloromethane, was a white powder,
and when illuminated with a 365 nm UV lamp, it emitted a
strong blue light, as shown in Figure 5 (named as the B-
Figure 3. Emission images of the compounds in pure THF (10 mm) and
95% water/THF solution (v/v) under UV light (365 nm) at room temper-
ature: a) VP3-(TPE)1, b) VP3-(TPE)2, and c) VP3-(TPE)3.
The photoluminescence quantum yields (FFL) were calcu-
lated for the compounds in mixtures of water and tetrahy-
drofuran in various proportions using 9,10-diphenylanthra-
cene (DPA) as the standard to obtain a quantitative estima-
tion of the AIE process. In Figure 4, the FFL values of the
Figure 5. VP3-(TPE)1 taken at room temperature under natural (left) and
UV light (right). Samples: (B1) as-synthesized sample; (G1) ground
sample; (B2) annealed sample; (G2) re-ground sample.
form). However, the ground samples are a light-green
powder and show green emission under the UV lamp
(named as G-form). This result indicates that the grinding
and annealing (or fuming) treatments can induce reversible
interchanges in emissions (green and blue) and molecular-
packing modes (G-form and B-form), exhibiting significant
piezofluorochromic behaviors.
The as-synthesized powder of VP3-(TPE)1 was spread on
a filter paper and a letter “M” was written on it with a
metal spatula. Under the UV lamp, a green “M” is observed
against the blue background (Figure 6).This result suggests
that the material has the potential for application as an opti-
cal recording material.
Figure 7 shows the PL spectra of VP3-(TPE)1 after grind-
ing and annealing treatments. The wavelength changes in
solid-state emission could be repeated many times, 454 nm
for the B-form and 482 nm for the G-form, indicative of an
excellent reversibility in the two-way switching process.
Except for the annealing treatment, the PL spectra of the B-
form can be completely restored when fuming the ground
sample with the vapor of a good solvent, such as dichloro-
methane.
Figure 4. Photoluminescence quantum yields (FFL) of the compounds in
water/THF mixtures with different volume fractions of water.
tetrahydrofuran solutions were very low and almost un-
changed before the compound molecules started to aggre-
gate markedly. However, the FFL values started to increase
swiftly upon addition of a certain amount of water. The
amount of water added to show the AIE effect was not the
same for each compound owing to the difference in solubili-
ty. The volume fractions of water in the water/tetrahydrofur-
an mixtures where the FFL could be observed with noticea-
ble change were 70%, 60%, and 50% for VP3-(TPE)1, VP3-
(TPE)2, and VP3-(TPE)3, respectively.
Similarly, the extent of the increase was not the same for
each compound. For example, when the volume fraction of
water in the water/tetrahydrofuran mixture was increased to
95%, the FFL of VP3-(TPE)2 rose to 11.2%, about 140-fold
higher than that in tetrahydrofuran (FFL =0.08%). For all
the compounds, the increasing extents of FFL in 95% water
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Figure 6. An image taken at room temperature under 365 nm UV light:
The as-synthesized powder of VP3-(TPE)1 was spread on a filter paper
and a letter “M” was written with a metal spatula.
Figure 8. WXRD of VP3-(TPE)1 : as-synthesized sample (B1); ground
sample (G1); annealed sample (B2); re-ground sample (G2); re-annealed
sample (B3); ground powder in dichloromethane vapor for 5 seconds
(BV).
destroyed by grinding can be restored by annealing or
fuming.
The first heating DSC results are shown in Figure 9. The
as-synthesized sample has a melting peak at around 2168C
from the first heating run. During the second heating (see
the Supporting Information, Figure S3), only a glass transi-
tion at around 908C was observed, with no melting peak;
this result indicates that it is difficult to crystallize from its
melt during the cooling procedure. Interestingly, the samples
after grinding reproducibly showed a cold-crystallization
transition peak at around 1178C, which indicates that there
is a metastable aggregation structure in the ground sample
which would transfer to the more-stable state. This relatively
stable aggregation structure refers to the crystal of the melt-
ing peak around 2168C. After treatment by annealing at
1508C or solvent fuming by dichloromethane for 5 seconds,
the cold-crystallization transitions disappeared and the
Figure 7. PL spectra of VP3-(TPE)1: as-synthesized sample (B1); ground
sample (G1); annealed sample (B2); re-ground sample (G2); re-annealed
sample (B3); ground powder in dichloromethane vapor for 5 seconds
(BV). Inset: repeated switching between blue and green states of the
emission by cycles of grinding and annealing.
