Reversible solid-state mechanochromic luminescence originated from aggregation-induced enhanced emission-active Donor?Acceptor cruciform luminophores containing triphenylamine

Article abstract of DOI:10.1016/j.dyepig.2019.107689

Two D-A type cruciform luminophores FB-CTPAEB and DFB-CTPAEB were designed and prepared. The two compounds exhibited unique intramolecular charge-transfer (ICT) and solid-state fluorescence behavior. The results show that both compounds exhibit AIEE behavior and high solid-state luminescence efficiency (up to 0.693 and 0.442, respectively). Moreover, FB-CTPAEB and DFB-CTPAEB show remarkable mechanofluorochromic (MFC) properties. Once the as-prepared samples were subjected to grinding, the emission of the two cruciform luminophores was red shifted by 52 nm and 46 nm, correspondingly, their fluorescence colors changed from yellow and reddish orange to red, respectively. The fluorescence emission changes induced by grinding can be restored by solvent-fuming and regenerated by grinding. PXRD analysis revealed that the MFC behavior of FB-CTPAEB and DFB-CTPAEB came from the transformation between crystalline and amorphous states upon external stimuli. Meanwhile, the extension in molecular conjugation caused by planarization of molecular conformation and subsequent planar intramolecular charge transfer (PICT) process were responsible for the red-shifts in the PL spectra.

Full text of DOI:10.1016/j.dyepig.2019.107689

Accepted Manuscript
Reversible solid-state mechanochromic luminescence originated from aggregation-
induced enhanced emission-active Donor−Acceptor cruciform luminophores
containing triphenylamine
Ying Wang, Dandan Cheng, Hongke Zhou, Xingliang Liu, Yonghui Wang, Aixia Han,
Chao Zhang
PII:
S0143-7208(19)31306-3
DOI:
https://doi.org/10.1016/j.dyepig.2019.107689
Article Number: 107689
Reference:
DYPI 107689
To appear in: Dyes and Pigments
Received Date: 4 June 2019
Revised Date: 16 June 2019
Accepted Date: 1 July 2019
Please cite this article as: Wang Y, Cheng D, Zhou H, Liu X, Wang Y, Han A, Zhang C, Reversible solid-
state mechanochromic luminescence originated from aggregation-induced enhanced emission-active
Donor−Acceptor cruciform luminophores containing triphenylamine, Dyes and Pigments (2019), doi:
https://doi.org/10.1016/j.dyepig.2019.107689.
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Table of Contents
Reversible solid-state mechanochromic luminescence
originated
from
aggregation-induced
enhanced
emission-active Donor−Acceptor cruciform luminophores
containing triphenylamine
Ying Wang, Dandan Cheng, Hongke Zhou, Xingliang Liu,* Yonghui Wang, Aixia
Han, Chao Zhang

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Reversible solid-state mechanochromic luminescence
originated
from
aggregation-induced
enhanced
emission-active Donor−Acceptor cruciform luminophores
containing triphenylamine
Ying Wang,a Dandan Cheng,b Hongke Zhou,a Xingliang Liu,a* Yonghui Wang,a
Aixia Han,a Chao Zhanga
aSchool of Chemical Engineering, Qinghai University, Xining 810016, China
bSchool of Life Science, Wuchang University of Technology, Wuchang, Wuhan
430223, China
E-mail address: liuxl1219@163.com.
Abstract
Two D-A type cruciform luminophores FB-CTPAEB and DFB-CTPAEB were
designed and prepared. The two compounds exhibited unique intramolecular
charge-transfer (ICT) and solid-state fluorescence behavior. The results show that
both compounds exhibit AIEE behavior and high solid-state luminescence
efficiency (up to 0.693 and 0.442, respectively). Moreover, FB-CTPAEB and
DFB-CTPAEB show remarkable mechanofluorochromic (MFC) properties. Once
the as-prepared samples were subjected to grinding, the emission of the two
cruciform luminophores was red shifted by 52 nm and 46 nm, correspondingly, their
fluorescence colors changed from yellow and reddish orange to red, respectively.
The fluorescence emission changes induced by grinding can be restored by
solvent-fuming and regenerated by grinding. PXRD analysis revealed that the MFC
behavior of FB-CTPAEB and DFB-CTPAEB came from the transformation
between crystalline and amorphous states upon external stimuli. Meanwhile, the

