High contrast mechanofluorochromic behavior of new tetraphenylethene-based Schiff base derivatives

Article abstract of DOI:10.1016/j.dyepig.2019.107619

New tetraphenylethene-based Schiff base ligand (TPE-ND) and its corresponding boronated complex (TPE-NDB) with aggregation-induced emission were designed and synthesized. Results showed that the inhibited C=N isomerization by N, O-chelated BF₂

Full text of DOI:10.1016/j.dyepig.2019.107619

Accepted Manuscript
High contrast mechanofluorochromic behavior of new tetraphenylethene-based Schiff
base derivatives
Ting Sun, Dandan Cheng, Yongshuai chai, Jian Gong, Mengyu Sun, Feng Zhao
PII:
S0143-7208(19)30973-8
DOI:
https://doi.org/10.1016/j.dyepig.2019.107619
Article Number: 107619
Reference:
DYPI 107619
To appear in: Dyes and Pigments
Received Date: 6 May 2019
Revised Date: 28 May 2019
Accepted Date: 3 June 2019
Please cite this article as: Sun T, Cheng D, Yongshuai chai , Gong J, Sun M, Zhao F, High contrast
mechanofluorochromic behavior of new tetraphenylethene-based Schiff base derivatives, Dyes and
Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.107619.
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High Contrast Mechanofluorochromic Behavior of New Tetraphenylethene-
based Schiff Base Derivatives
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Ting Sun ,a Dandan Cheng,b Yongshuai chai,c Jian Gong,a Mengyu Sun,a Feng Zhao,a,
d,
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*
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College of Chemistry and Chemical Engineering, Anyang Normal University,
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Anyang 455000, P. R. China
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b School of Life Science Wuchang University of Technology, No.16, Jiangxia Avenue,
Wuchang, Wuhan 430223, P.R.China
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C Interdisciplinary Research Center on Biology and Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences,Shanghai 200032, P.R. China
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Henan Province Key Laboratory of New Opto-Electronic Functional Materials,
Anyang 455000, P. R. China
* Corresponding author. E-mail addresses: zhaofeng@aynu.edu.cn
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Abstract
New tetraphenylethene-based Schiff base ligand (TPE-ND) and its corresponding
boronated complex (TPE-NDB) with aggregation-induced emission were designed
and synthesized. Results showed that the inhibited C=N isomerization by N, O-
chelated BF2 caused the significant intramolecular charge transfer features, and more
dramatic solvatochromism. In particular, the solid sample of TPE-NDB exhibited an
obvious mechanofluorochromic behavior. Upon grinding with a spatula, the as-
prepared powder sample illustrated a remarkable red shift of 97 nm, with considerable
color contrast from bright green (498 nm) to orange (595 nm). Its fluorescence color
can be reversibly switched by repeating the grinding-fuming process. The
mechanochromism is attributed to the phase transformation between amorphous and
crystalline states. The practical application indicated that TPE-NDB has excellent
mechanofluorochromic properties, and it can be utilized as optical recording materials.
Keywords:
Tetraphenylethene-based
Schiff
base,
boron
complex,
mechanofluorochromism, intramolecular charge transfer, aggregation-induced
emission.
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1. Introduction
Luminescent organic dyes have attracted much attention due to their potential for
sensors, memory storage, organic light-emitting devices, and data security protection
[1-4]. To date, design and creation of efficient luminescent dyes in the solid and
aggregate states is still a hot research topic. A formidable challenge to this
development is the notorious aggregation-caused quenching (ACQ) effect, namely,
the emission of a conventional luminophore is usually weakly emissive or completely
quenched upon aggregation in the solid state because of the formation of excimers
and exciplexes [5]. Fortunately, in 2001, Tang’s group developed a series of
propeller-shaped silole compounds showing aggregation-induced emission (AIE)
effect, which is exactly opposite to the troublesome ACQ phenomenon, and thus
could overcome fluorescence quenching in aggregation [6]. Since then, numerous
compounds with AIE properties have been explored, including tetraphenylethenes [7],
fulvenes [8], 9,10-divinylanthracene derivatives [9], pyran derivatives [10],
conjugated polymers [11] and others. In 2010, Park et al. reported the cyano-
distyrylbenzene derivative with stimuli-responsive and AIEE properties, which has
opened a new avenue of mechanofluorochromic (MFC) compounds. Then a number
of both MFC and AIE dyes were synthesized by Tang, Chi, Yang and their co-
workers [12-15]. In general, AIE-active compounds possess the strongly twisted
skeleton bearing rotatable aryl units, which can afford loose packing patterns in the
crystal states, could be easily destroyed under mechanical stimuli and cause the
emitting color change. Currently, extended π-conjugated molecules with a D–A
structure was very significant for the organic dyes to show MFC characteristics [16]
and some D–A typed molecules always contribute to the realization of fluorescence
change under mechanical force [17]. Tetraphenylethene (TPE) is a typical AIE unit,
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which takes a nonplanar configuration and has electron donating characteristics. It to
be a useful building block for the construction of D–A type dyes [12c, 13c, 18].
Despite a variety of AIE systems possessing reversible MFC features have been
developed, most of the MFC dyes so far exhibit spectral shifts within tens of
nanometers upon simple mechanical stimuli [18a, 19]. Reports of the dyes with very
large MFC shifts (>90 nm) and obvious color contrast are still rare. The prominent
color contrast before and after the application of mechanical stimulus are great
important for the effective application of MFC materials. Moreover, these dyes are
often prepared through complicated synthetic routes. It is highly demand to exploit
high contrast MFC dyes using facile synthetic procedures. Schiff bases have the
advantages of a rather facile synthesis with high yield and flexible structure
modification. They have been explored extensively as catalysts [20], ions sensors [21],
and pharmacological components [22]. However, the exploration of their MFC
potential is still in the early stages [23].