To further explore the aggregate structures of these differ-
ent forms of VP3-(TPE)1, WAXD experiments were con-
ducted (Figure 8). According to the WAXD measurements,
VP3-(TPE)1 forms different molecular aggregates before
and after grinding treatment. The diffraction curves of the
as-synthesized, annealed, and fumed samples (B-form) dis-
play sharp, intense reflections. This result indicates that the
B-form is crystalline. However, after grinding, many reflec-
tion peaks weakened or disappeared, and the diffraction
curves became diffuse. But two peaks at 19.38 and 20.18 still
remained. This result indicates that after grinding, most of
the ordered structures have been destroyed, but the grinding
sample still has some crystallinity. The WAXD results indi-
cate that the grinding treatment leads to a change in the
mode of solid-state molecular packing from high-order to
low-order. From Figure 8, the B1, B2, B3, and Bv curves are
almost the same, which indicates that the ordered structures
Figure 9. DSC of VP3-(TPE)1: as-synthesized sample (B1); ground sample
(G1); annealed sample (B2); re-ground sample (G2); re-annealed sample
(B3); ground powder in dichloromethane vapor for 5 seconds (BV).
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Piezofluorochromism and Aggregation-Induced Emission Compounds
shapes of the two DSC curves were almost the same as that
obtained from the as-synthesized sample. These results fur-
ther indicate that the grinding treatment causes the change
in morphology of the compound; and also that the change
in morphology can easily be restored through annealing or
fuming.
Time-resolved emission-decay behaviors of the compound
sample before and after grinding were studied. The time-re-
solved fluorescence curves are illustrated in Figure 10. The
However, for the ground sample, the A1 value is 0.56, show-
ing an obvious difference. The weighted mean lifetimes
<t> of the as-synthesized and ground samples are 1.24 ns
and 1.90 ns, respectively. This result indicates that the mole-
cule in the G-form has a longer lifetime than that in the B-
form and the change in <t> is believed to be caused by a
change of aggregation structure after grinding.
A single crystal of VP3-(TPE)1 was successfully grown
from a tetrahydrofuran/methanol solution and its crystalline
structure was determined by single-crystal X-ray diffraction
analysis. The selected crystallographic data are given in the
Supporting Information, Table S1. VP3-(TPE)1 crystallized
in the monoclinic space group P21/n. The crystal structure
of VP3-(TPE)1 shows four molecules in a crystal unit
(Figure 11, bottom).
Figure 10. Time-resolved emission-decay curves of VP3-(TPE)1.
lifetime data were processed according to the literature
method.[9] Decay in the fluorescence intensity (I) with time
(t) was fitted by a double-exponential function:
I ¼ A1expð-t=t1ÞþA2expðꢀt=t2Þ
ð1Þ
where t1 and t2 are the lifetimes of shorter- and longer-lived
species, respectively, and A1 and A2 are their respective am-
plitudes. The weighted mean lifetime (<t>) was calculated
according to Equation (2):
< t >¼ ðA1t1þA2t2Þ=ðA1þA2Þ
ð2Þ
Figure 11. The molecular conformation in the single crystal with defined
planes (top), and the molecular packing in a crystal unit (bottom).
The lifetime data are summarized in Table 1. As can be
seen from the table, there are two relaxation pathways in
their fluorescence decays. This observation implies that the
time-resolved PL spectra of the compound include inde-
pendent emissions from the segments with different p-conju-
gation lengths because multiple lifetimes were detected. For
the two samples, different pathways played different pre-
dominant roles. The excited molecules of the as-synthesized
sample mainly decay through the first pathway (A1 =0.84).
The geometry of the isolated free molecule of the com-
pound in the ground state was optimized based on B3LYP/
6-31G calculations using the GAUSSIAN 03 package pro-
gram.[10] Based on the optimized geometry, the selected di-
hedral angles between the aryl rings as defined in Figure 11
AHCTUNGTREG(NNNU top) as well as those in the single crystal were calculated
and collected in Table 2. The values of the dihedral angles
in the free and crystalline states are greater than 59 and 618,
respectively, thus indicating that the molecule takes a highly
twisted conformation either in the free or crystalline state.