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extension in molecular conjugation caused by planarization of molecular
conformation and subsequent planar intramolecular charge transfer (PICT) process
were responsible for the red-shifts in the PL spectra.
Keywords:
cruciform;
triphenylamine;
intramolecular
charge-transfer;
aggregation-induced enhanced emission; mechanofluorochromism
1. Introduction
Mechanofluorochromic (MFC) materials are a class of ‘‘smart’’ materials that
change their solid-state emission color in response to external force stimuli (such as
grinding, pressing, shearing, deformation, etc.) [1]. As the switchable emissions
could be easily obtained by destroying their relative loose structure arrangement
under the external stimulus instead of altering the chemical structures, these
materials have attracted wide attention due to their potential applications in optical
data storage devices [2], mechano-sensors [3], security inks [4], optoelectronic
devices [5] and other fields. To date, many MFC materials, such as
3,6-bis(aryl)-1,4-diketo-pyrrolo[3,4-c]pyrroles (DPPs) [6], tetraphenylethene [1f,7],
oligo(p-phenylene vinylene) [8], 9,10-divinylanthracene [9], triphenylacrylonitrile
[5c,7e,10] and organoboron complexes [11,12], have been developed. However,
MFC materials that exhibit high solid-state luminescence efficiency and an obvious
color contrast are still rare, partly because many organic luminescent dyes usually
suffer from aggregation caused quenching (ACQ) [13], which leads to poor solid
state emission, and then it is disadvantageous to the observation of MFC
phenomenon. In 2001, Tang et al. reported on an important class of landmark

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materials with anti-ACQ effects, namely aggregation-induced emission (AIE)
materials [14], which are nonluminescent in the dilute solutions, but become highly
emissive in the aggregate states. In the next year, Park's group reported on
aggregation-induced enhanced emission (AIEE) materials [15], and then in 2010,
this group published the cyano-distyrylbenzene derivative that exhibited both AIE
activity and MFC behavior [16], which made people realize that there was a certain
relationship between AIE and MFC properties of the luminophore. Thus, great
attention has been paid to the design and synthesis of new AIE- or AIEE-MFC
luminophores, and a number of AIE or AIEE fluorophores with MFC behavior have
been reported in Chi's laboratory and by other groups [7,17]. However, the
development of organic AIE- or AIEE-MFC materials that possess high contrast
mechanofluorochromism, excellent reversibility, and high solid-state fluorescence
efficiency is still strikingly desirable. In addition, MFC phenomena and mechanisms
are still debated, and there is still no clear general design strategy to design AIE- or
AIEE-MFC materials at present. It is difficult to predict in advance whether a new
AIE fluorescent chromophore has MFC performance. Therefore, it is important to
develop new MFC dyes for an in-depth understanding of the underlying mechanism
of AIE- or AIEE-MFC behavior at the molecular level and establish the
structure–property relationships of these compounds.
As an important kind of fluorescent chromophores, cruciform fluorophores have
gained an increasing interest because of their unique molecular structures and
fascinating optical properties [18]. Recently, some cruciform fluorophores

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exhibiting both obvious AIE character and high contrast MFC properties have been
reported by our group [19]. In the current work, we have designed and synthesized
another structural type of cruciform molecules FB-CTPAEB and DFB-CTPAEB
(Scheme 1). In these two molecules, two different arms with different functions
intersected at a central benzene core: in the middle of one arm is
electron-withdrawing dicyanovinylbenzene segment, and at both ends of this arm is
electron-donating triphenylamine unit. Thus, the spatially separated frontier
molecular orbitals (FMOs) in species could be realized. Meanwhile, in the other arm
there are two benzene rings, on which the substituents can effectively regulate the
electronic structure of the whole molecule. On the other hand, the space congestion
caused by four large substituents on the central benzene ring forces them distort
from the ring plane, which would depress the molecular close π–π packing and
enhance solid luminescence efficiency. As expected, FB-CTPAEB and
DFB-CTPAEB possess significant AIEE characteristics with high solid state
efficiency of 0.693 and 0.442, respectively. The crystalline to amorphous phase
transition of the two luminophores by grinding resulted in excellent MFC
performance and well distinguishable color contrast with emission peak shifts of 52
nm and 46 nm was obtained, respectively.
Scheme 1 The molecular structures of FB-CTPAEB and DFB-CTPAEB.

