Piezofluorochromism of triphenylamine-based triphenylacrylonitrile derivative with intramolecular charge transfer and aggregation-induced emission characteristics

Article abstract of DOI:10.1016/j.tetlet.2018.04.039

New triphenylacrylonitrile derivative (DPPA) with triphenylamine moiety as electron donor group has been synthesized. Its emission wavelengths were strongly affected by solvent polarity, indicating intramolecular charge transfer (ICT) transitions. It was found that DPPA was almost non-emissive in tetrahydrofuran (THF), and the emission could be intensified obviously when a great amount of H₂O was added, illustrating aggregation induced emission property. It is interesting that the pristine crystal of DPPA emitted green light, and the ground made its emitting color to change into orange, which could be recovered under fumed with dichloromethane (DCM) vapor. The piezofluorochromic behavior might be switched reversibly under the stimulus of external force because of crystalline-amorphous phase transformation, which could be confirmed from the results of PXRD patterns and DSC curves in different solid states. This work might open a new strategy to provide a broad perspective for the development of AIE PFC materials.

Full text of DOI:10.1016/j.tetlet.2018.04.039

Tetrahedron Letters xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Piezofluorochromism of triphenylamine-based triphenylacrylonitrile
derivative with intramolecular charge transfer and aggregation-induced
emission characteristics
⇑
Xin Wang, Yunhan Wang, Yong Zhan , Peng Yang, Xiaojing Zhang, Yongnan Xu
Key Laboratory of Structure-Based Drug Design and Discovery, Ministry of Education, Shenyang Pharmaceutical University, Shenyang 110016, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
New triphenylacrylonitrile derivative (DPPA) with triphenylamine moiety as electron donor group has
been synthesized. Its emission wavelengths were strongly affected by solvent polarity, indicating
intramolecular charge transfer (ICT) transitions. It was found that DPPA was almost non-emissive in
tetrahydrofuran (THF), and the emission could be intensified obviously when a great amount of H2O
was added, illustrating aggregation induced emission property. It is interesting that the pristine crystal
of DPPA emitted green light, and the ground made its emitting color to change into orange, which could
be recovered under fumed with dichloromethane (DCM) vapor. The piezofluorochromic behavior might
be switched reversibly under the stimulus of external force because of crystalline-amorphous phase
transformation, which could be confirmed from the results of PXRD patterns and DSC curves in different
solid states. This work might open a new strategy to provide a broad perspective for the development of
AIE PFC materials.
Received 14 March 2018
Revised 12 April 2018
Accepted 17 April 2018
Available online xxxx
Keywords:
Piezofluorochromism
Aggregation-induced emission
Intramolecular charge transfer
Triphenylamine
Triphenylacrylonitrile
Ó 2018 Published by Elsevier Ltd.
Introduction
reversibility in the solid state, is still challenging. Furthermore, to
enlarge the family of specific AIE PFC compounds, it is necessary
Piezofluorochromic (PFC) materials,1 whose emitting colors
change in response to external mechanical stimuli (grinding, rub-
bing or crushing), have attracted much attention owing to their
potential applications in mechanosensors,2 fluorescence switch,3
and data storage.4 Much effort has been devoted to the design
and synthesis of PFC materials. Strong Solid-state emission and
obvious color contrast play important roles for PFC materials.5
However, the aggregation-caused quenching (ACQ) effect results
in a poor solid-state emission efficiency due to the formation of
excimers or delocalized excitons.6 Consequently, the PFC phe-
nomenon becomes difficult to observe. In 2001, Tang and co-work-
ers reported unique fluorescent dyes, namely aggregation-induced
emission (AIE) materials, which are highly emissive in the aggre-
gated states compared with those in isolated states.7 Thus, AIE-
active dyes are promised to be PFC materials. Till now, several
studies have focused on AIE luminogens, such as the derivatives
of tetraphenylethene,8 cyanostilbene,9 anthracene,10 boron11 and
gold(I) complexes,12 because these compounds exhibit excellent
PFC properties. However, creating such organic AIE PFC materials,
which combine high fluorescence quantum yield, excellent
to carry out more extensive explorations of new AIE-active PFC
chromophores in this field.