The values of the dihedral angles in the single crystal are
greater than those in the isolated free molecule and the dif-
ference suggests that a high twist-stress exists for the mole-
Table 1. Fluorescence decay parameters of VP3-(TPE)1 before and after
grinding.
t1 [ns]
A1
t2 [ns]
A2
<t> [ns]
Pre-grinding
Ground
1.03
1.42
0.84
0.56
2.33
2.50
0.16
0.44
1.24
1.90
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Table 2. The dihedral angles of the selected planes of the molecule in
single crystal and free molecule.
stress, which is shown by the increase of molecular conjuga-
tion thus resulting in a red-shift of PL spectrum from
454 nm to 482 nm (Figure 7). This molecular mechanism
may explain the piezofluorochromic behavior of this PAIE
compound.
Plane
D-C
A-B
B-C
E-G
F-G
Free molecule[a]
Single crystal[b]
76.2
87.9
77.2
81.1
75.7
78.3
59.2
61.8
74.8
81.7
[a] Geometry determined from calculations using Gaussian 03; [b] Geom-
etry determined by single-crystal X-ray diffraction.
Why then do VP3-(TPE)2 and VP3-(TPE)3 not also exhibit
piezofluorochromic behavior? We can get the answer from
the WAXD and DSC studies of the three as-synthesized
samples. As shown in Figure 13 and 14, VP3-(TPE)2 and
VP3-(TPE)3 must be amorphous because there are not any
cule in the crystalline state. Such twist stress is believed to
be released when triggered by external pressure, which
probably causes the loss of the long-range order of the crys-
tal structure (i.e. destruction of the crystalline structure
leads to the planarization of molecular conformation result-
ing in a red shift of the PL spectrum).
As mentioned above, owing to the twisted conformation,
the backbone of the molecule largely deviates from a plane
ꢀ
and typical coplanar p p stacking becomes impossible.
From Figure 11, it can be seen that the molecules are
ꢀ
packed via weak C Hgp interactions in a crystal cell.
This result suggests that the VP3-(TPE)1 crystal has a low
lattice energy. Because of the twisted conformation and
ꢀ
weak p p interactions, the molecular packings are relatively
loose and there are some defects (cavities), as shown in
Figure 12. Clearly, the cavities are the most-feeble parts of
the crystalline structures. The two structural features, the
low lattice energy and the existence of cavities, make the
crystal readily destroyable by external pressure. The destruc-
tion of the crystalline structure leads to planarization of the
molecular conformation because of the release of twist
Figure 13. WAXD curves of the as-synthesized samples.
Figure 14. The first heating DSC curves of the as-synthesized samples.
sharp and intense reflections in their WAXD curves and no
melting peaks in their heating DSC curves. From Figure 11
(top), it can be seen that the conformation of tetraphenyl-
ethylene moiety is highly twisted. Clearly, the higher the
number of tetraphenylethylene moieties in a molecule, the
harder it becomes to efficiently pack the molecules, resulting
in an amorphous structure. However, for VP3-(TPE)1, there
were many sharp reflection peaks in its WAXD curve and a
sharp melting peak around 2168C in its DSC curve, thereby
Figure 12. Molecular packing in the single crystal: wireframe (top) and
space-fill representations (bottom).
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Piezofluorochromism and Aggregation-Induced Emission Compounds
compound were grown from tetrahydrofuran/methanol mixtures. X-ray
crystallographic intensity data were collected at 110 K using a Bruker
Smart 1000 CCD diffractometer equipped with graphite monochromated
Enhance (Mo) X-ray source (l=0.71073 ꢂ). The structures were solved
by the direct methods following difference Fourier syntheses, and refined
indicating that VP3-(TPE)1 is a crystalline compound. Given
that the piezofluorochromic behavior depends on a packing
change from a high-order aggregation state to a low-order
aggregation state, the packing structures of VP3-(TPE)2 and
VP3-(TPE)3 were amorphous to begin with, and accordingly
it is impossible to make further changes under external pres-
sure; therefore, the two compounds are not PAIE-active.
We can now conclude that piezofluorochromism is a general
characteristic of crystalline AIE materials.
2
by the full-matrix least-squares method against F0 using SHELXTL soft-
ware.[13] Ground samples were prepared by grinding using a pestle and
mortar. Annealing experiments were done on a hot-stage with an auto-
matic temperature-control system. The water/THF mixtures with differ-
ent water fractions were prepared by slowly adding distilled water into
solutions of the samples in THF under sonication at room temperature.
