Aggregation-induced emission and remarkable piezochromism based on tetraphenylethene modified cyanostilbene derivative

Article abstract of DOI:10.1016/j.tetlet.2021.153136

The D-π-A-π-D type tetraphenylethene-based cyanostilbene derivative TPEAN had been designed and synthesized. Solvatochromic experiment and theoretical calculation illustrated that TPEAN occurred unique intramolecular charge transfer (ICT) process. Luminog

Full text of DOI:10.1016/j.tetlet.2021.153136

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Aggregation-induced emission and remarkable piezochromism based on
tetraphenylethene modified cyanostilbene derivative
Yong Zhan a,c, , Qian Zhao , Wenjun Yang
⇑
d
b
a
College of Life Science, China Jiliang University, Hangzhou 310018, PR China
b
c
Shandong Key Laboratory of Biochemical Analysis, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China
Fujian Key Laboratory of Photoelectric Functional Materials (Huaqiao University), Xiamen, Fujian 361021, PR China
College of Materials and Chemistry, China Jiliang University, Hangzhou 310018, PR China
d
a r t i c l e i n f o
a b s t r a c t
Article history:
The D-p-A-p-D type tetraphenylethene-based cyanostilbene derivative TPEAN had been designed and
Received 26 February 2021
Revised 16 April 2021
Accepted 25 April 2021
Available online xxxx
synthesized. Solvatochromic experiment and theoretical calculation illustrated that TPEAN occurred
unique intramolecular charge transfer (ICT) process. Luminogen TPEAN also exhibited AIE activity. The
nanoparticles gradually formed and emitted intense fluorescence at high water content in a mixture of
THF/water. Interestingly, TPEAN displayed remarkable piezochromism. A emission color change from
bright green to dark yellow occurred for the pristine solid of TPEAN after grinding treatment. This emit-
ting color change could recover to its original state after fuming. The current work could be help us to
effectively design molecular structure to achieve an obvious fluorescence response to external force.
Ó 2021 Elsevier Ltd. All rights reserved.
Keywords:
Tetraphenylethene
Cyanostilbene
Piezochromism
Aggregation-induced emission
Fluorescence response
Introduction
group found and reported the relationship between AIE and PFC
phenomena. Although some progress about PFC materials has been
Over the past decade, piezofluorochromism (PFC) is defined as
the phenomenon in which such molecules show reversible changes
in emitting color in responsive to mechanical force such as press-
ing, grinding and stretching [1–3]. As a ‘‘smart” materials, they
have attracted more attention because of their potential applica-
tions in advanced optical technologies [4], such as mechano-sen-
sors, photoswitches, optical recording and anti-counterfeiting
inks [5]. So far, a certain amount of PFC molecules has been
reported and studied, including the derivatives of tetrapheny-
lethene [6], b-ketoiminate boron complexes [7], phenothiazine
made, but designing molecules with remarkable solid-state fluo-
rescence response to mechanical force remain a challenge [17].
There is a lack of effective molecular design strategies [18].
Tetraphenylethene (TPE), a typical aggregation-induced emis-
sion (AIE) luminogen. 1,4-Phenylenediacetonitrile moiety often
used to construct AIE-active compounds [19]. Hence, we are trying
to combine them to build AIE-active PFC molecule with remarkable
piezochromic luminescence. The dumbbell (2Z,2 Z)-2,2 -(1,4-phe-
nylene)bis(3-(4-(1,2,2-triphenylvinyl)phenyl)acrylo-nitrile)
(TPEAN) with tetraphenylethene moiety as electron donor and
cyanostilbene unit as electron acceptor had been designed and pre-
pared using Suzuki-Miyaura coupling reaction and Knoevenagel
condensation reaction (Scheme 1). It was found that TPEAN exhib-
ited intramolecular charge transfer (ICT) property and obvious AIE
characteristic. SEM and DLS data confirmed that the molecule
TPEAN aggregated to form nanoparticles at high water content in
a mixture of THF/water. Notably, TPEAN exhibited remarkable
PFC performance. XRD and DSC proved that reversible piezochro-
mism was originated from phase transformation between crys-
talline and amorphous states.
0
0
[
8], Schiff base [9], pyrene [10], triphenylacrylonitrile [11], anthra-
cene [12], carbazole [13], triphenylamine [14], and gold(I) complex
15]. However, the fluorescence emission of great organic molecule
[
is usually decreased, even quenched in the aggregated state. This
phenomenon is known as aggregation-caused quenching (ACQ),
which greatly limits the development of PFC materials. A class of
important anti-ACQ materials were reported by Tang’s research
group in 2001, namely aggregation-induced emission (AIE) materi-
als. Although they don’t emit light in solution, but emit intense flu-
orescence in aggregated state [16]. Subsequently, Part’s and Chi’s
⇑
Corresponding author at: College of Life Science, China Jiliang University,
Hangzhou 310018, PR China.
E-mail address: zhanyong@cjlu.edu.cn (Y. Zhan).
https://doi.org/10.1016/j.tetlet.2021.153136
0
040-4039/Ó 2021 Elsevier Ltd. All rights reserved.
Please cite this article as: Y. Zhan, Q. Zhao and W. Yang, Aggregation-induced emission and remarkable piezochromism based on tetraphenylethene mod-
ified cyanostilbene derivative, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2021.153136

