Mechanochromic and thermochromic fluorescent properties of cyanostilbene derivatives

Article abstract of DOI:10.1016/j.dyepig.2013.03.017

Two isomers (TPA-CNa and TPA-CNb) consisting of twisted triphenylamine and cyanostilbene were synthetized by Knoevenagel condensation and C-N coupling reaction. The fluorescent colours of two isomers in crystalline state were sky-blue for TPA-CNa and gree

Full text of DOI:10.1016/j.dyepig.2013.03.017

Dyes and Pigments 98 (2013) 486e492
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Mechanochromic and thermochromic fluorescent properties
of cyanostilbene derivatives
,
Yujian Zhang a b, Guilin Zhuang a, Mi Ouyang a, Bin Hu a, Qingbao Song a, Jingwei Sun a,
,
c
c
c,
*
Cheng Zhang a , Cheng Gu , Yuanxiang Xu , Yuguang Ma
*
a State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials Science,
Zhejiang University of Technology, Hangzhou, PR China
b Department of Materials Chemistry, Huzhou University, Xueshi Road 1#, Huzhou, PR China
c State Key Lab of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two isomers (TPA-CNa and TPA-CNb) consisting of twisted triphenylamine and cyanostilbene were
synthetized by Knoevenagel condensation and CeN coupling reaction. The fluorescent colours of two
Received 14 December 2012
Received in revised form
18 March 2013
Accepted 20 March 2013
Available online 28 March 2013
isomers in crystalline state were sky-blue for TPA-CNa and green for TPA-CNb with quantum yields (Ff
)
of 44.9% and 7.7%, respectively. The single crystal X-ray diffractometry revealed the fully different mo-
lecular conformations and packing modes of TPA-CNa (planar) and TPA-CNb (twisting), which might be
the reason for different Ff. The reversible mechanochromic and thermochromic fluorescence switching
was observed in crystalline TPA-CNa powders. The sky-blue crystals were changed into the green-
emissive solids after grinding, and they recovered their original state upon heating at 60 ꢀC over
2 min. The reversible conversion of fluorescence colour was also realized by a pure thermal process.
Powder X-ray diffractometry clearly demonstrated that the mechanochromic and thermochromic
behaviour could be attributed to the crystal-to-amorphisation phase transition.
Keywords:
Cyanostilbene derivatives
Isomers
Mechanochromic
Thermochromic
Crystal structure
Phase transition
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
intermolecular multiple secondary bonds [13]. The altering of
dipoleedipole interaction and intermolecular pep interaction was
Stimuli-responsive florescence switching, such as mechano-,
thermo- and vapochromism was thought to be very important for
developing sensors, memories and rewritable optical media [1e3].
In particular, mechanochromic fluorescent (MCF) materials are of
great importance in practical applications [4,5]. The rapid devel-
opment was achieved in this field in recent years. For instance, the
mechanochromic behaviour was observed for liquid-crystalline
molecules and crystalline solids containing pyrene core, which
were easily distinguished by the naked eye [6e9]. The tetraphe-
nylethylene or cyanostilbene-based crystalline organic solids were
also capable of exhibiting mechanochromic fluorescence [10e14].
Moreover, it was proposed that the MCF properties was related to
the planarization of molecular conformation in Xu’s group [10,15].
Recently, Park group demonstrated that a reversible fluorescence
switching could be attritued to two-directional shear-sliding
ability of adjacent molecular sheets, which are formed through
also suggested to account for MCF behaviours [16e19]. At present,
whereas the understanding of MCF mechanisms remains unclear at
the molecular-level, which requires further debates.
For MCF materials, their luminescent colours can be also
reversibly switched by other environmental stimulus including
organic vapour, heating, acid and base. For example, acid and base
treatments effectively cause the fluorescence transition between
blue and red colours of CF3-substituted cyanostilbene derivatives
[20]. Tang et al. have revealed thermally induced fluorescence
switching properties accompanied by the controlling poly-
morphism [16,21]. However, the reversible colours transformation
achieved by a pure thermal treatment has been rarely reported in
MCF systems, which would widen the range of application in op-
tical memory systems. Herein, we prepared two electron donor-
acceptor (DeA) structured isomers containing the twisty aryl-
amine and cyanostilbene derivatives (see Fig. 1). The twisty
conformation inducing the weak intermolecular interaction in
aggregated state may facilitate the phase transformation [10]. The
change in the position of cyano group had great effects on molec-
ular packing and optical properties. The crystal structure of
* Corresponding authors.