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2. Experimental section
2.1. Materials and Measurements
1
General. The H (400 MHz) and 13C NMR (100 MHz) spectra were obtained
with a Bruker-Avance III spectrometer with CDCl3 as the solvent and
tetramethylsilane (TMS) as the internal standard. The high-resolution mass
spectrometry (HRMS) was carried out with a MALDI-TOF MS Performance
instrument (Shimadzu, Japan). C, H, and N elemental analyses were performed with
a Perkin-Elmer 240C elemental analyzer. The UV-vis absorption spectrum was
recorded on a Shimadzu UV-2550 spectrophotometer. Fluorescent measurements
were obtained on a Cary Eclipse Fluorescence Spectrophotometer. The absolute
fluorescence quantum yields for the as-prepared solids were measured on an
Edinburgh FLS920 steady state spectrometer using an integrating sphere. Powder
X-ray diffraction (XRD) was performed on a Bruker D8 Focus Powder X-ray
diffraction instrument. Dynamic light scattering (DLS) measurements were
performed on the BI-200SM Laser Light Scattering System (Brookhaven). Density
functional theory (DFT) calculations were performed on FB-CTPAEB and
DFB-CTPAEB at the B3LYP/6-31G(d) level using the Gaussian 09W program
package.
Preparation of the samples for AIEE study. A stock solution of luminogen in
THF with a concentration of 1.0 × 10-3 M was prepared. An aliquot (0.1 mL) of the
stock solution was transferred to a 10 mL volumetric flask. After the appropriate
amount of THF was added, distilled water was added slowly under sonication to

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give 1.0 × 10-5 M solution with different fractions of water. The PL measurement of
the resultant solutions was performed immediately.
Preparation of the samples for mechanofluorochromism study. The ground
powders were prepared by grinding the initial powders with a pestle and mortar.
The fumed samples were obtained by fuming the ground powders with DCM for 2
min.
Materials. Ethanol was distilled under normal pressure over sodium under
nitrogen before use. The other chemicals were used as received without further
purification.
2.2. Synthesis
4,4''-bis(trifluoromethyl)-[1,1':4',1''-terphenyl]-2',5'-dicarbaldehyde (3a)
2,5-Dibromoterephthalaldehyde 1 (4.00 g, 13.70 mmol), 2a (7.30 g, 38.44
mmol), Na2CO3 (8.00 g, 75.48 mmol) and Pd(PPh3)4 (150 mg, 0.130 mmol) were
added into toluene/H2O (300 mL, v/v = 3/2). Then the mixture was heated to reflux
and stirred for 48 h under a nitrogen atmosphere. After cooling to room temperature,
the organic layer was separated, the aqueous layer extracted with DCM, the
combined organic layer dried over anhydrous Na2SO4. Upon evaporating off the
solvent, the crude product was subjected to purification by column chromatography
(silica gel; petroleum ether/DCM, v/v = 2/1), affording a light yellow solid (5.15 g).
1
Yield 89%. H NMR (400 MHz, CDCl3) δ 10.08 (s, 2H), 8.14 (s, 2H), 7.83 (d, J =
8.0 Hz, 4H), 7.60 (d, J = 7.6 Hz, 4H) (Fig. S5); 13C NMR (100 MHz, CDCl3) δ
190.49, 143.57, 139.91, 136.57, 130.64, 130.58, 130.30, 125.84, 125.81 (Fig. S6);

