New thermally stable piezofluorochromic aggregation-induced emission compounds

Article abstract of DOI:10.1021/ol102872x

New piezofluorochromic compounds with high thermal stabilities and aggregation-induced emission behavior were developed. The spectroscopic properties and morphological structures of these compounds were reversed upon pressing (or grinding)/annealing (or fuming). The switchable color change feature and aggregation-induced emission make the compounds promising candidates for optical recording, pressure-sensing, and light-emitting systems.

Full text of DOI:10.1021/ol102872x

ORGANIC
LETTERS
2011
Vol. 13, No. 4
556–559
New Thermally Stable Piezofluorochromic
Aggregation-Induced Emission
Compounds
Haiyin Li, Xiqi Zhang, Zhenguo Chi,* Bingjia Xu, Wei Zhou, Siwei Liu, Yi Zhang, and
Jiarui 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
chizhg@mail.sysu.edu.cn; xjr@mail.sysu.edu.cn
Received October 17, 2010
ABSTRACT
New piezofluorochromic compounds with high thermal stabilities and aggregation-induced emission behavior were developed. The
spectroscopic properties and morphological structures of these compounds were reversed upon pressing (or grinding)/annealing (or fuming).
The switchable color change feature and aggregation-induced emission make the compounds promising candidates for optical recording,
pressure-sensing, and light-emitting systems.
Although modification of molecular structures is the
most common approach to controlling fluorescence prop-
erties, materials having dynamically reversible fluores-
cence with high efficiency and speed are quite limited
because most chemical reactions in the solid state fre-
quently result in insufficient conversion, irreversible reac-
tions, or loss of fluorescence.1 To overcome this problem,
an attractive approach is to control the solid fluorescence
properties dynamicallybyaltering the solidstate molecular
packing without changing the chemical structure of the
constituent molecules. If a material has a fluorescent color
change based on pressure-dependent molecular packing,
then it is called a piezochromic fluorescent (PCF) material.
Many piezochromic materials based on the change of ab-
sorption characteristics under pressure have been reported.2
As fluorescence can be detected with high sensitivity,
materials exhibiting piezochromic fluorescence have a
wide variety of applications, such as optical recording
and strain- or pressure-sensing systems. However, PCF
materials are exceedingly rare.3 Most of the pure organic
(3) (a) Sagara, Y.; Kato, T. Nat. Chem. 2009, 1, 605. (b) Mutai, T.;
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Nature 2002, 420, 759. (b) Papaefstathiou, G. S.; Zhong, Z.; Geng, L.;
MacGillivray, L. R. J. Am. Chem. Soc. 2004, 126, 9158.
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r
10.1021/ol102872x
Published on Web 01/20/2011
2011 American Chemical Society