E-mail addresses: czhang@zjut.edu.cn (C. Zhang), ygma@jlu.edu.cn (Y. Ma).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.03.017

Y. Zhang et al. / Dyes and Pigments 98 (2013) 486e492
487
2.3. Synthesis and crystal parameters
(Z)-2-(4-(diphenylamino)phenyl)-3-(4-methoxyphenyl)-acrylo
nitrile (TPA-CNa): 3-(4-bromophenyl)-2-(4-methoxy phenyl)
acrylonitrile (CSB-Br) (3.1 g, 10 mml), diphenylamine (25 mmol,
4.23 g), Pd(OAc)2 (45 mg, 0.18 mmol), (t-Bu)3P (0.18 mmol), sodium
tert-butoxide (0.86 g, 9.0 mmol) and toluene (45 mL) were mixed in
three-necked flask kept under nitrogen. The mixture was heated at
reflux for 24 h. after the reaction finished, the mixture was cooled
to room temperature and the solvent was removed under vacuum.
Subsequently, the residue was extracted with chloroform. The
extraction solution was washed with brine. And then, the organic
layer was dried over MgSO4 and filtered. The crude product was
purified by columm chromatography using dichloromethane/hex-
anes (1/50) mixture as eluent to obtain the desired compound in
Fig. 1. Molecular structures of two isomers and their corresponding crystal images
under UV irradiation (lex ¼ 365 nm).
84.5% yield (3.4 g). 1H NMR (500 MHz, CDCl3)
d(ppm) 7.87 (d,
J ¼ 9.0 Hz, 2H), 7.51 (d, J ¼ 9.0 Hz, 2H), 7.38 (s, 1H), 7.32e7.28 (m,
4H), 7.14(d, J ¼ 7.5 Hz, 4H), 7.10 (d, J ¼ 8.5, 4H), 6.98(d, J ¼ 8.5, 2H),
3.88 (s, 3H); 13C NMR (500 MHz, CDCl3);
d 161.2, 148.5, 147.3, 139.7,
TPA-CNa with a “pinwheel” motif was edge-to-face packing, while
that of TPA-CNb was parallel stacking. Additionally, the emission
colour and efficiency switching of dye TPA-CNa can be reversibly
converted by grinding-vapour or only heating process.
131.0, 129.5, 128.2, 126.9, 126.7, 125.2, 125.0, 123.7, 122.8, 118.7,
114.4, 108.5, 55.5; MS(EI): m/e 402.2 (Mþ); Anal. Calcd for
C28H22N2O: C, 83.56; H, 5.51; N, 6.96; O, 3.98. Found: C, 83.47; H,
5.62; N, 6.89; O, 4.02.
2. Experimental
(Z)-3-(4-(diphenylamino) phenyl)-2-(4-methoxyphenyl) acrylo-
nitrile (TPA-CNb) was prepared from the Knoevenagel condensation
reaction of 4-(diphenylamino)- benzaldehyde with 2-(4-methoxy
phenyl)acetonitrile in anhydrous ethanol in the presence of so-
2.1. Chemicals and instruments
dium hydroxide. 1H NMR (500 MHz, CDCl3)
d
7.76 (d, J ¼ 9.0 Hz, 2H),
Tetrahydrofuran (THF) was purchased from Aladdin and
distilled from Na/benzophenone under N2 prior to use. Chinese li-
quor (56ꢀ, Peking Erguotou wine) and medicinal alcohol (75%) were
purchased from Wal-Mart supermarket in Hangzhou. All other
chemicals were purchased from Alfa Aesar or Aladdin and used as
received.
7.59 (d, J ¼ 9.0 Hz, 2H), 7.35 (s, 1H), 7.31e7.34 (m, 4H), 7.17 (d,
J ¼ 7.5 Hz, 4H), 7.13 (t, J ¼ 7.5 Hz, 2H), 7.07 (d, J ¼ 9.0 Hz, 2H), 6.96 (d,
J ¼ 9.0 Hz, 2H), 3.87 (s, 3H); 13C NMR (500 MHz, CDCl3);
d 160.0,
149.6,146.8,139.8,130.3,129.7,129.5,127.6,127.0,126.9,125.6,124.2,
118.9, 114.4, 107.6, 55.4; MS(EI) m/e 402.2[Mþ]. Anal. Calcd for
C28H22N2O: C, 83.56; H, 5.51; N, 6.96; O, 3.98. Found: C, 83.51; H,
5.55; N, 6.91; O, 4.03.