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Scheme 1. Synthetic routes of TPE-ND and TPE-NDB.
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In this perspective, we focus on the combination of tetraphenylethene (TPE) unit
with conventional Schiff base functional segment to develop new MFC dyes with D-
A structure. We synthesized the tetraphenylethene-based Schiff base ligand (TPE-ND)
and its corresponding boronated complex (TPE-NDB), and their synthetic routes are
shown in Scheme 1. AIE and MFC characteristics of the two fluorescent dyes were
systematically investigated, and the results indicated that the ligand TPE-ND
demonstrated the intramolecular charge transfer (ICT) nature and AIE property,
whereas boronated complex TPE-NDB exhibited strong ICT feature, and obvious
AIE and MFC characteristics. More importantly, MFC behaviors of TPE-NDB
displayed a large spectral shift of 97 nm before and after mechanical stimuli. The
remarkable MFC characteristics are desirable owing to their practical applications in
mechano-sensors, memory devices, and forensic science [24-26].
2. Experimental
2.1. Materials and instrumentation
All reagents and solvents were purchased commercially (AR grade) and used without
further purification unless otherwise noted. Tetrahydrofuran (THF) and toluene were
distilled from sodium and benzophenone ketyl in a nitrogen atmosphere.
Dichloromethane (DCM) was distilled from calcium hydride. The THF/H2O mixtures
with different water fractions were prepared by slowly adding distilled water into the
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THF solution of samples under ultrasound at room temperature. H NMR and C
NMR spectra were collected on a Bruker–400 MHz spectrometer with TMS as an
internal standard. Mass spectra were measured on a Shimadzu MALDI-TOF MS
spectrometer. UV-vis spectra were recorded on Shimadzu UV-2550
spectrophotometer. Emission spectra were performed by a HITACHI fluorescence
spectrometer (F-4600). The absolute fluorescence quantum yields and fluorescence
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lifetime were measured on an Edinburgh FLS980 steady state spectrometer using an
integrating sphere, and the lifetimes were calculated with the F900 Edinburgh
instruments software. Crystal data of compound was selected on a Bruker D8 Focus
Powder X-ray diffraction diffractometer. The geometry of the sample molecule was
fully optimized using density functional theory (DFT) at the B3LYP/6-31G* level,
employing the Gaussian 09W suit of programs. Dynamic light scattering (DLS)
measurements were performed on the BI-200SM Laser Light Scattering System
(Brookhaven). The morphologies of the nanoaggregates were investigated using a
Tecnai G2S-Twin F20 TEM at an accelerating voltage of 200 kV.
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2.2. Synthesis
2.2.1. Synthesis of Compounds 2 and 3
The intermediates 2 and 3 were prepared according to the previously reported method
[27], with all their characterization data matching the literature data.
2.2.2. Synthesis of Compound TPE-ND
Compound 3 (0.34 g, 1.0 mmol) and 2-hydroxy-1-naphthaldehyde (0.17 g, 1.0 mmol)
in ethanol (30 mL) were refluxed for 10 min, then an orange solid was precipitated.
The product was filtered and washed several times with ethanol and then collected
with a high yield of 94 %. Mp: 203–204 °C. 1H NMR (400 MHz, DMSO-d6): δ 15.75
(d, J = 5.1 Hz, 1H), 9.59 (d, J = 5.2 Hz, 1H), 8.45 (d, J = 8.5 Hz, 1H), 7.91 (d, J = 9.2
Hz, 1H), 7.78 (d, J = 7.9 Hz, 1H), 7.52 (t, J = 7.7 Hz, 1H), 7.43 (d, J = 8.2 Hz, 2H),
7.34 (t, J = 7.4 Hz, 1H), 7.22 –7.11 (m, 9H), 7.10 – 6.94 (m, 9H). (Fig. S6). 13C NMR
(101 MHz, DMSO-d6): δ 171.53, 155.12, 143.61, 143.51, 142.15, 141.94, 141.39,
140.32, 137.41, 133.62, 132.42, 131.20, 131.14, 131.09, 129.43, 128.53, 128.46,
128.35, 128.26, 127.19, 127.12, 127.07, 127.03, 123.92, 122.79, 120.81, 120.29,
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108.99. (Fig. S7). HRMS (MALDI-TOF) m/z: [M+H]+ Calcd for C37H27NO 502.2126;
Found 502.2176 (Fig. S8).
2.2.3. Synthesis of Compound TPE-NDB
The corresponding boron complex TPE-NDB was synthesized by reaction of TPE-
ND with boron trifluoride etherate in the presence of triethylamine. The residual
crude product was purified by silica gel chromatography to obtain a yellow boron
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complex. Yield: 89%. Mp: 215–216 °C H NMR (400 MHz, CDCl3): δ 9.06 (s, 1H),
8.10 (d, J = 9.1 Hz, 1H), 8.06 (d, J = 8.4 Hz, 1H), 7.85 (d, J = 8.0 Hz, 1H), 7.70 –
7.62 (m, 1H), 7.50 (t, J = 7.5 Hz, 1H), 7.36 (d, J = 8.3 Hz, 2H), 7.32 – 7.26 (m, 2H),
7.20 – 7.04 (m, 16H). (Fig. S9).13C NMR (101 MHz, CDCl3): δ 162.63, 157.40,
144.85, 143.32, 143.19, 143.14, 142.35, 140.90, 140.83, 139.46, 132.54, 131.46,
131.35, 131.27, 131.22, 129.76, 129.43, 128.08, 127.97, 127.86, 127.68, 126.93,
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126.75, 126.69, 125.09, 122.78, 120.51, 119.16, 108.74. (Fig. S10). F NMR (376
MHz, CDCl3): δ -134.95, -134.98, -135.03, -135.06. (Fig. S11). HRMS (MALDI-
TOF) m/z: [M+H]+ Calcd for C37H26BF2NO 550.2109; Found 550.2183 (Fig. S12).