CCDC 810816 (VP3-(TPE)1) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Conclusions
Synthesis of VP3-Br2
A new series of aggregation-induced emission compounds
containing triphenylethylene and tetraphenylethylene moiet-
ies has been synthesized. Only compound VP3-(TPE)1 ex-
hibited piezofluorochromic behavior. The spectroscopic
properties and morphological structures exhibited reversible
changes upon grinding and annealing (or fuming) processes.
The piezofluorochromic behavior is caused by a change be-
tween two modes of solid-state molecular packing under ex-
ternal pressure. The twisted conformation of the aggrega-
tion-induced emission compound leads to the formation of a
metastable crystal lattice with some cavities which are easily
destroyed under external pressure. The planarization of mo-
lecular conformation leads to an increase in molecular con-
jugation and a red-shift in the PL spectrum from 454 to
482 nm. Piezofluorochromism is a general characteristic of
crystalline AIE materials.
A solution of bis(4-bromophenyl)methanone (2.00 g, 5.8 mmol) and di-
ethyl benzylphosphonate (1.98 g, 8.7 mmol) in anhydrous THF (40 mL)
was stirred under an argon atmosphere at room temperature. Then potas-
sium tert-butyloxide (1.17 g, 10.4 mmol) was added and the mixture was
stirred for 4 h. The reaction mixture was concentrated and purified by
column chromatography on silica gel using n-hexane as the eluent. White
crystalline powder was obtained in a yield of 88% (2.10 g). 1H NMR
(300 MHz, CDCl3): d=7.42 (d, 4H), 7.14 (m, 5H), 7.02 (d, 4H),
6.92 ppm (s, 1H); FT-IR (KBr): n˜ =3058, 3023, 1585, 1485, 1446, 822,
745, 692, 576 cmꢀ1; HRMS (EI), m/z: 414 ([M]+, calcd for C20H14Br2,
414); Anal. calcd for C20H14Br2: C 58.00, H 3.41; found: C 57.91, H 3.48.
Synthesis of VP3-(TPE)1
TPE-B (0.36 g, 0.95 mmol) and VP3-Br (0.29 g; 0.86 mmol) were dis-
solved in toluene (30 mL), and then 2m aqueous K2CO3 solution
(0.5 mL) and TBAB (0.1 g) were added. The mixture was stirred for
40 min under an argon atmosphere at room temperature. Then the [Pd-
AHCTUNGTRENNUNG
Experimental Section
Materials and Measurements
Bis(4-bromophenyl)methanone, benzophenone, diethyl benzylphospho-
nate, diethyl 4-bromobenzylphosphonate, potassium tert-butyloxide, n-bu-
tyllithium in hexane (2.2m), diphenylmethane, tetrakis (triphenylphos-
phine) palladium(0), trimethylborate, para-toluenesulphonic acid, and
tetrabutyl ammonium bromide (TBAB) were all purchased from Alfa
Aesar and were used as received. All other reagents and solvents were
purchased as analytical grade from Guangzhou Dongzheng Company
(China) and used without further purification. Tetrahydrofuran (THF)
was distilled from sodium/benzophenone. Ultra-pure water was used in
the experiments. VP3-Br,[11] VP3-Br3[12], and TPE-B[6] were synthesized ac-
cording to literature methods. 1H and 13C NMR spectra were measured
on a Mercury-Plus 300 spectrometer (CDCl3 as solvent and tetramethylsi-
lane, TMS as the internal standard). High-resolution mass spectra
(HRMS) were measured on a Thermo MAT95XP-HRMS spectrometer.