1H and 13C NMR spectra of the desired products were recorded
on a Bruker AVANCE III 500-MHz instrument (Bruker, Switzerland)
using TMS as the internal standard and the chloroform-d (CDCl3) as
the solvent. Mass spectroscopy was recorded with a Thermo LCQ
Fleet MS spectrometer. Elemental analyses were performed using
the Thermo-Finnigan Flash EA-1112 (CE, Italy) instrument. The UVe
vis spectrum was recorded on a Perkin Elmer Lambda 35 spectro-
photometer. Fluorescent measurements were recorded on a Per-
kineElmer LS-55 luminescence spectrophotometer. The Ff of
crystal was determined by using a calibrated integrating sphere
system. Powder XRD measurements were conducted on X’Pert PRO
Crystallographic data for TPA-CNa: C28H22N2O, M ¼ 402.48 g/
ꢀ
ꢀ
mol, monoclinic, a ¼ 22.475(3) A, b ¼ 8.7817(10) A, c ¼ 10.7156(13)
3
ꢀ
ꢀ
ꢀ
A;
a
¼ 90.00ꢀ,
b
¼ 96.981(2)ꢀ and
g
¼ 90.00 , V ¼ 2099.3(4) A ,
T ¼ 133(2) K, space group P21/c, DC ¼ 1.273 Mg mꢁ3, Z ¼ 4, 14805
reflections collected, 4575 unique reflection (Rint ¼ 0.0392), The
final R indices were R1 ¼ 0.0454, wR2 ¼ 0.1253 [I > 2
CCDC ¼ 872836.
s (I)],
Crystallographic data for TPA-CNb: C28H22N2O, M ¼ 402.48 g/
ꢀ
ꢀ
3
ꢀ
ꢀ
mol, triclinic, a ¼ 6.7788(7) A, b ¼ 17.4776(19) A, c ¼ 20.892(3) A;
ꢀ
a
¼ 65.356(2)ꢀ,
b
¼ 89.988(3) , V ¼ 2237.5(5) A , T ¼ 293(2) K,
diffractometer (CuKa) in the range 5 < 2q < 30 (PANalytical,
Netherlands). X-ray crystallographic intensity data were collected
using a Xcalibur, Eos, Gemini Ultra CCD diffractometer equipped
space group P-1, DC ¼ 1.195 g cmꢁ3, Z ¼ 4, 11833 reflections
collected, 8189 unique reflection (Rint ¼ 0.0241), The final R indices
with a graphite monochromated Enhance (Mo) X-ray source
were R1 ¼ 0.0854, wR2 ¼ 0.2308 [I > 2
s
(I)], CCDC ¼ 872835.
ꢀ
(l
¼ 0.71073 A). The time-resolved fluorescence lifetime experi-
ments were obtained by time-correlated single photo-counting
technique with an Flsp920 spectro-fluorometer. Differential scan-
ning calorimetry (DSC) was performed on a TA Instruments
DSC2920 at a heating rate of 10 ꢀC min-1. Field-emission scanning
electron microscopy (SEM) measurements were taken by using a
Hitachi S-4800 scanning electron microscopy (Hitachi, Japan).
Digital photographs were taken by Canon 550D (Canon, Japan)
digital cameras.
3. Results and discussion
3.1. Synthesis and characterization
The molecular structures of cyanostilbene derivatives TPA-
CNa and (Z)-3-(4-(diphenylamino) phenyl)-2-(4-methoxyphenyl)
acrylonitrile (TPA-CNb) were illustrated in Fig. 1. The desired dye
TPA-CNb and the key intermediate (Z)-3-(4-bromophenyl)-2-(4-
methoxyphenyl) acrylonitrile (CSB-Br) were prepared by Knoe-
venagel condensation reaction of benzyl cyanide derivatives with
corresponding aromatic aldehydes. Subsequently, the palladium-
catalysed aromatic CeN coupling reaction of CSB-Br with
diphenylamine was carried out at 110 ꢀC in toluene, to obtain
TPA-CNa with the yields of more than 84.5%. Finally, the resulting
compounds were fully characterized by 1H NMR, 13C NMR, MS
spectral and elemental analyses. Briefly, a mass of isomerically-
2.2. Sample preparation
Single crystals were obtained from the mixture of n-hexane and
dichloromethane at room temperature. Crystal samples of TPA-
CNa and TPA-CNb were prepared by recrystallization from hot
ethanol solutions as sheet-like and needle-like crystallites
respectively.