2.3. Preparation of the samples for AIE measurements
A 10−3 M stock solution of target molecules in THF was prepared. Aliquots (100 µL)
of the stock solution were added to 10 mL volumetric flasks and diluted to volume
with water and THF in the proper ratios under sonication at room temperature, the
concentration was maintained at 1.0×10-5 M. The fluorescence emission spectral
measurement of the mixture was performed immediately.
2.4. Preparation of the samples for mechanofluorochromism study
The grinding powders were obtained by grinding the as-synthesized crystals with a
pestle in the mortar. The fumed samples were prepared by fuming the grinding
powders with DCM for 2 min.
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Fig. 1. Normalized UV-vis absorption of TPE-NDB (a) and TPE-ND (b), and PL
spectra of TPE-NDB (c) and TPE-ND (d) excited at 400 nm in different solvents (1.0
× 10-5 mol L-1). Photographs of TPE-NDB (e) and TPE-ND (f) taken under UV
illumination in different solvents, from left to right: hexane, cyclohexane, toluene,
THF, chloroform, DMF, and DMSO.
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3. Results and discussion
3.1. Synthesis
The target ligand is conveniently synthesized by the reaction of compound 3 with 2-
hydroxy-1-naphthaldehyde in a high yield (94%). TPE-ND was allowed to react with
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boron trifluoride etherate in the presence of triethylamine to give corresponding boron
complex TPE-NDB with yield 89%.
3.2. Optical Properties in solution
In order to probe the optical properties of Schiff base ligand and boron complex in
solvents with different polarities, UV-vis and fluorescence spectroscopy were carried
out. The UV-vis absorption and fluorescence emission spectra were shown in Fig. 1,
and the corresponding photophysical data are summarized in Table S1. The UV-vis
absorption spectra of TPE-NDB (Fig. 1a) shown an absorption band from λ = 280nm
to 350 nm. This band did not shift with increasing polarity of the solvents, which was
attributed to π–π* local electron transitions of the conjugate system. The absorption
band at λ = 360 – 520 nm exhibited a trend of red-shifted with the increasing solvent
polarity, assigned to the ICT transition. From the absorption spectra of the ligand (Fig.
1b), TPE-ND also had two main absorption bands at ca. 330 nm and ca. 410 nm in
different solvents, which can be assigned to the π-π* transition of the conjugated
system and ICT transitions, respectively [28]. The solvent-dependent PL spectra of
TPE-NDB were shown in Fig.1c, it is clear that the emission bands were dramatically
red-shifted with increasing solvent polarity, and was accompanying with large Stokes
shift. In hexane, the TPE-NDB emission band located at 544 nm and its Stokes shift
was 5948 cm-1, and with increasing polarity of the solvents, its emission band and
Stokes shift reached 627 nm and 8621 cm-1 in DMSO, respectively. The result
suggests that ICT transitions of TPE-NDB take place at excited state in more polar
solvents [29]. It is worth mentioning that the fluorescence spectra of TPE-NDB
became structured in non-polar solvents, such as hexane and cyclohexane, which
indicated that the emission was typical from the locally excited (LE) state [30]. The
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Scheme 2. Illustration of inhibited C=N isomerization by hydrogen bonding (TPE-
ND) and N, O-chelated boron complex (TPE-NDB)
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PL spectra of TPE-ND ligand demonstrated a small red shift of emission upon
increased solvent polarity. The emission band located at 503 nm in hexane, and
reached to 516 nm in DMSO. The results indicated that the ICT propterty of ligand
was weaker than boron complex. Moreover, TPE-NDB exhibited significantly red-
shifted absorption bands as compared with the ligand in the same solution. For
example, in THF the emission peak of ligand was centered at 509 nm (Φf = 0.03), by
contrast, the emission band of boron chelated complex localed at 583 nm (Φf = 0.08),
displayed a large red-shift of 74 nm. The extended π-conjugation skeleton produced
by the embedded boron atom was responsible for this phenomenon (Scheme 2) [31].
In order to obtain a better insight of spectroscopic properties, we calculated frontier
molecular orbital by using the density functional theory (DFT) method at the
B3LYP/6-31G* level with the Gaussian 09W program. Fig. 2 showed the electron
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Fig. 2. Energy levels of HOMO and LUMO, energy gaps, and electron cloud
distributions calculated by the B3LYP/6-31G* program, and the optimized
conformation structures of TPE-NDB and TPE-ND.
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distribution of the highest occupied molecular orbital (HOMO) and the lowest
occupied molecular orbital (LUMO) of TPE-NDB and TPE-ND [32]. It was found
that the HOMO of TPE-NDB mostly localized on the major donor (i.e., the TPE unit),
while the LUMO shifts to the right part of the molecule because of the strong electron
withdrawing ability of the naphthalene boron moiety in terms of its strong push–pull
structure, implying that light excitation would lead to ICT from the donor unit to the
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acceptor unit as occurs in most D–A type molecules. The HOMO of TPE-ND ligand
was mostly located at the whole molecule, and the LUMO was mainly distributed at
naphthalene boron unit, resulting in a weak ICT performance. This information is
helpful in understanding the absence of the MFC behavior of TPE-ND. The
asymmetrical electron cloud distribution of the HOMO and LUMO of TPE-NDB
resulted in an energy gap of 3.03 eV, which was lower than ligand alone (3.33eV),
further confirming the ICT property of TPE-NDB was stronger than TPE-ND. The
decrease in the HOMO−LUMO energy band gap of TPE-NDB boron complex
compared to the ligand was due to that the C=N isomerization was inhibited by the
complexation of B(III) , produced more rigid boron-bridged π-framework (Scheme 2).