Y. Zhan, Q. Zhao and W. Yang
Tetrahedron Letters xxx (xxxx) xxx
Scheme 1. The synthetic routes of luminogen TPEAN.
Results and discussion
The luminogen TPEAN is prepared by a simple two step
approach and corresponding synthetic routes are depicted in
Scheme 1. Firstly, the synthesis of key intermediate 4-(1,2,2-triph-
enylvinyl)benzaldehyde 3 was using the Suzuki-Miyaura coupling
reaction, which involved the reaction of (2-bromoethene-1,1,2-
triyl)tribenzene 1 with (4-formylphenyl)boronic acid 2 catalyzed
by Pd(PPh
Knoevenagel condensation reaction between intermediate 3 and
,4-phenylenediacetonitrile 4 in a molar weigh radio of 2/1 cat-
3 4
) under alkaline condition [20]. After that, the classical
1
alyzed by sodium methoxide in ethanol produced the molecule
TPEAN in a yield of 78%. Finally, the product TPEAN was character-
1
13
ized by H NMR, C NMR and high resolution mass spectra. The
detailed characterization data are listed in ESI. TPEAN is soluble
in common organic solvents, such as CH
uble in water and ethanol.
2 2 3
Cl , CHCl , THF, but insol-
The normalized UV–vis absorption and photoluminescence
spectra of the compound TPEAN are investigated in solvents with
different polarities (10 lM) as shown in Fig. 1, and the correspond-
ing optical data are illustrated in Table S1 (ESI). The absorption
spectra of TPEAN showed two characteristic absorption bands at
ca. 300 nm and ca. 402 nm, the former one could be ascribed from
Fig. 1. Normalized UV–vis absorption spectra (a) and emission spectra (b) of AIEgen
ꢀ
5
TPEAN in solvents with different polarities. (Concentration is 1.0 ꢁ 10 M, excited
wavelength is 400 nm.)
p-skeleton [23]. Moreover, as shown in Fig. 2b, we could find that
p-p transition, and the latter one might be derived to charge trans-
molecule TPEAN possessed almost symmetrical structure and
adopted twisted conformations, which were beneficial to exhibit
fantastic luminescent characteristic, such as piezochromism [24].
Although the molecule TPEAN is very weak emission in THF
solution, it could emit intense fluorescence in solid state
fer (CT) transition. With increasing the solvent polarities, the
absorption spectra showed a slight shift, which indicated that a
small change of dipole moment at the ground state in different sol-
vents [21]. In contrast, the maximum emission peaks of TPEAN
were red-shifted obviously with increasing solvent polarities. For
example, emission peak appeared at 496 nm in hexane, and grad-
ually red-shifted to 554 nm in acetonitrile. Obvious solva-
tochromism suggested that TPEAN possessed evident D-A
structure [22]. Furthermore, with increasing solvent polarities,
the Stokes shifts of TPEAN clearly increased, and they were
(
Fig. S1). Thus, we deduced that compound TPEAN might show
AIE characteristic [25]. To verify this point, the fluorescent emis-
sion spectra with different water content (f , vol%) in a mixture
of THF/water were investigated and shown in Fig. 3a. TPEAN was
almost non-emissive in THF solution or in THF/water with f below
increased to 70%, strong emission peak centered at
25 nm was observed, and its intensity was gradually increased
. As can be seen from Fig. 3b, it was found
that the maximum emission intensity in THF/water mixture with
of 90% was approximately 20.3 times higher than that in THF
solution, and the increased to 0.35 (Table S2). The result further
w
w
6
5
0%. When f
w
ꢀ1
ꢀ1
4
714 cm in hexane and 6713 cm in acetonitrile (Table S1).
The emission bands were bathochromic shifted and broadened
from non-polar hexane to polar acetonitrile, which was typical
ICT effect. The fluorescent intensity of TPEAN in dilute solution
was very weak (Fig. S1), and the fluorescence quantum yields
with the increasing f
w
f
w
U
F
(U
F
) were less than 0.01 using 9,10-diphenylanthracene in ben-
confirmed its AIE performance [26]. We deemed that the
intramolecular rotations would be restricted in aggregated states.
Simultaneously, the radioactive decay path opened, resulting in a
obvious emission enhancement [27].
In order to understand the AIE nature of luminogen TPEAN, the
aggregate morphology and particle size of aggregated TPEAN in
THF/water mixtures with fw of 90% were observed by scanning
electron microscopy (SEM) and dynamic light scattering (DLS)
experiments. As shown in Fig. 4, it clearly confirmed aggregated
form. The luminogen TPEAN tended to aggregate in the form of
zene ( = 0.85) as standard.
U
F
To explore the ICT characteristic and electronic structure of
TPEAN, the DFT calculation was performed at the B3LYP/6-31G
d) level by Gaussian 09 W program. As shown in Fig. 2a, it was
found that the electron cloud density of HOMO was mainly dis-
tributed at two tetraphenylethene units, whereas the electron
cloud density of LUMO was located at cyanoethylene moiety. The
significant difference of electron cloud densities of HOMO and
LUMO suggested the occurrence of the ICT process and D-A-D type
(
2