488
Y. Zhang et al. / Dyes and Pigments 98 (2013) 486e492
pure cyanostilbene derivatives can be efficiently and easily
configurational change could result in the different fluorescence
obtained.
emission behaviours.
3.3. Stimuli-responsive fluorescence switching
3.2. Photophysical properties
Aside from the higher Ff, another notable difference was that
the dye TPA-CNa exhibited mechanochromic fluorescent behav-
iour. As shown in Fig. 4, the sky-blue powders changed into rela-
tively weak green-emissive solids (Ff ¼ 21.2%, lem ¼ 502 nm) after
grinding by a spatula. Interestingly, this change occurred only at
the pressed area. Fig. 4c indicated photographic images of TAP-
CNa, shaped in ‘ZFL’ on a quartz plate, after pressing with a spatula
under natural light (left) and 365 nm UV light (right), respectively.
Clearly, the ground dye TPA-CNa exhibited higher lem and lower Ff
than that in the crystalline state (Ff ¼ 44.9%, lem ¼ 470 nm). These
ground powders could revert to the blue-emitting solids upon
heating at about 60 ꢀC. The emission spectrum indicated a broad
fluorescent band with an obvious peak at 491 nm (Fig. S2).
Moreover, the ground sample exhibited the evident vapochromic
fluorescence. Fig. S3 showed the change of fluorescence from
green to sky-blue was also achieved by the addition of various
organic solvents such as ethanol, hexane and acetonitrile. As
depicted in Fig. S4, the reversibility of fluorescence conversion was
confirmed by the subsequent grinding-vapour exposure processes.
The emission peaks did not exhibit notable degradation and shift,
even after 5 cycles. Interestingly, when the blue-emitting crystals
of TPA-CNa were heated to the melted state and rapidly cooled
down at room temperature in air. The solidified powders, termed
as crystal-m, showed the light green fluorescence (Fig. 5,
Ff ¼ 16.7%, lem ¼ 519 nm). Moreover, if the ground powders were
thermally melted and then cooled down rapidly, its fluorescence
was also changed into light blue similar to crystal-m. Upon
heating at 80 ꢀC for 5 h, the fluorescence of the solidified powders
returned to original sky-blue. The fluorescence spectra of crystal-h
exhibited the emission peak at 495 nm with a shoulder peak at
around 473 nm, which resemble that of the ground powder with
anneal treatment (Fig. 5, Fig. S2). These observations indicated that
TPA-CNa exhibited vapo-, machano- and thermochromic fluores-
cent behaviours. Uniquely, the harmless ethanol-water mixtures
also resulted in fluorescence conversion. As shown in Fig. 6aed,
below 30% volume fractions of water (fW), the fluorescence could
completely return to the original state. When fW was progressively
increased, the ground sample would partially convert to its orig-
inal phase with cyan fluorescence (Fig. 6f, g). However, the ground
TPA-CNa did not show a pronounced colour change under UV light
via the addition of mixture with fW >50% (Fig. 6h) due to the in-
hibition of the crystallization. It was noteworthy that the mixtures
such as medicinal alcohol (75%) and Chinese liquor (56ꢀ) were
widely used in our daily life, thus the practicality could be further
improved.
As shown in Fig. S1, the absorption spectra of TPA-CNa indi-
cated an obvious peak at 299 nm along with a shoulder peak at
around 379 nm, which was assigned to the
pep* transition. For
TPA-CNb, the shoulder peak became prevailing absorption band at
388 nm. However, their emission spectra were almost the same.
Moreover, two isomers were almost nonfluorescent when molec-
ularly dissolved in THF, while highly fluorescent when aggregated.
The remarkable fluorescence enhancement can be unambiguously
considered as the aggregation-induced emission phenomenon
[22e24]. Fig. 1 revealed the fluorescent colours of two dyes
recrystallized from ethanol solution were sky-blue for TPA-CNa
and green for TPA-CNb. Both TPA-CNa and TPA-CNb in crystalline
state emitted an intense light with quantum yields (Ff) of 44.9%
and 7.7%, respectively. Upon photoexcitation, the emission peaks
(
lem) of their crystals were located at 470 nm (TPA-CNa) and
507 nm (TPA-CNb) (see Fig. 2). The fluorescence spectra of blend
films of two isomers dispersed in poly (methyl methacrylate)
(PMMA, 0.05wt %) were also obtained in inset of Fig. 2. The
emission spectrum of TPA-CNa got close to that of crystals. In
comparison, the fluorescence peak of TPA-CNb in PMMA at
461 nm was obviously blue-shifted from that in crystalline state.