It was assumed that more rigid structure significantly reduces the energies of singlet
excited states and a much lower singlet-triplet (ST) energy splitting for TPE-NDB.
The optimized geometries of TPE-NDB and TPE-ND showed twisted nonplanar
shapes, respectively. Dihedral angles between the O−H···N hydrogen bonds ring A
and phenyl group B in TPE-ND was 32.2°, and it increases to 41.2° after the
covalently bridged C=N structure by N, O-chelated BF2 of TPE-NDB. The dihedral
angles between phenyl rings in tetraphenylethene units, including B–C, B–D, and B–
E remain unchanged (Tab. S3). The introduction of boron group thus leaded to the
enhanced the degree of molecular distortions. The above theoretical results further
illustrate the occurrence of the ICT process, which is consistent with the observed
optical property (Fig. 1). The features of twisted spatial conformation and ICT
transition of TPE-NDB and TPE-ND may endow them with vivid color emission in
the condensed/solid state [33].
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Fig. 3. PL spectra of TPE-NBD (a) and TPE-ND (c) in THF–water with different
water fractions (vol., 0-95%), λex = 400 nm. Normalized fluorescent emission
intensities of TPE-NBD (b) and TPE-NB (d) in THF–water with different fw. The
inset graphs in (b) and (d) are the solutions of TPE-NBD and TPE-ND in THF-water
under irradiation of UV lamp at 365 nm, from left to right (fw): 0%, 10%, 20%, 30%,
40%, 50%, 60%, 70%, 80%, 90%, 95%.
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3.3. Aggregation induced emission (AIE)
The AIE effects of the synthesized ligand and boron complex were examined by
comparing the fluorescence emission spectra of THF/H2O solution with different
water fraction (fw, the volume percentage of water). As shown in Fig. 3, in dilute THF
solution, the emission band of TPE-NDB was broad with a center at 586 nm, and Φf
is very low (0.08), which might be due to active intramolecular rotations of the
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compound. The boron complex TPE-NDB served as relaxation channels for the
excited state in this situation. When an appropriate amount of water was added to the
THF solution, the emission peaks decreased and eventually disappeared. The
fluorescence quenching was attributed to the ICT effect in polar solvent for TPE-
NDB with D–A structure [15e]. When fw was above 70%, TPE-NDB displayed a
sudden increase in the emission intensity due to molecular aggregation. As fw was
95%, the emission peak located at 571 nm, and the PL intensity is approximately 2.5
times higher than that in the pure THF. The fluorescence enhancement phenomenon
could be ascribed to the restriction of the intramolecular rotations, which blocked the
non-irradiative channels and caused clear enhancement of emission in aggregated
states [15]. The emission of TPE-ND exhibited an enhancement in the emission
intensity with gradual increase from fw = 0% to fw = 50%, and showed a rapid decrease
at fw = 70%. At an fw of 80%, a significant increase in the emission intensity was
observed, which can be attributed to the AIE effect caused by the formation of
molecular aggregates when water is added into the THF solution. The PL intensity for
TPE-ND showed a zig-zag pattern (Fig. 1d). This phenomenon was often observed in
the determination of the AIE effect, but the reasons remain unclear [34]. There are
two possible explanations for this phenomenon [35]. First, when water is added, the
solute molecules can aggregate into crystal particles or amorphous particles
suspensions [36]. The crystal particles suspension results in an enhancement in the PL
intensity, in contrast, the amorphous particles suspension leads to a decrease of PL
intensity. Second, only the molecules on the surface of the nanoparticles emitted light
after the aggregation, which lead to a decrease in PL intensity. However, the
restriction of intramolecular rotations of the aromatic rings in the aggregation state
could enhance light emission. The net output of these antagonistic processes depends
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on which process plays a key role in affecting the fluorescent behavior of the
aggregated molecules [37]. Thus, the measured PL intensity often shows no regularity
in high water content. To further reveal the AIE behavior of TPE-NBD and TPE-ND,
dynamic light scattering (DLS) and transition electron microscopy (TEM) were
employed to study the microstructures in the aggregate state. As shown in Fig. S1,
TPE-NDB aggregates were obtained when fw was 95%, with the size from 20 to 120
nm. When fw was 80%, TPE-ND aggregates also emerged with the size of 20−150 nm.
The TEM images indicated that the nanoparticles were in amorphous state both for
TPE-NDB and TPE-ND (Fig. S2). These results clearly implied that TPE-NBD and
TPE-ND are AIE-active molecules.
In many works, the Schiff base ligands don’t possess AIE behavior due to the
intramolecular rotations of C=N bond, which could produce the non-irradiative
channels [38]. In this work, the Schiff base ligand TPE-ND exhibited AIE
characteristics, although the fluorescence is weak. This phenomenon could be
ascribed to the intramolecular hydrogen bond O−H···N is formed to yield a six-
membered pseudo ring, such structure helps the molecules to further rigidify their
conformation and to restrict molecular vibration, thus producing the AIE feature
(Scheme 2). When the covalently bridged C=N structure by N, O-chelated BF2, the
rigid molecular structure is improved. The rigid boron-bridged π-frameworks tend to
undergo very rapid nonradiative decay and intensify the AIE performance.