PCF compounds contain long flexible alkyl or alkyloxy
side chains in their molecular structures, which means they
should not have good thermal stabilities such as high glass
transition temperature and high decomposition tempera-
ture. Recently, many fluorescent compounds with high
thermal stabilities and aggregation-induced emission (AIE)
have been synthesized in our laboratory. Interestingly, most
ofthe AIE compoundsare PCF materials and are hereafter
labeled by us as PAIE materials. Hence, a relationship in
molecular structure between the AIE and the PCF effect is
hypothesized. AIE materials are an important class of
antiaggregation-caused quenching materials first reported
by Tang in 2001.4 Since then, a large number of AIE com-
pounds have been developed by various research groups.5
Combining the AIE and PCF properties should be very
useful in expanding their applications.
An-1, An-2, An-1a, and An-2a are 196, 201, 223, and
242 ꢀC, respectively. Among them, the Tg of An-2a is the
highest. The melting point (Tm) of An-2 is 369 ꢀC, which is
∼21ꢀC higher than that of An-1; the Tm of An-1a is336 ꢀC,
which is lower than those of An-1 and An-2 because of the
introduction of triphenylamine and dendritic structures in
An-1a. However, the Tm of An-2a cannot be detected,
indicating the as-synthesized An-2a is completely amor-
phous. Their decomposition temperatures (Td, defined
as the temperature at which a minimum of 5% weight loss
is observed) were >491 ꢀC. The results indicate that the
compounds have high thermal stabilities. The maximum
photoluminescence (PL) emission wavelengths (λem)
of An-2 are blue-shifted relative to An-1 by 53 nm in the
solid state and 107 nm in dichloromethane solution.
This indicates that the introduction of diphenylanthracene
decreases molecular conjugation. A similar effect was
observed in the analogues An-1a and An-2a. An obvious
blue shift (∼32 nm) in the λem of An-1 is observed by
changing the state from solution to solid; An-2, by con-
trast, shows a 22 nm red shift. An-1a and An-2a exhibit
slight red shifts of 6 and 2 nm, respectively. By comparing
the emission spectra of An-1 and An-1a, the λem of An-1a is
noticeably longer in the solid state than that of An-1. The
reverse is observed in solution.
Scheme 1. Chemical Structure of the Compounds
The PL spectra of 10 μM An-1a in water/tetrahydrofur-
an (THF) mixtures with various water content are shown
in Figure S1 in the SI. The compound in pure THF exhibits
a very weak PL intensity. However, the fluorescence
intensity is significantly enhanced when the water fraction
exceeds 10%. The PL intensity in pure THF is 5.7, and it
increases to ∼58.5 in 30% water/THF mixture, a 10-fold
enhancement. This indicates that the compound has sig-
nificant AIE effect. The AIE effect is caused by the form-
ation of molecular aggregates upon adding water to the
solution. Similar results were obtained for An-1, An-2, and
An-2a (Figures S2-S4, SI), indicating that all of them are
AIE compounds. The PL quantum yields were calculated
for the compounds in the water/THF mixtures with dif-
ferentwater fractions, usingquinine sulfate asthereference
(Figure S5, SI), which had the similar change trendency
with the PL intensity with different water fraction.
The common structural feature of the reported AIE
compounds, such as the triphenylethylene,7 tetraphenyl-
ethylene,8 silole,9 cyanodistyrylbenzene,10 and distyrylan-
thracene11 derivatives, is that multiple peripheral phenyl
groups are linked to an olefinic core via rotatable car-
bon-carbon single bonds to form an AIE moiety. The steric
effect between the phenyl rings forces the AIE moieties or
the molecules to take a twisted conformation. Due to the
However, only one PAIE compound containing two
butoxy groups (DBDCS, Scheme 1) has been reported to
date in the literature.6 In this communication, we report
our preliminary investigations on the new PAIE com-
pounds An-1 and An-1a with wholly aromatic structures.
For comparison, an isomer of An-1 (i.e., An-2) and an
analogue of An-1a (i.e., An-2a) were also investigated.
Their chemical structures are also shown in Scheme 1.
An-1 and An-1a are linked by 9,10-anthracene, whereas
An-2 and An-2a are linked by 9,10-diphenylanthracene.
These differences, however, lead to distinct differences in
thermal and photophysical properties (Table S1 in the
Supporting Information, SI). All compounds possess very
high glass transition temperatures (Tg). The Tg values of
(4) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.;
Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun.
2001, 1740.
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4332. (b) Zhao, Y. S.; Fu, H. B.; Peng, A. D.; Ma, Y.; Xiao, D. B.; Yao,
J. N. Adv. Mater. 2008, 20, 2859. (c) Yang, Z.; Chi, Z.; Yu, T.; Zhang, X.;
Chen, M.; Xu, B.; Liu, S.; Zhang, Y.; Xu, J. J. Mater. Chem. 2009, 19,
5541. (d) Zhang, X.; Yang, Z.; Chi, Z.; Chen, M.; Xu, B.; Wang, C.; Liu,
S.; Zhang, Y.; Xu, J. J. Mater. Chem. 2010, 20, 292.
(7) Xu, B.; Chi, Z.; Yang, Z.; Chen, J.; Deng, S.; Li, H.; Li, X.; Zhang,
Y.; Xu, N.; Xu, J. J. Mater. Chem. 2010, 20, 4135.
(8) Kim, S. K.; Park, Y. I.; Kang, I. N.; Park, J. W. J. Mater. Chem.
2007, 17, 4670.
(9) Dong, Y. Q.; Lam, J. W. Y.; Qin, A. J.; Li, Z.; Sun, J. Z.; Dong,
Y. P.; Tang, B. Z. J. Inorg. Organomet. Polym. 2007, 17, 673.
(10) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem.
Soc. 2002, 124, 14410.
(6) Yoon, S. J.; Chung, J. W.; Gierschner, J.; Kim, K. S.; Choi, M. G.;
Kim, D.; Park, S. Y. J. Am. Chem. Soc. 2010, 132, 13675.
(11) He, J. T.; Xu, B.; Chen, F. P.; Xia, H. J.; Li, K P.; Ye, L.; Tian,
W. J. J. Phys. Chem. C 2009, 113, 9892.
Org. Lett., Vol. 13, No. 4, 2011
557