The results indicated that strong intermolecular interactions
existed in crystals of TPA-CNb. Additionally, the position change of
cyano group resulted in different degrees of distortion between
the vinylene and neighbour phenyl ring in crystalline states. Fig. 3a
indicated that torsion angles of TPA-CNb are 28.5ꢀ and 16.2ꢀ be-
tween the vinylene and adjacent phenyl ring. However, the cor-
responding torsion angles for TPA-CNa were reduced to 9.0ꢀ and
0.7ꢀ, respectively (Fig. 3b). Thus, the TPA-CNa had longer effective
conjugation than TPA-CNb, with more extended planarity. Based
on the resulting optical properties, two interesting features were
as follows: (1) dye TPA-CNb possessed significantly lower Ff than
TPA-CNa due to less coplanarity and face-to-face stacking inter-
action (Fig. 3c, d) [25,26]. (2) In terms of the fluorescence spectra of
two dyes, the direction of spectral shift was different. As crystals
were formed, the peak maximum of TPA-CNb was red-shifted
relative to that in THF, while that of TPA-CNa was blue-shifted
(see Fig. S1b). The phenomenon indicated even
a tiny
As shown in Fig. 7A, some irregular plates recrystallized from
hot ethanol indicated relatively smooth surfaces. The surfaces of
plates after grinding became rough and showed disordered gran-
ules (Fig. 7B). This rough surface could return to the original state
after heating or solvent vapour treatment (Fig. 7C and D). Namely,
mechanical stimuli resulted in the change of surface morphology,
which was recoverable through annealing or fuming treatment.
The various surface morphologies could be attributed to the rear-
rangement of molecules in the crystal lattice [13]. This proposal was
evidenced by time-resolved fluorescence spectroscopy and X-ray
diffraction (XRD) analyses. The XRD pattern of crystals TPA-CNa
exhibited clear reflection peaks due to highly ordered microcrys-
talline structures (see Fig. 8A). After grinding, the crystal lattice was
significantly disturbed as shown by obviously decreased peak in-
tensities along with several broad peaks. The results indicated
Fig. 2. Fluorescence spectra of TPA-CNa (C) and TPA-CNb (-) in crystal, grinding
treatment and dispersed in PMMA (Inset).

Y. Zhang et al. / Dyes and Pigments 98 (2013) 486e492
489
Fig. 3. The conformations and torsion angles in single-crystal structures of TPA-CNa (a) and TPA-CNb (b); scheme of the CꢁH/O interactions and
adjacent molecules for TPA-CNa(c) and TPA-CNb (d), respectively.
p∙∙∙
p
interactions between two
machanochromic behaviour was related to the conversion of a
well-ordered phase to a poorly organized phase. Additionally,
Fig. S5 exhibited the excited state yielded a single-exponential
3.4. X-ray crystal structure
In addition to photophysical properties, the different molec-
ular packing motifs were also observed in their single crystal.
The crystal structure of TPA-CNa containing one discrete mole-
cule was monoclinic with space group P21/c. The two-
dimensional structure with edge-to-face interactions was an
achiral “pinwheel” pattern on the bc plane (see Figs. 3c and 9A),
similar to DSB (Distyrylbenzene) [26]. The good coplanarity of
cyanostilbene and absence of face-to-face
resulted in the high Ff in crystal state [26e28]. In the
“pinwheel” stacking, Fig. 9A showed the CeH/N interaction,
decay (
s
¼ 5.98 ns) in the crystalline state. Upon grinding, the
sample decayed bi-exponentially with the lifetime shortened to
3.24 ns. These results indicated the ground powders possessing two
distinguished states was a more disordered state, in agreement
with the partial amorphisation discussed above. After heating
treatment, XRD of the sample obviously indicated the peaks orig-
inated from the original crystals along with those from the ground
powder, revealing a mixture of two packing modes (see Fig. 8A).