3.4. Mechanochromic properties
Generally, the compound possessing non-planar geometries and D-A conjugated
systems with ICT properties is expected to undergo MFC behavior. The fluorescent
performance of the solid sample TPA-NDB in response to external pressure was
investigated. The crude product was purified on a silica-gel column to afford a yellow
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Fig. 4. (a) Normalized fluorescent spectra of TPE-NDB in different solid-states: as-
prepared, grinding and fuming, λex = 365 nm. Photographs of TPE-NDB color
changes upon grinding and fuming stimuli by naked eyes (b) and under UV light (365
nm) (c).
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powder. As shown in Fig. 4, the as-prepared TPE-NBD powder demonstrated bright
green emission under 365 nm UV-light illumination. Interestingly, after grinding with
a mortar and pestle, the powder changed its emission color into orange. The force-
induced color switch could be fully restored by fuming with DCM for 2 minutes at
room temperature. The fluorescence converted rapidly to bright green, similar to their
original powders. The color changes could be observed with the naked eye (Fig. 4b).
In addition, this process of fluorescence color change could be repeated many times
(Fig. S3). The dye displayed excellent reversibility without any fatigue in response
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throughout the six cycles, demonstrating the obvious and reversible MFC behavior of
the TPE-NDB. The PL spectra measurement was applied to monitor such a reversible
color switching under external stimuli. As depicted in Fig. 4a, the emission peak of
as-prepared powder centered at 498 nm, and red-shifted to 595 nm after grinding,
which suggested that the grinding treatment has induced a significant spectral red-
shift of 97 nm. The Φf of as-prepared and ground sample for TPE-NDB in solid state
were 0.35 and 0.19, respectively (Tab. S2). High intensity solid-state emission and
the prominent color contrast before and after the application of mechanical stimulus
are very significant for the effective application of MFC materials. The MFC
properties of TPE-ND were also tested by grinding treatment. However, there was
almost no change in the main fluorescence peak of the compound TPE-ND and could
not lead to the color change before and after grinding, suggesting that TPE-ND has
no MFC activity. These observations are consistent with the assumption that while
boron has immensely enhanced the rigid molecular structure since it can result in the
increase of molecular distortion degree and ICT feature, which endows TPE-NDB
with obvious MFC behavior, in contrast, the ligand TPE-ND has no MFC property.
These phenomena are accord with the observed optical property and DFT result.
The luminescent decay profiles of TPE-NDB in solid state were carried out (Fig.
S5) and the corresponding data were illustrated in Tab. S2. The lifetime of TPE-NDB
and TPE-ND were 4.53 ns and 2.10 ns, respectively. The excited-state decay of as-
prepared TPE-NDB fitted one exponential function. The data of ground sample TPE-
NDB fitted a double-exponential decay, which revealed the mixture of two
distinguished emission states in the amorphous phase.
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Fig. 5. Photos of the luminescence writing/erasing process of TPE-NDB on filter
papers under UV light (365 nm): (a) fluorescence emission of as-synthesized powder;
(b) mechanochromic fluorescence of the letter of “A” was written with a spatula; (c)
the paper was erased by vapor fuming (the letter “A” becoming invisible); (d)
rewritable mechanochromic fluorescence of the letter of “Y” generated with a spatula.
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Switchable mechanochromic dye of TPE-NDB with a large shift of 97 nm
prompts us to evaluate it as a kind of smart material with numerous potential
applications. An example of such applications is demonstrated in Fig. 5, after being
simply pressed by streaking a metal spatula on a piece filter paper with sprayed as-
synthesized powder, an orange letter appeared on the bright green background due to
the amorphization of TPE-NDB in the written “A” area under UV light illumination
(Fig. 5b). Interestingly, after vapor fuming the letter “A” can be merged in the
background because of the crystallization of TPE-NDB in area of “A” (Fig. 5c), and a
clear orange letter “Y” can be written again (Fig. 5d). Such writing and erasing
process can be repeated many times through repeating writing and fuming processes.
On the basis of its excellent MFC properties, TPE-NDB may be utilized as optical
recording materials.
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Fig. 6. XRD patterns of TPE-NDB in different solid-states: as-prepared, grinding and
fuming.
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With the aim of getting insight into the mechano-induced emission color changes,
powder X-ray diffraction (XRD) was used to study the synthesized TPE-NDB in
different solid states. As shown in Figure 6, many sharp and intense reflection peaks
were observed in the diffraction pattern of the untreated sample, indicating that the as-
prepared TPE-NDB was well-ordered arrangement crystalline structure. In sharp
contrast, all of the diffraction peaks displayed diffuse and depressed reflections after
grinding, verifying that the ground sample was amorphous. The transformation from
the crystalline structure into an amorphous state took place under an external force
and, thus, led to a change in the emitting color from bright green to orange. When
fumed with DCM, sharp reflection peaks resemble to those of the as prepared powder
emerge out, suggesting the ground sample can be readily converted back into an
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ordered crystalline lattice. In addition, the H NMR spectrum of TPE-NDB after
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grinding treatment was shown in Fig. S13, the result is similar with that the as-
prepared sample obtained (Fig. S9), implying that TPE-NDB converts the emission
color without changing its chemical structure during grinding process. Accordingly to
these results, the mechanochromism of TPE-NDB should be attributed to the
crystalline-amorphous phase transformations, which greatly influences photophysical
properties.
4. Conclusions
In summary, an efficient strategy to design novel tetraphenylethene-based Schiff base
ligand TPE-ND and the corresponding boron complex TPE-NDB were developed.