Y. Zhan, Q. Zhao and W. Yang
Tetrahedron Letters xxx (xxxx) xxx
Fig. 2. (a) Highest occupied molecular orbital and lowest unoccupied molecular orbital spatial distributions and (b) optimized geometric models in the ground state of AIEgen
TPEAN.
Fig. 3. (a) Fluorescent emission spectra of AIEgen TPEAN in the mixture of THF/
water with different water fraction (f
w
) (0%–90%); (b) I/I
0
value of AIEgen TPEAN in
Fig. 4. SEM images (a, b) of AIEgen TPEAN in the mixture of THF/water with water
fraction of 90%. The concentration is 10 M.
different f . The insert is photoluminescence under 365 nm light irradiation (the
w
l
left is 0%, the right is 90%.)
TPEAN in dilute THF solution are shown in Fig. S2. There are no
nano-aggregates formed in dilute THF solution.
In the general, the AIE luminogen usually show the strongly
twisted molecular skeleton, which can lead to intermolecular loose
spindle-shaped nanoparticles with a average maximum diameter
approximately 200 nm in the mixtures of THF/water with 90%
water fraction. The DLS experimental data in Fig. 5 further indi-
cated that the existence of the nanoparticles in the solvent mixture
at fw = 90%. For comparison, the SEM and DLS images of AIEgen
packing and strong
p-p interaction, and then endow such AIEgen
with tunable solid-state fluorescence response to mechanical force
3