Thus, this thermochromic transformation may be an incomplete
process, which was further confirmed by the broad fluorescent
spectrum of the annealed sample (see Fig. S2). Fig. 8B depicted that
the solidified powders (crystal-m) exhibiting very broad diffraction
peak was the amorphous states. After heating, the sample crystal-
m became well-ordered aggregates with sharp and intensive
reflection peaks. The results indicated the crystal and amorphous
phase were efficiently switched by only thermal stimulation, which
induced the change in fluorescence colours. Moreover, the ground
sample did not experience an exo/endothermic process at the
corresponding temperature (Fig. S6). And its DSC curve was quite
similar to that before grinding. Thus, this “abnormal” behaviour
was attributed to the absence of (meta)stable state at room pres-
sure [9].
pep interaction
ꢀ
which was longer than the Van der Waals (VDW) radii of 2.75 A
[29], existed between two adjacent molecules. The strong Ce
ꢀ
H/O interaction with short distance of 2.507 A and angle of
151.7ꢀ were also observed. As for dye TPA-CNb, the single crystal
ꢁ
consisting of two molecules belonged to Pı triclinic space group.
As shown in Fig. 3d and Fig. S7, there was a face-to-face slipped
p
-stacking motif with a roll angle of 58.7ꢀ in the crystal. Another
Fig. 4. The photographs of TPA-CNa particles taken at R.T under 365 nm UV light: (A)
heating or organic vapour treatments; (B) ground sample; (C) compound TPA-CNa is
casted on a quartz substrate and written “ZFL” with a metal spatula under ambient
light (left) and UV light (right).
Fig. 5. Fluorescence spectra and photo of dye TPA-CNa under 365 nm UV light in the
different states; Crystal-m: heating powders to melt and solidification at R.T.; Crystal-
h: the solidified powders annealed above 80 ꢀC for 5 h.

490
Y. Zhang et al. / Dyes and Pigments 98 (2013) 486e492
Fig. 6. Photographs showing dye TPA-CNa on an agate mortar under UV irradiation (lex ¼ 365 nm), (A)e(H) TPA-CNa crystals after grinding with a pestle; (a)-(h) the corresponding
fluorescence by dropwise addition of the ethanol/water mixtures onto the centre of the ground powders.
notable feature was that the CeH/N distance in TPA-CNb,
2.54(1) A fell well within the sum of VDW radii (see Fig. 9B). In
each other, were also observed in Fig. 9B. However, such aro-
matic CeH/ and CeH/N interactions did not existed in
ꢀ
p
addition, two types of aromatic CeH/
making the adjacent molecules of the twisted TPA connect to
p
interactions (I and II),
crystal of TPA-CNa (see Fig. S8). These various multiple sec-
ondary bonding interactions in TPA-CNb enhanced the
Fig. 7. SEM images of TPA-CNa: (A) crystals, (B) ground crystals, (C) annealing sample B at 60 ꢀC for 5 min and (D) fuming sample B in ethanol vapor for 30 s.

Y. Zhang et al. / Dyes and Pigments 98 (2013) 486e492
491
Fig. 8. XRD profiles of TPA-CNa in different states.
molecular rigidity and stability in crystal lattice [30,31]. This
proposal was further supported by a higher melting point of
TPA-CNb (m.p. 139.7 ꢀC) than that of TPA-CNa (m.p. 131.2, ꢀC see
Fig. S6). Moreover, the single crystal with a “pinwheel” motif
(edge-to-face), relative to parallel stacking, was quite unstable
(see Fig. S7). Briefly, the crystals of TPA-CNa with edge-to-face
stacking were readily damaged by force stimuli in the absence
of various secondary bonding interactions. Thus, the dye
TPA-CNa presented MCF properties, rather than TPA-CNb.
4. Conclusions
In summary, two isomerically-pure isomers exhibiting
aggregation-induced enhanced emission behaviour were easily
obtained with high yields. The desired dye TPA-CNa presented high
Ff (44.9%) and weak intermolecular interactions relative to TPA-
CNb. Moreover, its luminescent efficiency and colour can be
reversibly transformed by grinding-vapour or a pure thermal
treatment. Based on the PXRD and fluorescence lifetime studies,
the mechanochromic and thermochromic fluorescent characteris-
tics were related to the phase transition between the crystalline
phase and amorphous phase. In crystalline state, the intermolec-
ular interaction of TPA-CNa was relatively weaker than that of TPA-
CNb, which induced the phase transformation. This material with
reversible colour conversion may be employed to prepare tem-
perature- or pressure-sensing in future.
Acknowledgements
The authors gratefully thank the supporting of National Basic
Research Program of China (2010CB635108, 2011CBA00700) and
International S&T Cooperation Program, China (2012DFA-51210).
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.dyepig.2013.03.017.
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