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HRMS (MALDI-TOF) m/z: [M]+ Calcd for C22H12F6O2 422.0741; Found 422.3416
(Fig. S7). Anal. Calcd (%) for C22H12F6O2: C 62.57, H 2.86; Found: C 62.63, H
2.78.
3,3'',5,5''-tetrakis(trifluoromethyl)-[1,1':4',1''-terphenyl]-2',5'-dicarbaldehyde
(3b)
By following the synthetic procedure for 3a, 3b was synthesized by using
2,5-Dibromoterephthalaldehyde 1 (4.00 g, 13.70 mmol), 2b (9.91 g, 38.44 mmol),
Na2CO3 (8.00 g, 75.48 mmol) and Pd(PPh3)4 (150 mg, 0.130 mmol) as the reagents.
The crude product was purified by column chromatography (silica gel,
1
DCM/petroleum ether, v/v = 1/1), affording a yellow solid (6.36 g). Yield 83%. H
NMR (400 MHz, CDCl3) δ 10.08 (s, 2H), 8.16 (s, 2H), 8.08 (s, 2H), 7.92 (s, 4H)
13
(Fig. S8); C NMR (100 MHz, CDCl3) δ 189.27, 142.30, 138.35, 136.58, 132.74,
132.41, 131.73, 129.73, 129.70, 124.23, 122.98, 121.50 (Fig. S9); HRMS
(MALDI-TOF) m/z: [M]+ Calcd for C24H10F12O2 558.3158; Found 558.3179 (Fig.
S10). Anal. Calcd (%) for C24H10F12O2: C 51.63, H 1.81; Found: C 51.70, H 1.89.
(2Z,2'Z)-3,3'-(4,4''-bis(trifluoromethyl)-[1,1':4',1''-terphenyl]-2',5'-diyl)bis(2-(4
-(diphenylamino)phenyl)acrylonitrile) (FB-CTPAEB)
Compound 3a (0.74 g, 1.75 mmol) and 2-(4-(diphenylamino)phenyl)acetonitrile
(1.25 g, 4.40 mmol) were added into dry ethanol (50 mL), and then CH3ONa (0.25 g,
4.63 mmol) was added quickly. The mixture was refluxed with stirring for 6 h under
a nitrogen atmosphere. After cooling to room temperature, the resulting precipitate
was collected by filtration, and dried under vacuum. The crude product was purified

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by column chromatography (silica gel, DCM/petroleum ether, v/v = 1/1), affording
a bright orange red solid (1.46 g). Yield 87%. 1H NMR (400 MHz, CDCl3) δ 8.24 (s,
2H), 7.78 (d, J = 8.0 Hz, 4H), 7.67 (d, J = 8 Hz, 4H), 7.42 (d, J = 8.8 Hz, 4H),
13
7.36-7.30 (m, 10H), 7.15-7.10 (m, 12H), 7.06 (d, J = 8.8 Hz, 4H) (Fig. S11); C
NMR (100 MHz, CDCl3) δ 149.42, 146.80, 142.52, 140.38, 136.85, 133.67, 130.74,
130.57, 130.35, 130.24, 129.55, 126.98, 126.18, 125.69, 125.65, 125.40, 124.16,
122.70, 121.83, 117.92, 114.67 (Fig. S12); HRMS (MALDI-TOF) m/z: [M+H]+
Calcd for C62H41F6N4 955.3235; Found 955.1653 (Fig. S13 and S14). Anal. Calcd
(%) for C62H40F6N4: C 77.98, H 4.22, N 5.87; Found: C 77.91, H 4.18, N 5.94.
(2Z,2'Z)-3,3'-(3,3'',5,5''-tetrakis(trifluoromethyl)-[1,1':4',1''-terphenyl]-2',5'-di
yl)bis(2-(4-(diphenylamino)phenyl)acrylonitrile) (DFB-CTPAEB)
By following the synthetic procedure for FB-CTPAEB, DFB-CTPAEB was
synthesized
by
using
3b
(1.00
g,
1.79
mmol),
2-(4-(diphenylamino)phenyl)acetonitrile (1.27 g, 4.48 mmol) and CH3ONa (0.26 g，
4.81 mmol) as the reagents. The crude product was purified by column
chromatography (silica gel, DCM/petroleum ether, v/v = 3/2), affording a bright
1
orange red solid (1.68 g). Yield 86%. H NMR (400 MHz, CDCl3) δ 8.29 (s, 2H),
8.02 (s, 6H), 7.41 (d, J = 8.8Hz, 4H), 7.34-7.28 (m, 10H), 7.16-7.11 (m, 12H), 7.05
(d, J = 8.4Hz, 4H) (Fig. S15); 13C NMR (100 MHz, CDCl3) δ 149.70, 146.75,
140.79, 139.15, 135.14, 133.94, 132.77, 132.44, 132.10, 131.77, 130.75, 130.04,
129.54, 126.99, 125.79, 125.42, 124.42, 124.21, 122.21, 122.17, 122.14, 121.78,
121.71, 118.99, 117.60, 116.27 (Fig. S16); HRMS (MALDI-TOF) m/z: [M]+ Calcd