The ligand TPE-ND showed a typical ICT characteristic in addition to an AIE
behavior. The boron complex TPE-NDB resulted in enhanced and red-shifted
emission, more dramatic solvatochromism, bright AIE phenomenon, and reversible
high contrast MFC behavior due to the inhibition of C=N isomerization in ligands.
Upon grinding the as-prepared sample of TPE-NDB, the emission color changed
from bright green to orange, accompanied with the remarkable spectral shift from 498
to 595 nm. It should be mentioned that such a large red-shift of 97 nm during MFC
process has been rarely reported for organic compounds. Moreover, the ground
powder of TPE-NDB that emitted orange could be switched into its as-prepared state
emitting bright green light by fuming with DCM for 2 min. The results of this work
will facilitate the rational design of new MFC dyes with high−contrast performance
and the exploration of their potential applications in high-tech fields.
Conflict of Interest
The authors declare no conflict of interest.
Acknowledgements
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We are grateful to the Program for Innovative Research Team of Science and
Technology in the University of Henan Province (18IRTSTHN004), and Anyang
Normal University Program (AYNU-2017-B16, AYNUKP-2018-B20) for financial
support of this research.
References
[1] Chen Q, Wang JX, Yang F, Zhou D, Bian N, Zhang XJ, Yan CG, Han BH.
Tetraphenylethylene-based fluorescent porous organic polymers: preparation, gas
sorption properties and photoluminescence properties. J Mater Chem 2011; 21:
13554−60.
[2] Mei J, Hong YN, Lam JWY, Qin AJ, Tang YH, Tang BZ. Aggregation-induced
emission: the whole is more brilliant than the parts. Adv Mater 2014; 26: 5429−79.
[3] (a) Liu B, Dan T, Bazan GC. Collective response from a cationic tetrahedral
fluorene for label-free DNA detection. Adv Funct Mater 2007; 17: 2432−8;
(b) Toal SJ, Jones KA, Magde D, Trogler WC. Luminescent silole nanoparticles as
chemoselective sensors for Cr (VI). J Am Chem Soc 2005; 127: 11661−5;
(c) Wu YT, Kuo MY, Chang YT, Shin CC, Wu TC, Tai CC, et al. Synthesis, structure,
and photophysical properties of highly substituted 8,8a-dihydrocyclopenta[a]indenes.
Angew Chem Int Ed 2008; 47: 9891−4.
[4] Sun H, Liu S, Lin W, Zhang KY, Lv W, Huang X, et al. Smart responsive
phosphorescent materials for data recording and security protection. Nat Commun
2014; 5: 4601/1–4601/9.
[5] Wang J, Mei J, Hu R, Sun JZ, Qin A, Tang BZ. Click synthesis, aggregation-
induced emission, E/Z isomerization, self-Organization, and multiple chromisms of
pure stereoisomers of a tetraphenylethene-cored luminogen. J Am Chem Soc 2012;
134: 9956−66
21

ACCEPTED MANUSCRIPT
417
418
419
420
421
422
[6] Luo J, Xie Z, Lam JWY, Cheng L, Chen H, Qiu C, et al. Aggregation-induced
emission of 1-methyl- 1, 2, 3, 4, 5-pentaphenylsilole. Chem Commun 2001; 18:
1740–1.
[7] Wang ZM, Nie H, Yu ZQ, Qin AJ, Zhao ZJ, Tang BZ. Multiple stimuli-responsive
and reversible fluorescence switches based on a diethylamino-functionalized
tetraphenylethene. J Mater Chem C 2015; 3: 9103–11.
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
[8] (a) Tong H, Dong YQ, Hong YN, Haussler M, Lam JWY, Sung HHY et al.
Aggregation-induced emission:ꢀ effects of molecular structure, solid-state
conformation, and morphological packing arrangement on light-emitting behaviors of
diphenyldibenzofulvene derivatives. J Phys. Chem C 2007; 111: 2287–94;
(b) Gu XG,Yao JJ, Zhang GX, Yan YL, Zhang C, Peng Q, et al. Adv Funct Mater
2012; 22: 4862-72;
(c) He ZK, Zhang LQ, Mei J, Zhang T, Lam JWY, Shuai ZG, et al. Polymorphism-
dependent and switchable emission of butterfly-like bis(diarylmethylene)-
dihydroanthracenes. Chem Mater 2015; 27: 6601−7.
[9] (a) Zhang X, Chi Z, Li H, Xu B, Li X, Zhou W, et al. Piezoflfluorochromism of an
aggregation-induced emission compound derived from tetraphenylethylene. Chem
Asian J 2011; 6: 808−11;
(b) Dong YJ, Xu B, Zhang JB, Tan X, Wang LJ, Chen JL, et al. Piezochromic
luminescence based on the molecular aggregation of 9,10-Bis((E)-2-(pyrid-2-
yl)vinyl)anthracene. Angew Chem Int Ed 2012; 51: 10782−5;
(c) Sagara Y, Yamane S, Mutai T, Araki K, Kato T. A stimuli-responsive,
photoluminescent, anthracene-based liquid crystal: emission color determined by
thermal and mechanical processes. Adv Funct Mater 2009; 19: 1869−75.
22

ACCEPTED MANUSCRIPT
441
442
[10] Shi XY, Wang H, Han TY, Feng X, Tong B, Shi JB, et al. A highly sensitive,
single selective, real-time and “turn-on”fluorescent sensor for Al3+ detection in
443
444
aqueous media. J Mater Chem 2012; 22: 19296−302.
[11] Hu R, Leung NLC, Tang BZ. AIE macromolecules: syntheses, structures and
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
functionalities. Chem Soc Rev 2014; 43: 4494−562.