twisted conformation and the weaker π-π interactions,
the molecular packings are relatively loose, leading to
defects (cavities). Thus, the crystals are readily destroyed,
which may trigger planarization of molecular conforma-
tion or slip deformation under external pressure. The
collapse of the crystalline structure leads to the planariza-
tion of molecular conformation because of the release of
twist stress, which is considered as one of the possible
reasons for the increased molecular conjugation, thus
resulting in a red shift of the PL spectrum and the PAIE
behavior. Thus, we propose that if an AIE compound
exists in two different pressure-dependent stable or meta-
stable states, it will exhibit piezochromic fluorescence.
Figure 2. WAXD curves of the sample An-1a: (a) as-synthesized
sample; (b) after pressing at 1500 psi for 5 min; (c) after
annealing the (b) sample at 300 ꢀC for 5 min; (d) after pressing
the (c) sample at 1500 psi for 5 min.
S6-S16, SI). The A and B aryl rings in An-2 and An-2a
(Figure S17, SI) are more twisted at ∼76ꢀ dihedral angle,
whereas in An-1 and An-1a, this angle was ∼60ꢀ. The more
twisted bridge structure hinders molecular packing and
decreases the degree of crystallization, which was con-
firmed by wide-angle X-ray diffraction (WAXD). The
WAXD results of the as-synthesized An-1 and An-1a
samples exhibit sharp and intense reflections at 2θ < 25ꢀ
(Figure 2a, and Figure S18a in the SI), indicating some
crystalline order. However, no such reflections can be
observed in the as-synthesized An-2 and An-2a samples
(Figures S19a and S20a, SI); hence, these samples may
have poor crystallinity or may be amorphous. Given that
PAIE behavior depends on the packing change from the
crystalline to the amorphous state, the packing structures
of initially amorphous An-2 and An-2a do not change
under pressure; thus, the two compounds are not PAIE-
active. In other words, a relatively stable crystalline state is
necessary for PAIE compounds.
Figure 2 shows the WAXD curves of the An-1a samples
obtained under different conditions. An-1a exhibits differ-
ent molecular aggregation structures before and after the
pressing treatment. The diffraction curves of the as-synthe-
sized and annealed powders display sharp and intense
reflections at 2θ = 11.6ꢀ and 20.2ꢀ, indicating some
crystalline order. The diffraction curves of the pressed or
ground sample show a weak, broad, and diffuse peak,
indicating an amorphous structure. The amorphous sample
obtained by pressing can be reverted to the crystalline state
by annealing for a short time (i.e., 5 min) or fuming with a
good solvent vapor such as CH2Cl2. Similar results were
obtained for An-1 (Figure S18, SI). However, there is almost
no difference in WAXD before and after pressing for An-2
and An-2a (Figures S19 and S20, SI). These results indicate
that the interchange between crystalline-amorphous pack-
ing modes causes the PAIE effect in An-1 and An-1a.
The differential scanning calorimetry (DSC) results of
An-1a (Figure 3) have a reproducible cold-crystallization
Figure 1. The images of (a) An-1 and (b) An-1a taken at room
temperature under 365 nm UV light: (left) as-synthesized sam-
ples or annealed samples (at 300 ꢀC, for 5 min); (right) pressed
(1500 psi for 5 min) or ground sample. An-1 (c) and An-1a (d)
were cast on filter paper and “SU” and “AIE” were written with
a metal spatula at room temperature under ambient light (left)
and UV light (right).
An-1 and An-1a exhibit significant piezofluorochromic
properties. The annealed and fumed (CH2Cl2 vapor
exposed) samples of An-1 and An-1a show strong green
and yellow emissions, respectively, under 365 nm UV light,
and after being pressed or ground they show strong yellow
and orange-red emissions, respectively (Figure 1a,b). The
two emission colors are completely reversible through pres-
sing (or grinding) and annealing (or fuming with a good
solvent vapor). When the sample is pressed by streaking a
metal spatula across filter paper containing it, a color path
was observed (Figure 1c,d). The color change occurred only
at the streaked area. Under ambient or UV light, the
marked “SU” and “AIE” can be clearly seen on the strea-
ked An-1 and An-1a samples on paper, respectively. The
results suggest that these materials have a color-switchable
feature that may be a potential for application in optical
recording and temperature- or pressure-sensing materials.
However, An-2 and An-2a do not show piezochromic
behavior although they possess AIE properties. Opposite
behaviors were observed in these compounds (Figures
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Org. Lett., Vol. 13, No. 4, 2011