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for C64H38F12N4 1090.2905; Found 1090.5599 (Fig. S17 and S18). Anal. Calcd. (%)
for C64H38F12N4: C 70.46, H 3.51, N 5.14; Found: C 70.51, H 3.44, N 5.21.
3. Result and discussion
3.1. Synthesis of FB-CTPAEB and DFB-CTPAEB
The preparations of the cruciform luminophores FB-CTPAEB and
DFB-CTPAEB are summarized in Scheme 1. Suzuki-Miyaura coupling of
2,5-dibromoterephthalaldehyde 1 with the aromatic boric acid 2a or 2b was carried
out in the mixed solvent of toluene and H2O by using Pd(PPh3)4 as a catalyst in high
yields of 89% and 83%, respectively. Then a subsequent standard Knoevenagel
condensation catalyzed by sodium methoxide produced the target compounds
FB-CTPAEB and DFB-CTPAEB with good yields of 87% and 86%, by treating
the intermediate compounds 3a or 3b with 2-(4-(diphenylamino)phenyl)acetonitrile,
respectively. Both FB-CTPAEB and DFB-CTPAEB have good solubility in
organic solvents such as chloroform, DCM, THF, benzene and toluene, but show
poor solubility in alcohols and aliphatic hydrocarbon solvents, such as cyclohexane,
n-hexane, methanol and ethanol. The structures of all the intermediates and target
molecules were fully characterized by different spectroscopic methods, including 1H
and 13C NMR spectroscopy and MALDI-TOF high resolution mass spectrometry, as
well as C, H, N elemental analysis.

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Scheme 2 Synthetic routes of FB-CTPAEB and DFB-CTPAEB.
3.2. UV-vis absorption and fluorescent emission spectra in solutions
Due to the existence of typical electron-donating triphenylamine and
electron-accepting cyano groups, FB-CTPAEB and DFB-CTPAEB become typical
D-A molecules, and it offers ready ICT process in these two molecules [18d,19a].
Since photophysical properties of the D−A conjugates in solutions are strongly
dependent on the solvent polarity, we thus examined the absorption and emission
properties of both compounds in varying solvents, and the corresponding
photophysical data were collected in Table S1 and S2. As shown in Fig. 1a, each
compounds displayed two intense absorption bands, the one that located at
approximately λ < 360 nm was attributed to the π–π* transition delocalized along
the π-electronic system, while the other absorption band ranging from 380 nm to
550 nm with a high molar extinction coefficient (104 M-1 cm-1) was assigned to the
intramolecular charge transfer (ICT) transition from electron rich triphenylamine
units to the dicyanovinylbenzene segments, it can be confirmed by the
solvent-dependent absorption spectra, which blue-shifted slightly with increasing
solvent polarity. For example, the ICT bands of FB-CTPAEB and DFB-CTPAEB

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emerged at 450 nm and 461 nm in cyclohexane, and blue-shifted to 435 nm and 436
nm in DMF, respectively (Table S1 and S2). The above phenomenon might be due
to the less conjugation degree of the compounds resulted from a larger dihedral
angle in more polar solvents [11d, 20].
The emission spectra of FB-CTPAEB and DFB-CTPAEB in different solvents
showed obvious positive solvatochromicity properties (Fig. 1c and d, Table S1 and
S2). In nonpolar cyclohexane, the two compounds could emit green and yellow
lights located at 540 nm and 556 nm, respectively, and the emission bands gave the
vibrational structures, which is the LE (locally excited) ones [21]. With the
increasing solvent polarity, the emission bands of FB-CTPAEB and
DFB-CTPAEB red-shifted significantly. For example, they emitted both red lights
centered at 624 nm and 642 nm in the moderately polar THF, respectively, and in
highly polar DMF, the lights that the two compounds emitted were extremely weak
with the emission wavelengths of 658 nm and 667 nm. The above results are
consistent with the strong ICT character of the solvent relaxed emissive state. The
solvatochromic behaviors of FB-CTPAEB and DFB-CTPAEB can be further
quantitatively described by the Lippert−Mataga equation [22]. The Stokes shifts (∆ν)
of the two compounds in varying solvents were calculated and listed in Table S1
and S2. From the plots of ∆υ – ∆f (Fig. S1), we could find that the slopes of the
fitting line for FB-CTPAEB and DFB-CTPAEB were as high as 12546 and 14772,
exhibiting significant solvatochromism effect. In addition, in the same solvent, the
maximum ICT absorption and emission wavelengths of FB-CTPAEB is smaller