[12] (a) Luo X, Li J, Li C, Heng L, Dong Y,Liu Z, et al. Reversible switching of the
emission of diphenyldibenzofulvenes by thermal and mechanical stimuli. Adv Mater
2011; 23: 3261–5;
(b) Yuan WZ, Tan YQ, Gong YY, Lu P, Lam JWY, Shen XY, et al. Synergy between
twisted conformation and effective intermolecular interactions: strategy for efficient
mechanochromic luminogens with high contrast. Adv Mater 2013; 25: 2837–43.
[13] (a) Chi Z, Zhang X, Xu B, Zhou X, Ma C, Zhang Y, et al. Recent advances in
organic mechanofluorochromic materials. Chem Soc Rev 2012; 41: 3878–96;
(b) Zhang X, Chi Z, Zhang J, Li H, Xu B, Li X, et al. Piezoflfluorochromic properties
and mechanism of an aggregation-induced emission enhancement compound
containing N-hexyl-phenothiazine and anthracene moieties. Phys Chem B 2011; 115:
7606–11;
(c) Li H, Chi Z, Xu B, Zhang X, Li X, Liu S, et al. Aggregation-induced emission
enhancement compounds containing triphenylamine-anthrylenevinylene and
tetraphenylethene moieties. J Mater Chem 2011; 21: 3760–7;
(d) Lu Q, Li X, Li J, Yang Z, Xu B, Chi Z, et al. Influence of cyano groups on the
properties of piezofluorochromic aggregation-induced emission enhancement
compounds derived from tetraphenylvinyl-capped ethane. J Mater Chem C 2015; 3:
1225–34;
23

ACCEPTED MANUSCRIPT
465
466
467
(e) Xu B, Chi Z, Zhang J, Zhang Z, Li H, Li X, et al. Piezofluorochromic and
aggregation-induced-emission compounds containing triphenylethylene and
tetraphenylethylene moieties. Chem Asian J 2011; 6: 1470–8.
468
469
470
471
472
473
474
475
476
477
478
479
480
[14] (a) Liu W, Wang Y, Sun M, Zhang D, Zheng M, Yang W. Alkoxy-position
effects on piezofluorochromism and aggregation-induced emission of 9,10-
bis(alkoxystyryl)anthracenes. Chem Commun 2013; 49: 6042–4;
(b) Zheng M, Zhang D, Sun M, Li Y, Liu T, Xue S, et al. Cruciform 9,10-distyryl-2,6-
bis(p-dialkylamino-styryl)anthracene homologues exhibiting alkyl length-tunable
piezochromic luminescence and heat-recovery temperature of ground states. J Mater
Chem C 2014; 2: 1913–20;
(c) Bu L, Sun M, Zhang D, Liu W, Wang Y, Zheng M, et al. Solid-state fluorescence
properties and reversible piezochromic luminescence of aggregation-induced
emission-active 9,10-bis[(9,9-dialkylfluorene-2-yl)vinyl]anthracenes. J Mater Chem C
2013; 1: 2028–35;
(d) Wang Y, Liu W, Bu L, Li J, Zheng M, Zhang D, et al. Reversible piezochromic
luminescence of 9,10-bis[(N-alkylcarbazol-3-yl)vinyl]anthracenes and the
481
482
dependence on N-alkyl chain length. J Mater Chem C 2013; 1: 856–62.
[15] (a) Gao H, Xu D, WangY, Wang Y, Liu X, Han A, et al. Effects of alkyl chain
483
484
485
length on aggregation-induced emission, self-assembly and mechanofluorochromism
of tetraphenylethene modified multifunctional β-diketonate boron complexes. Dyes
Pigm 2018; 150: 59–66;
24

ACCEPTED MANUSCRIPT
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
(b) Zhou L, Xu D, Gao H, Han A, Liu X, Zhang C,et al. Triphenylamine
functionalized β -Ketoiminate boron complex exhibiting aggregation-induced
emission and mechanofluorochromism. Dyes Pigm 2017; 137: 200-7;
(c) Gao H, Xu D, Liu X, Han A, Zhou L, Zhang C, et al. Tetraphenylethene modified
β-ketoiminate boron complexes bearing aggregation-induced emission and
mechanofluorochromism. RSC Adv 2017; 7: 1348–56;
(d) Gao H, Xu D, Liu X, Han A, Zhou L, Zhang C, et al. Tetraphenylethene-based β-
diketonate boron complex: Efficient aggregation-induced emission and high contrast
mechanofluorochromism. Dyes Pigm 2017; 139: 157–65;
(e) Zhou L, Xu D, Gao H, Han A, Yang Y, Zhang C, et al. Effects of cyano groups on
the properties of thiazole-based b-ketoiminate boron complexes: aggregation-induced
emission and mechanofluorochromism. RSC Adv 2016; 6: 69560–8;
(f) Wang Y, Xu D, Gao H, Wang Y, Liu X, Han A, et al. Twisted Donor−Acceptor
Cruciform Luminophores Possessing Substituent-Dependent Properties of
Aggregation-Induced Emission and Mechanofluorochromism. J Phys Chem C 2018;
122: 2297−306.
[16] (a) Ooyama Y, Kagawa Y, Fukuoka H, Ito G, Harima Y.
Mechanofluorochromism of a series of benzofuro [2, 3-c] oxazolo [4, 5-a] carbazole-
type fluorescent dyes. Eur J Org Chem 2009; 5321–6;
(b) Ooyama Y, Harima Y. Molecular design of mechanofluorochromic dyes and their
solid-state fluorescence properties. J Mater Chem 2011; 21: 8372–80.