Elemental analyses (EA) were performed with an Elementar Vario EL
elemental analyzer. FT-IR spectra were obtained on a Nicolet NEXUS
670 spectrometer (KBr pellet). UV/Vis absorption spectra (UV) were de-
termined on a Hitachi U-3900 spectrophotometer. Fluorescence spectra
(PL) were measured on a Shimadzu RF-5301PC spectrometer with a slit
width of 1.5 nm for excitation and 3 nm for emission. Thermal behaviors
were determined by differential scanning calorimetry (DSC) at heating
Synthesis of VP3-(TPE)2
TPE-B (0.48 g, 1.3 mmol) and VP3-Br2 (0.22 g; 0.53 mmol) were dissolved
in toluene (30 mL), and then 2m aqueous K2CO3 solution (0.7 mL) and
TBAB (0.1 g) were added. The mixture was stirred for 40 min under an
argon atmosphere at room temperature. Then the [PdACTHUNTRGNE(UNG PPh3)4] catalyst
was added and the reaction mixture was stirred at 808C for 16 h. After
cooling to room temperature, the product was concentrated and purified
by column chromatography on silica gel with dichloromethane/n-hexane
(v/v=1:8) as eluent. Light green power of VP3-(TPE)2 was obtained in
60% yield (0.29 g). 1H NMR (300 MHz, CDCl3): d=7.52 (d, 2H), 7.48
(d, 2H), 7.39 (d, 2H), 7.33 (d, 2H), 7.20 (s, 1H), 7.16–6.99 (m, 2H),
6.98 ppm (s, 1H); 13C NMR (75 MHz, CDCl3): d=143.45, 142.70, 142.58,
142.03, 141.52, 140.89, 140.25, 139.48, 139.19, 138.96, 137.98, 137.12,
131.59, 131,09, 131,52, 130.60, 129.33, 127.78, 127.51, 127.43, 126.67,
126.32, 126.24, 125.77 ppm; FT-IR (KBr): n˜ =3023, 1600, 1495, 1446, 817,
760, 700 cmꢀ1; HRMS (EI), m/z: 916 ([M]+, calcd for C72H52, 916); Anal.
calcd for C72H52: C 94.29, H 5.71; found: C 94.21, H 5.73.
and cooling rates of 108Cminꢀ1 under
a N2 atmosphere using a
NETZSCH thermal analyzer (DSC 204F1). Time-resolved emission-
decay behaviors were measured on an Edinburgh Instruments Ltd spec-
trometer (FLSP920). Wide-angle X-ray diffraction (WAXD) measure-
ments were performed using a Bruker X-ray diffractometer (D8 AD-
VANCE, Germany) with an X-ray source of CuKa (l=0.15406 nm) at
40 kV and 40 mA, at a scan rate of 48 (2q) minꢀ1. Single crystals of the
Synthesis of VP3-(TPE)3
TPE-B (1.0 g, 2.7 mmol) and VP3-Br3(0.29 g, 0.59 mmol) were dissolved
in toluene (30 mL), and then 2m aqueous K2CO3 solution (1.5 mL) and
Chem. Asian J. 2011, 6, 1470 – 1478
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1477

Z. Chi, J. Xu et al.
FULL PAPERS
TBAB (0.1 g) were added. The mixture was stirred for 40 min under an
argon atmosphere at room temperature. Then the [PdACTHUNTRGNE(UNG PPh3)4] catalyst
was added and the reaction mixture was stirred at 808C for 16 h. After
cooling to room temperature, the product was concentrated and purified
by column chromatography on silica gel with dichloromethane/n-hexane
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1
54% yield (0.40 g). H NMR (300 MHz, CDCl3): d=7.53 (d, 2H), 7.48
N
2H), 7.39 (d, 2H), 7.34 (d, 3H), 7.33–7.28 (t, 4H), 7.25 (d, 2H), 7.16–
6.96 ppm (m, 55H); 13C NMR (75 MHz, CDCl3): d=143.46, 142.70,
142.59, 142.50, 141.97, 141.53, 140.88, 140.25, 139.47, 139.26, 139.02,
138.46, 137.98, 136.08, 131.53, 131.09, 130.60, 129.71, 127.76, 127.43,
126.75, 126.24, 126.05, 125.78, 125.59 ppm; FT-IR (KBr): n˜ =3023, 1600,
1495, 1445, 817, 760, 700 cmꢀ1; HRMS (FAB), m/z: 1247 ([M+H]+,
calcd for C98H70, 1246); Anal. calcd for C98H70: C 94.34, H 5.66; found:
C 94.26, H 5.62.
Acknowledgements
The authors gratefully acknowledge the financial support from the Na-
tional Natural Science Foundation of China (Grant numbers: 50773096
and 51073177), the Start-up Fund for Recruiting Professionals from the
“985 Project” of SYSU, the Science and Technology Planning Project of
Guangdong Province, China (Grant numbers: 2007A010500001-2,
2008B090500196), the Construction Project for University–Industry coop-
eration platform for Flat Panel Display from The Commission of Econo-
my and Informatization of Guangdong Province (Grant numbers:
20081203), the Fundamental Research Funds for the Central Universities,
and the Opening Fund of Laboratory Sun Yat-sen University
(KF201026).
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Chem. Asian J. 2011, 6, 1470 – 1478