The peak at the longer wavelength increases and the
shorter one decreases with increasing excitation wave-
length. However, the emission wavelength of the as-synthe-
sized and annealed samples shows almost no change.
Time-resolved emission decay behaviors of the annealed
and pressed samples were studied. The time-resolved
fluorescence curves were illustrated in Figures S28-S31
in the SI, and fluorescence decay parameters are summar-
ized in Table S2 in the SI. As can be seen from the table,
there are two relaxation pathways in the fluorescence
decays. This implies that the time-resolved PL spectra of
the compounds include independent emissions from the
segments with different π-conjugation lengths because
multiple lifetimes have been detected. The two PAIE
compounds, An-1 and An-1a, showed significant changes
beforeand after pressing in the weightedmeanlifetimesÆτæ,
from 0.96 to 1.33 ns and from 1.11 to 0.95 ns, respectively.
The An-2 and An-2a exhibited little changes in Æτæ. It is
suggested that the changes of Æτæ of the PAIE compounds
are caused by the changes of their aggregation structures
after pressing.
Figure 3. DSC heating curves of the samples: (a) the as-synthe-
sized sample; (b) pressing at 1500 psi for 5 min; (c) annealing the
(b) sample at 300 ꢀC for 5 min; (d) pressing the (c) sample at 1500
psi for 5 min.
In summary, two new thermally stable piezofluorochro-
mic compounds showing aggregation-induced emission
were developed. The Tg is as high as 242 ꢀC for compound
An-1a. The spectroscopic properties and morphological
structures are reversed upon pressing or annealing. The PL
spectra of the two pressed samples depend on the excita-
tion wavelength, and red shift with increase in excitation
wavelength. The piezochromic fluorescent nature is generated
through a crystalline-amorphous phase transformation.
This switchable color feature makes the compounds pro-
mising candidates for optical recording, temperature- or
pressure-sensing, and light-emitting systems.
transition at ∼290 ꢀC, indicating that the aggregation
structure in the pressed sample is a metastable state that
transforms into a more stable packing structure through
annealing or fuming. The as-synthesized or annealed sam-
ples have no such transition. After pressing, the glass tran-
sition becomes sharper. The results reveal that the mor-
phological change of the compound induced by pressing is
reversible. Similar results were obtained for An-1 (Figure
S21, SI). The cold-crystallization transition in the heating
DSC curves of the pressed An-2 and An-2a samples is very
weak or unnoticeable (Figures S22 and S23, SI). Thus, the
cold-crystallization cannot cause a large change in the
mode of the packing structure. This finding agrees with
the result of WAXD. On the basis of the WAXD results,
the annealed samples were crystalline whereas the pressed
samples wereamorphous. Theamorphouspressed samples
were converted exothermically to the more stable crystal-
line state (i.e., annealed samples) after heating beyond the
transition temperature. The thermal behaviors are as-
cribed to an amorphous-crystalline transition, which
confirms the mechanism of the stimuli-responsive smart
and rewritable natures of An-1 and An-1a.
The PL spectra of the as-synthesized and pressed sam-
ples via different excitation wavelengths are shown in
Figures S24-S27 in the SI. The spectra of the samples
are very different. The emission wavelengths of the pressed
samples of An-1and An-1a significantly red-shifted with
the increase in excitation wavelength. The red-shifted
emission may be due to the fact that they contain both
amorphous and small crystal aggregation states. Two
peaks were present in the fluorescence emission spectra.
Acknowledgment. The authors gratefully acknowledge
the financial support from the National Natural Science
Foundation of China (50773096, 51073177), the Start-up
Fund for Recruiting Professionals from the “985 Project”
of SYSU, the Science and Technology Planning Project of
Guangdong (2007A010500001-2, 2008B090500196), the
Construction Project for University-Industry cooperation
platform for Flat Panel Display from The Commission of
Economy and Informatization of Guangdong (20081203),
the Open Research Fund of State Key Laboratory of
Optoelectronic Materials and Technologies, and the Fun-
damental Research Funds for the Central Universities.
Supporting Information Available. Experimetal details
and Figures S1-S52 (including PL spectra, PL quantum
yields, WAXD curves, heating DSC curves, Time-
resolved emission decay curves, and NMR, HRMS,
and FT-IR spectra). This material is available free of
charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 4, 2011
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