Recently, p-conjugated molecules with a D-p-A structure have
attracted great interest because of their flexibility for creating
organic emitters with tunable emission.13 Harima and co-workers
demonstrated that the D-A structure was important for the organic
molecule to have PFC behaviors.14 Therefore, ICT dyes with AIE-
active compounds are expected to become a foundation for PFC
materials. In our previous study, we found that D-p-A molecules
with non-planar conjugated moieties exhibited significant PFC
behaviors.15 With these results in mind, we designed and synthe-
sized new donor-p-acceptor type triphenylamine (TPA) functional-
ized triphenylacrylonitrile (TPAN) molecule DPPA (Scheme 1). The
molecular design was based on the following points. First, the TPA
moiety was introduced due to the strong electron donating ability
and propeller-like conformation would helpful for generating PFC
properties.16 Second, TPAN derivatives usually exhibited AIE prop-
erties and better-withdrawing ability.17 Third, conjugated connect
of TPA and TPAN in molecular backbone would construct D-p-A
type AIE-active fluorescence molecule. Its charge transfer, AIE
and PFC properties have been systematically investigated. Optical
research and theoretical calculations indicated that ICT occurred
from electron donor TPA to acceptor TPAN. Moreover, DPPA
⇑
Corresponding author.
E-mail address: zhanyong2046@126.com (Y. Zhan).
https://doi.org/10.1016/j.tetlet.2018.04.039
0040-4039/Ó 2018 Published by Elsevier Ltd.
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X. Wang et al. / Tetrahedron Letters xxx (2018) xxx–xxx
Scheme 1. The synthetic routes of DPPA.
showed evident AIE characteristics. Interestingly, the solid-state
emitting colors of DPPA from green to orange under the treatment
of grinding/fuming with DCM, exhibiting piezofluorochromism.
The PXRD and DSC results illustrated that the piezofluo-
rochromism was derived from the transformation between crys-
talline and amorphous states. This work will be helpful for the
design of AIE PFC materials.
Results and discussion
The synthetic routes to the triphenylamine modified tripheny-
lacrylonitrile derivative DPPA are shown in Scheme 1. Firstly, com-
pound 3 2-(4-bromophnyl)-3,3-diphenylacrylonitrile was synthesized
as a key intermediate in 89% yield through Knoevenagel condensation
between benzophenone 1 and 4-bromophenylacetonitrile 2.18a Then,
the compound 5 (N-phenyl-N-(4-vinylphenyl)benzenamine) was
synthesized via Wittig reaction with 4-(diphenylamino)
benzaldehyde 4 and methyltriphenylphosphonium iodide in a
yield of 92%.18b Finally, the Heck reaction between compounds 3
and 5 afforded compound DPPA, catalyzed by Pd(OAc)2 at 110 °C
under nitrogen atmosphere overnight in a yield of 80%.19 The tar-
get compound DPPA was purified by column chromatography on
silica gel, and unambiguously characterized by 1H NMR, 13C
NMR, elemental analysis and high resolution mass spectroscopy.
Moreover, the peaks at 6.914 and 6.887 ppm in the 1H NMR spec-
trum from the vinyl group protons had a large coupling constant of
16.2 Hz, indicating that the vinyl group had a transformation.20
The photophysical properties of DPPA in solvents of varying
polarities at concentration of 1.0 Â 10À5 M were investigated by
UV–vis absorption and fluorescence emission spectroscopy
(Fig. 1), and corresponding photophysical data are summarized in
Table S1 (ESI). As shown in Fig. 1a, the absorption bands of DPPA
in dilute solutions appeared at ca. 307 nm and ca. 400 nm. The for-
Fig. 1. Normalized UV–vis absorption spectra (a) and fluorescence emission spectra
(b) of DPPA in different solvents excited at 400 nm (1.0 Â 10À5 M).
mer band could be ascribed to the
p–
p⁄ transitions, whereas the
latter was attributed to the intramolecular charge transfer (ICT)
transition, which could be supported by the solvent-dependent
absorption and fluorescence emission spectra changes. For exam-
ple, the maximal absorption band of DPPA located at 396 nm in
hexane red-shifted gradually with increasing the polarity of the
solvents, and it reached 403 nm in DMF (Table S1). These results
illustrated that an excited state with a larger dipole moment than
that in the ground state.21 In addition, the fluorescence emission
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X. Wang et al. / Tetrahedron Letters xxx (2018) xxx–xxx
3
bands of DPPA red-shifted significantly with increasing the solvent
polarity. For instance, the emission maximum of DPPA in hexane
appeared at 484 nm, which red-shifted to 578 nm in DMF (Fig.