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than that of DFB-CTPAEB, indicating the ICT degrees of the two compounds in
the excited state increased in a sequence of FB-CTPAEB < DFB-CTPAEB [19,23].
The fluorescence quantum yields (Φf) of FB-CTPAEB and DFB-CTPAEB in
different solvents are also determined by using quinine sulfate (Φf = 0.546, 0.5 mol
L-1 H2SO4) as the standard (Table S1 and S2). The Φf of the two compounds in
solution decreases from 0.328 and 0.270 (cyclohexane) to 0.131 and 0.104
(chloroform) and less than 0.001 (DMF), respectively, which is consistent with a
positive solvatokinetic effect [11d]. FB-CTPAEB and DFB-CTPAEB show weak
fluorescence in highly polar solvents due to the fast interconversion from the
emissive LE state to the low emissive ICT state; however, they exhibit much
stronger emission in nonpolar solvents due to restricted ICT transitions.
Fig. 1 Normalized UV-vis absorption spectra of FB-CTPAEB (a) and
DFB-CTPAEB (b), and normalized PL spectra of FB-CTPAEB (c, λex = 450 nm)
and DFB-CTPAEB (d, λex = 465 nm) in different solvents (1.0 × 10-5 mol L-1).

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To reveal the influence of the geometric and electronic structures of
FB-CTPAEB and DFB-CTPAEB on their photophysical properties, we performed
density functional theory (DFT) calculations on the two cruciforms with the
Gaussian 09W program using the DFT/B3LYP/6-31G(d,p) method. Fig. 2 shows
the optimized molecular configuration and the electron distribution of the HOMOs
and LUMOs for FB-CTPAEB and DFB-CTPAEB. Obviously, the overall electron
density of the HOMOs of FB-CTPAEB and DFB-CTPAEB is mainly localized on
the electron-donating triphenylamine frameworks, whereas the overall electron
density of LOMOs of the two compounds is distributed on the electron-accepting
dicyanovinylbenzene fragments. Generally, such electron distribution imparts the
dye molecules with an intrinsic ICT property, which is consistent with the measured
photophysical data. Furthermore, the above theoretical calculation results illustrate
that FB-CTPAEB and DFB-CTPAEB adopt a twisted spatial conformation at their
optimized lowest energy states. For FB-CTPAEB, both the dihedral angles between
two vinyl planes and the central benzene are 30.2o. And both dihedral angles
between two side benzene rings and the central core are 45.3o. In the case of
DFB-CTPAEB, the corresponding dihedral angles are 30.8o and 46.7o, respectively.
The reason for such large twist angles is that the crowding in the central benzene
ring forces the surrounding substituents to distort from the ring plane. The specially
twisted conformation of the two cruciforms would lead to effective depression of
molecular close π–π packing and enhancement of emission quantum yields in the
solid state, which may endow the two compounds with AIE characteristics and

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MFC behavior.
Fig. 2 The calculated HOMO and LUMO density maps and optimized molecular
geometry of FB-CTPAEB and DFB-CTPAEB at the B3LYP/6-31G(d) level.
3.3. Aggregation-induced enhanced emission (AIEE)
Keeping in mind the reports and our design strategy that constructing molecular
framework with large twisted conformation may contribute to interesting AIEE
behavior, we explored the AIEE properties of FB-CTPAEB and DFB-CTPAEB by
measuring their absorption and PL spectra in dilute mixtures of water–THF with
different water fractions (fw, the volume percentage of water in THF–water
mixtures). The absorption spectra of FB-CTPAEB and DFB-CTPAEB in the
THF/water mixtures are depicted in Fig. 3, which shows that the spectral profiles of
the two compounds are significantly changed when fw was above 50%, the whole