[17] (a) Lim SJ, An BK, Jung SD, Chung MA, Park SY. Photoswitchable organic
nanoparticles and a polymer film employing multifunctional molecules with enhanced
fluorescence emission and bistable photochromism. Angew Chem Int Ed 2004; 43:
6346−50;
25

ACCEPTED MANUSCRIPT
511
512
513
514
515
516
517
518
519
520
521
522
523
(b) Kishimura A, Yamashita T, Yamaguchi K, Aida T. Rewritable phosphorescent
paper by the control of competing kinetic and thermodynamic self-assembling events.
Nat Mater 2005; 4: 546−9.
[18] (a) Shen XY, Wang YJ, Zhao E, Yuan WZ, Liu Y, Lu P, et al. Effects of
substitution with donor−acceptor groups on the properties of tetraphenylethene trimer:
aggregation-induced emission, solvatochromism, and mechanochromism. J Phys
Chem C 2013; 117: 7334−47;
(b) Ma CP, Xu BJ, Xie GY, He JJ, Zhou X, Peng BY, et al. An AIE-active
luminophore with tunable and remarkable fluorescence switching based on the piezo
and protonation–deprotonation control. Chem Commun 2014; 50: 7374–7;
(c) Aldred MP, Zhang GF, Li C, Chen G, Chen T, Zhu MQ. Optical properties and
red to near infrared piezo-responsive fluorescence of a tetraphenylethene-
perylenebisimide-tetraphenylethene triad. J Mater Chem C 2013; 1: 6709–18.
524
525
526
527
528
529
530
531
[19] (a) Zhao J, Peng J, Chen P, Wang H, Xue P, Lu R. Mechanofluorochromism of
difluoroboron β-ketoiminate boron complexes functionalized with benzoxazole and
benzothiazole. Dyes Pigm 2018; 149, 276–83;
(b) Zhao Y, Lin H, Chen M, Yan D. Niflumic anion intercalated layered double
hydroxides with mechano-induced and solvent-responsive luminescence. Ind Eng
Chem Res 2014; 53: 3140−47;
(c) Li Y, Zhuang Z, Lin G, Wang Z, Shen P, Xiong Y, et al. A new blue AIEgen based
on tetraphenylethene with multiple potential applications in fluorine ion sensors,
26

ACCEPTED MANUSCRIPT
532
533
534
535
536
537
538
mechanochromism, and organic light-emitting diodes. New J Chem 2018; 42: 4089-
94;
(d) Chen M, Chen R, Shi Y, Wang J, Cheng Y, Li Y, et al. Malonitrile-functionalized
tetraphenylpyrazine: aggregation-induced emission, ratiometric detection of hydrogen
sulfide, and mechanochromism. Adv Funct Mater 2018; 28: 1704689;
(e) Lin C, Liu Y, Peng S, Shinmyozu T, Yang J. Excimer−monomer
photoluminescence mechanochromism and vapochromism of pentiptycene -
539
540
containing cyclometalated platinum(II) complexes. Inorg Chem 2017; 56: 4978-89.
[20] Das P, Linert W. Schiff base-Derived Homogeneous and Heterogeneous
541
542
543
544
545
546
547
548
Palladium Catalysts for the Suzuki−Miyaura Reaction. Coord Chem Rev 2016; 311:
1−23.
[21] Feng L, Shi W, Ma J, Chen Y, Kui F, Hui Y, et al. A novel thiosemicarbazone
Schiff base derivative with aggregation-induced emission enhancement characteristics
and its application in Hg2+ detection. Sensor Actuat B Chem 2016; 237: 563−9.
[22] Al-Zoubi W, Al-Hamdani AAS, Kaseem M. Synthesis and antioxidant activities
of schiff bases and their complexes: a review. Appl Organomet Chem 2016; 30:
810−7.
549
550
551
[23] (a) G Liu, Yang MD, Wang LK, Zheng J, Zhou HP, Wu JY et al. Schiff base
derivatives containing heterocycles with aggregation-induced emission and
recognition ability. J Mater Chem C 2014; 2: 2684−91;
27

ACCEPTED MANUSCRIPT
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
(b) Kathiravan A, Sundaravel K, Jaccob M, Dhinagaran G, Rameshkumar A, Ananth
DA, et al. Pyrene Schiff Base: Photophysics, Aggregation Induced Emission, and
Antimicrobial Properties. J Phys Chem B 2014; 118: 13573−81;
(c) Tang WX, Xiang Y, Tong AJ. Salicylaldehyde azines as fluorophores of
aggregation-induced emission enhancement characteristics. J Org Chem 2009; 74:
2163−6;
(d) Fang WY, Zhang GB, Chen J, Kong L, Yang LM, Bi H, et al. An AIE active probe
for specific sensing of Hg2+ based on linear conjugated bis-Schiff base. Sens
Actuators B 2016; 229: 338−46;
(e) Cheng JH, Li YX, Sun R, Liu J Y, Gou F, Zhou XG, et al. Functionalized salen
ligands linking with non-conjugated bridges: unique and colorful aggregation-induced
emission, mechanism, and applications. J Mater Chem C, 2015; 3: 11099−110.
[24] (a) Han T, Feng X, Chen D, Dong Y. A diethylaminophenol functionalized Schiff
base: crystallization-induced emission-enhancement, switchable fluorescence and
application for security printing and data storage. J Mater Chem C 2015; 3: 7446−54;
(b) Dong YQ, Lam JWY, Tang BZ. Mechanochromic luminescence of aggregation-
induced emission luminogens. J Phys Chem Lett 2015; 6: 3429−36.
28