1b). Moreover, the Stokes shifts gradually increased with increas-
ing solvent polarity, and they were 459 cmÀ1 and 751 cmÀ1 in hex-
ane and DMF, respectively. Thus, such significant bathochromic
shift and the broaden of the emission band with increasing the
polarity of the solvents indicated the ICT character for the excited
state.22
It is worth noting that, in addition to the main emission band,
compound DPPA exhibited another weaker emission band at the
higher-energy side in CH2Cl2, suggesting dual emission properties
of DPPA. The intensity radio of the two emission maxima at 463
nm and 567 nm of DPPA changed when the excitation wavelength
was varied (Fig. S1a). The excitation spectrum of the emission at
567 nm was distinctly red-shifted with respect to that of the emis-
sion at 463 nm (Fig. S1b). The dual emissions of DPPA were
believed to originate from a balance between the LE (local excited)
and ICT excited states.23
The fluorescence quantum yield (UF) of DPPA in different sol-
vents was investigated using 9,10-diphenylanthracene (UF = 0.85
in benzene, excited at 390 nm) as the standard. It was found that
DPPA showed very low UF (0.01–0.05) in the tested solutions
(Table S1). The very weak UF value in solutions could be ascribed
to the active intramolecular rotations of multiple phenyl rings,
which often caused non-irradiative decay.24 In contrast, the crys-
tals of DPPA showed high fluorescence quantum yield, the value
of UF reached 75.8%.
packing and good structural transformability, which might endow
DPPA with both AIE and PFC characteristics.25
As discussed above, DPPA was practically non-emissive when
molecularly dissolved in solvents due to active intramolecular
rotations and vibrations. Triphenylacrylonitrile (TPAN) is a well-
known building block in AIE compounds,26 so it is predicted that
the synthesized compound DPPA might exhibit AIE properties.
To investigate the AIE activity of DPPA, its fluorescence emission
behavior was studied in dilute solutions of THF/water with differ-
ent water fractions (fw, the volume percentage of water in THF/
water mixtures), and the corresponding fluorescence emission
spectra are shown in Fig. 3. The fluorescence spectra revealed no
significant changes with an increase in the content of water from
0 to 60%, because the DPPA molecule was fully dissolved in mix-
tures. A dramatic fluorescence enhancement was observed when
the water volume fraction (fw) increased up to 70%, and then emis-
sion of the mixture of DPPA increased to the maximum value at fw
= 90% in all measurements. At a fw = 90%, the bright yellow light
(centered at 558 nm) emission was obtained for the aggregates,
the corresponding emission intensity increased up to about 31-fold
higher than that in THF (fw = 0%). The weak emissive in THF and
aqueous mixtures with low fw should be ascribed to the active
intramolecular rotations of the genuinely dissolved luminogens
in these mixtures. However, at high fw, such intramolecular
rotations were restricted, which prevented the non-radiative
To obtain a better insight of the optical properties and solvent
effects of DPPA, density functional theory (DFT) calculations were
performed at the B3LYP/6-31G(d) level after optimizing its struc-
ture to the lowest energy spatial conformation with the Gaussian
09W program. The frontier orbital plots of the HOMO and LUMO
are shown in Fig. 2. We could find that the HOMO level of DPPA
was mainly located at the triphenylamine and vinyl moieties,
while the LUMO level was chiefly distributed at the acceptor of
the TPAN unit. This result demonstrated that DPPA adopted typical
D-p-A structure with the triphenylamine moiety as the electron
donor and the triphenylacrylonitrile unit as the acceptor. As a
result, the intramolecular charge transfer would occur in the D-
p-A type triphenylamine-based triphenylacrylonitrile detivative.