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absorption spectra start to increase, indicating the formation of nano-aggregates.
The aggregated particles obtained are characterized by dynamic light scattering
(DLS) and shows the existence of nano-aggregates as their main constituent in the
solvent mixtures with high water contents (Fig. S2 and S3). The emergence of the
increase in absorbance and levelling-off tail in the visible region of the absorption
spectra originates from the light scattering, or Mie effect, of the nanoaggregate
suspensions in the solvent mixtures, which effectively decreased the light
transmission in the mixture [19a]. As shown in Fig. 4, FB-CTPAEB and
DFB-CTPAEB show switched emission in THF–water mixture solutions with fw,
which directly elucidates the AIEE process. In pure THF, the weak reddish orange
and red emission for the two compounds with λmax at 625 nm and 643 nm,
respectively, was observed. However, their Φf is only 0.062 and 0.027. When the
proper amount of water is added into THF solution, the PL intensities are gradually
quenched and the emission peaks show red-shifts. This is because the addition of
water significantly increases the polarity of the mixed solvent, which induces the
quenching of fluorescence and the red-shifts of the emission band of the two D-A
type compounds. Meanwhile, when fw is over 50%, the emission peaks of the two
luminogens are recovered and the PL intensities start to increase rapidly,
respectively, at which solvating powers of the mixtures are so poor that the
luminogen molecules tend to aggregate, inside which the encapsulated the two
luminogen molecules locate in a nonpolar environment and the ICT process is
limited, thus the fluorescence is recovered. When fw is increased to 95%, the

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emission of solutions further blue-shifts to 603 nm and 618 nm, whose intensities
increase by ca. 3.5-fold and 5.5-fold relative to those of in pure THF solutions,
respectively, the bright orange and reddish orange light emitting for FB-CTPAEB
and DFB-CTPAEB is observed for the aggregates. The changes in the above
fluorescence spectra could be attributed to the AIEE effect derived from the
formation of molecular aggregates when adding water to the solution. The AIEE
nature of FB-CTPAEB and DFB-CTPAEB can also be further confirmed by the
vivid contrast fluorescent images of the solvent mixtures from 0% to 95% (insets of
Fig. 4b and d). To quantitatively evaluate the AIEE effect of FB-CTPAEB and
DFB-CTPAEB, the fluorescence quantum yields of the luminogens in solution (Φf,s)
and as prepared solid (Φf,aps) states are determined. The Φf,aps values for
FB-CTPAEB and DFB-CTPAEB (0.693 and 0.442) were much higher than Φf,s
(0.062 and 0.027), further suggesting the AIEE behavior of the two compounds.
Thus, we could get the AIEE factors (αAIEE = Φf,aps/Φf,s) of FB-CTPAEB and
DFB-CTPAEB as 11.3 and 16.4, respectively.
Fig. 3 UV-vis absorption spectra of FB-CTPAEB (a) and DFB-CTPAEB (b) in
THF/water mixtures with different fw.

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Fig. 4 PL spectra of FB-CTPAEB ((a), λex = 450 nm) and DFB-CTPAEB ((c), λex
= 470 nm) in THF/water mixtures with different fw. Plot of I/I0 versus water fraction
of FB-CTPAEB (b) and DFB-CTPAEB (d), where I and I0 represent the PL
intensities in a THF/water mixture with a specific fw and in pure THF, respectively.
The insets of b and d depict the fluorescence images of FB-CTPAEB and
DFB-CTPAEB with various water fractions (from 0% to 90%, excitation
wavelength: 365 nm), respectively.
3.5. Mechanofluorochromic (MFC) properties
Generally, the fluorophores that possess the AIE feature, ICT characteristics, and
twisted spatial conformation may show mechanochromic luminescence [12,19].
Herein, FB-CTPAEB and DFB-CTPAEB are greatly anticipated to be
mechanochromic active. Thus, the mechanochromic properties of the two
compounds are studied. As shown in Fig. 5c and d, the as-prepared samples of
FB-CTPAEB and DFB-CTPAEB could emit intense yellow and reddish orange
light under UV irradiation, and both their emitting colors changed into red when

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they are ground in a mortar or on a quartz substrate by using a pestle, indicating
obvious mechanochromic behavior of the two fluorophores. PL spectra are used to
monitor such a color transformation under grinding stimuli. As dispicted in Fig. 5a
and b, the emission of the as-prepared FB-CTPAEB and DFB-CTPAEB samples
was located at 574 nm and 593 nm, and red-shifted to 626 nm and 639 nm in the
ground powders, respectively. It means that the grinding treatment induces spectral
red-shifts of 52 nm and 46 nm for the two compounds. An excellent
mechanochromic material usually exhibits good reversible changes in color and
emission. To check the reversibility of the mechanochromic phenomenon observed
for the powders of FB-CTPAEB and DFB-CTPAEB, we treat their ground
samples by fuming with DCM vapor. The red emitting ground powders of the two
fluorophores could be transferred into yellow and reddish orange emitting solids,
respectively, similar to the as-prepared solids, after fuming with DCM for 2 min.
Meanwhile, their corresponding maximum emission wavelengths return to 578 nm
and 598 nm, respectively. The obtained results indicate the good reversibility of the
mechanochromic fluorescence for FB-CTPAEB and DFB-CTPAEB. Moreover,
the grinding-fuming fluorochromic process could be repeated many times without
any fatigue, suggesting the excellent reversibility in the switching processes (Fig.
S4).