Y. Zhan, Q. Zhao and W. Yang
Tetrahedron Letters xxx (xxxx) xxx
red-shifted to ca. 455 nm, indicating the extension of
tion after grinding [30].
p-conjuga-
As shown in Fig. 7a, according to the XRD experimental results,
we could find that the luminogen TPEAN formed different molec-
ular aggregates before and after grinding treatment. The diffraction
patterns of the initial solid powder showed sharp and strong
diffraction peaks, meaning the well-ordered crystalline morphol-
ogy. On the contrary, the weak diffraction peaks appeared upon
grinding the initial solid. After fuming, sharp and intense diffrac-
tion peaks once again appeared. During the PFC experiment, the
TPEAN undergone a crystalline-amorphous phase transformation
process [29]. Therefore, we could deem that the crystallization
ability remarkably influence their PFC performance. The DSC
curves of the initial and grinding powders were studied in Fig. 7b.
The crystalline state of AIEgen TPEAN showed an obvious
endothermic peak at 247.5 °C, which was the melting point. After
the treatment of grinding, an obvious exothermic peak (167.3 °C)
was observed before the melting point, it was ascribed to the
cold-crystallization upon annealing [31].
Fig. 5. DLS size distribution profile of AIEgen TPEAN in the mixture of THF/water
(
1:9, v/v).
that exhibit PFC properties [28]. Consequently, the AIEgen TPEAN
is anticipated to be PFC-active. As shown in Fig. 6a, it was found
that the initial solid of TPEAN emitted bright green light under
3
65 nm light irradiation. After grinding with a pestle, its emission
color immediately turned yellow. And then, its color gradually
changed into its initial state upon fumed with dichloromethane
vapor. This experimental phenomenon confirmed that it possessed
the PFC property. Fig. 6b showed the initial solid of AIEgen TPEAN
gave clear emission peak at 521 nm. Upon grinding, the maximum
emitting wavelength red-shifted to 548 nm. And then, when the
ground powder was fumed, the emission wavelength was recov-
ered to green light centered at 520 nm. This process could be
repeated many times, indicating that the PFC process was reversi-
ble (Fig. S3). Moreover, As shown in Fig. S4, a wide absorption
region centered at ca. 430 nm was found in the initial solid of
TPEAN. Upon the treatment of grinding, the absorption band
became wider, and the maximum absorption peak at ca. 440 nm
Fig. 6. (a) Photographs of AIEgen TPEAN color changes in initial and grinding states
under 365 nm light irradiation; (b) Normalized emission spectra of AIEgen TPEAN
in different solid states: initial, grinding and fuming (kex = 400 nm).
Fig. 7. (a) XRD diffraction patterns of AIEgen TPEAN in initial, grinding and fuming
states; (b) DSC curves of AIEgen TPEAN in initial solid and grinding powder under
N flow at a heating rate of 10 K/min.
2
4

Y. Zhan, Q. Zhao and W. Yang
Tetrahedron Letters xxx (xxxx) xxx
(
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In summary, new tetraphenylethene modified cyanostilbene
derivative TPEAN had been designed and synthesized. Spectral
characteristic and theoretical chemical calculation indicated that
the compound TPEAN possessed unique ICT property. The TPEAN
gave intense fluorescence in aggregated state on account of AIE.
Specially, the TPEAN exhibited remarkable piezochromism. The
emitting color and wavelength of TPEAN were reversible and
repeated in the grinding-fuming treatment. This PFC mechanism
was originated from the conversion between the well-defined
crystalline and unordered amorphous states. This property endow
TPEAN with a variety of promising applications in mechanical sen-
sor, anti-counterfeiting paper and so on.
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