In addition, the DPPA adopted a twisted skeleton in its optimized
lowest energy state. This conformation disfavored close molecular
packing in the solid state, leading to efficient fluorescence. We sug-
gested that DPPA in the solid state might have loose molecular
Fig. 3. (a) Fluorescence emission spectra of DPPA in the mixtures of THF and water
with different water fractions (fw); (b) normalized emission intensity of DPPA in
THF/water with different fw. Concentration: 1.0 Â 10À5 M; kex = 400 nm; Inset is the
fluorescence images of DPPA in THF (fw = 0%) and THF/water (fw = 90%) under UV
illumination.
Fig. 2. Electron density distributions in the HOMO and LUMO states and optimized
conformation of DPPA calculated by DFT in Gaussian 09W at the B3LYP/6-31G(d)
level.
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X. Wang et al. / Tetrahedron Letters xxx (2018) xxx–xxx
relaxation channels and thus made the aqueous mixture highly
emissive.27 Therefore, DPPA exhibited obvious AIE properties.
We found that the pristine crystal emitted green light and the
fluorescence color changed into orange when lightly pressed, indi-
cating piezofluorochromism.28 After that, the solid emission prop-
erties of DPPA were investigated. As shown in Fig. 4a, a maximum
emission peak at 526 nm appeared in the pristine crystal, allowing
it to exhibit green fluorescence with light irradiated at 365 nm. The
ground powder emitted orange fluorescence centered at 558 nm, a
red shift of 32 nm. When the ground powder was exposed to DCM
vapor for several seconds, the emission of the ground powder
rapidly recovered to bright green (Fig. 4b). This fluorescence color
switch could be repeatedly observed by grinding and then solvent
fuming (Fig. S2). These results display that the PFC behavior of
DPPA is reversible. After fuming, the NMR spectrum was investi-
gated, there was no peak at 5.3 ppm in CDCl3 at room temperature,
indicating that there was no DCM residue after fuming (Fig. S3).
To provide further insight into the MFC mechanism of DPPA,
the powder X-ray diffraction (PXRD) measurements for the sam-
ples were studied. As shown in Fig. 5, the pristine crystal exhibited
strong and sharp diffraction peaks in wide angle region, suggesting
the ordered arrangement form. In contrast, the ground powder
showed no diffraction peaks, indicating amorphous state. After
the ground powder was fumed with DCM vapor, sharp diffractions
emerged again, implying the recovery of an ordered crystalline lat-
tice. The PXRD results clearly indicated that piezofluorochromism
was highly associated with the molecular arrangement that greatly
influenced photophysical properties.29 Differential scanning
calorimetry (DSC) measurements were investigated to study ther-
mal properties of DPPA (Fig. 6). The pristine crystal of DPPA
showed one sharp endothermic peat at 166 °C, corresponding to
its melting point, while obvious exothermic transition peak at 95
°C appeared in its ground powder, which could be ascribed to the
transformation from amorphous to crystalline. Therefore, the PFC
Fig. 5. PXRD patterns of DPPA in different solid-states: pristine, ground and fumed.
Fig. 6. DSC curves of DPPA in the pristine crystal (blue line) and ground powder
(red line) under nitrogen atmosphere at a heating rate of 5 °C/min. (For interpre-
tation of the references to color in this figure legend, the reader is referred to the
web version of this article.)
behavior of DPPA originated from the phase transformation
between crystalline and amorphous states, and was not due to
any chemical change.30
Conclusions
In summary, we have synthesized a new donor-p-acceptor
triphenylamine functionalized triphenylacrylonitrile DPPA and
investigated its photophysical properties. It was found that DPPA
showed bathochromic shift in varying solvents due to the ICT
effect. More importantly, it exhibited obvious AIE and significant
PFC properties. The PFC behavior of DPPA (from green to orange
emission, kem from 526 to 558 nm) is reversible by repeating the
grinding-fuming process. The PFC mechanism is believed to
generate through the transformation between the crystalline and
amorphous states. The present study will be of great help in
designing new AIE PFC materials.
Fig. 4. (a) Normalized fluorescence spectra of DPPA in different solid-states:
pristine, ground and fumed, kex = 400 nm; (b) Photograph of DPPA color change
under ground and fumed stimuli.
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5
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(c) Xue P, Yao B, Liu X, et al. J Mater Chem C. 2015;3:1018.
This work was supported by Career Development Support Plan
for Young and Middle-aged Teachers in Shenyang Pharmaceutical
University (No. ZQN2016027).
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