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Fig. 5 (a) Normalized fluorescent spectra of FB-CTPAEB (a) and DFB-CTPAEB
(b) in different solid-states: as-prepared, grinding and fuming (λex = 390 nm).
Photographs of FB-CTPAEB (c) and DFB-CTPAEB (d) in different solid states
under UV light (365 nm).
Generally, the emission behavior of solid emitters depended on the molecular
packings. Thus, to gain further insight into the MFC phenomenon, the powder X-ray
diffraction (PXRD) patterns for FB-CTPAEB and DFB-CTPAEB in different solid
states are measured. As shown in Fig. 6, it is clear that the as-prepared solids of
FB-CTPAEB and DFB-CTPAEB exhibit many sharp and intense diffraction peaks,
indicative of the crystalline forms. By sharp contrast, the corresponding ground
powders show very weak, broad, diffused or no diffraction peaks, indicating most of
the ordered structures are disrupted, and the disordered molecular packing or
amorphous samples are obtained. Moreover, when fumed with DCM vapor, sharp
diffractions, similar to those of the as-prepared solids, emerge again, because of the
transformation from an amorphous state back to a crystalline state. Therefore, the
mechanochromism of FB-CTPAEB and DFB-CTPAEB originates from the
transformations between the ordered crystalline and amorphous states. It is a general

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mechanism for many other mechanochromic compounds. Moreover, we believe that
the large red-shifts of FB-CTPAEB and DFB-CTPAEB after the external force
applied to the as-prepared samples might come from the conformational change and
rotate to a position more parallel to the coplanar under external pressure, leading to
extended conjugation and subsequent planar intramolecular charge transfer (PICT)
[18d], which can be confirmed by the UV-vis absorption spectra of as-prepared and
ground samples (Fig. 7). The as-prepared samples of FB-CTPAEB and
DFB-CTPAEB exhibit strong absorption peaks at 444 nm and 455 nm, after
grinding, they red-shift to 449 nm and 450 nm, indicating the extension of
π-conjugation after grinding [5c].
Fig. 6 PXRD patterns of FB-CTPAEB (a) and DFB-CTPAEB (b) in different
solid-states: as-prepared, grinding and fuming.
Fig. 7 Normalized UV-vis absorption spectra of as-prepared and ground samples of
FB-CTPAEB (a) and DFB-CTPAEB (b), respectively.

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4. Conclusion
In summary, we have demonstrated the molecular design, synthesis, and
properties of two twisted donor–acceptor cruciform luminophores FB-CTPAEB
and DFB-CTPAEB, and studied the impact of molecular structure on photophysical
properties from the perspective of electronic and steric effects. FB-CTPAEB and
DFB-CTPAEB possess highly distorted spatial conformation and significant ICT
process
from
electron-rich
triphenylamine
unit
to
electron-poor
dicyanovinylbenzene fragment. The two cruciform luminophores show typical
AIEE behavior with high solid state efficiency of 0.693 and 0.442, respectively.
Moreover, FB-CTPAEB and DFB-CTPAEB exhibit high contrast MFC properties
with good reversibility. Under external force stimuli, both yellow and reddish
orange emitting as-prepared powders of the two fluorophores could be transferred
into red emitting solids. The increase in molecular conjugation caused by
planarization of molecular conformation and subsequent PICT process lead to the
red-shifts in the PL spectra from 574 nm and 593 nm to 626 nm and 639 nm,
respectively. All these excellent properties make the compounds promising
stimuli-responsive, smart, luminescent materials for mechanical sensors, rewritable
record media, security protection and light-emitting device applications.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of
China (NNSFC, No. 21662028 and 21362027), the Qinghai Science & Technology
Department of China (Grant No. 2018-HZ-817), and Qinghai Province “High-end

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Innovative Thousand Talents Plan”.
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