Synthesis, aggregation-induced emission properties and mechanofluorochromic behavior of sulfur connected bis(tetraphenylethene) luminogens

Article abstract of DOI:10.1016/j.dyepig.2020.108978

Three novel bis(tetraphenylethene) (BTPE) derivatives 1–3 were efficiently synthesized. All the materials exhibited aggregation-induced emission characteristics, and the dynamic light scattering measurments proved that the average diameter of the oxidised analogues 2 and 3 was less than that of the thioether precursor 1 due to the oxidation of the linking S-atom. Linking two tetraphenylethylenes via a sulfur atom with different oxidation states offered compounds which showed reversible mechanofluorochromic behavior. The crystal-to-amorphous phase conversion process was responsible for the mechanofluorochromism. Most notably, sulfone-based BTPE 3 presented polymorphism due to different packing modes, and the three observable morphologies could be interconverted.

Full text of DOI:10.1016/j.dyepig.2020.108978

Journal Pre-proof
Synthesis, aggregation-induced emission properties and mechanofluorochromic
behavior of sulfur connected bis(tetraphenylethene) luminogens
Jiang Wu, Jun Yin, Sheng Chen, Hongyan Xiao, Bin Yan, Qin Yang
PII:
S0143-7208(20)31675-2
DOI:
https://doi.org/10.1016/j.dyepig.2020.108978
DYPI 108978
Reference:
To appear in: Dyes and Pigments
Received Date: 8 September 2020
Revised Date: 29 October 2020
Accepted Date: 29 October 2020
Please cite this article as: Wu J, Yin J, Chen S, Xiao H, Yan B, Yang Q, Synthesis, aggregation-induced
emission properties and mechanofluorochromic behavior of sulfur connected bis(tetraphenylethene)
luminogens, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.108978.
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Author statement
Jiang Wu: Conceptualization, Methodology, Investigation, Formal
analysis, Writing-original draft. Jun Yin: Conceptualization. Sheng Chen:
Visualizaiton, Formal analysis. Hongyan Xiao: Investigation, Validation,
Resources. Bin Yan: Supervision, Funding acquisition, Visualization. Qin
Yang: Conceptualization, Writing - review & editing, Funding
acquisition.

Synthesis,
mechanofluorochromic
bis(tetraphenylethene) luminogens
aggregation-induced
emission
of
properties
and
behavior
sulfur
connected
a
b,c
a
a
a,
a,
Jiang Wu , Jun Yin , Sheng Chen , Hongyan Xiao , Bin Yan *, Qin Yang *

Synthesis,
mechanofluorochromic
bis(tetraphenylethene) luminogens
aggregation-induced
emission
of
properties
and
behavior
sulfur
connected
a
b,c
a
a
a,
a,
Jiang Wu , Jun Yin , Sheng Chen , Hongyan Xiao , Bin Yan *, Qin Yang *
a College of Biomass Science and Engineering, Sichuan University, Chengdu
6
10065, China
b Key Laboratory of Pesticide and Chemical Biology, Ministry of Education,
College of Chemistry, Central China Normal University, Wuhan 430079, China
c State Key Laboratory of Chemical Oncogenomics, Key Laboratory of Chemical
Biology, the Graduate School at Shenzhen, Tsinghua University, Shenzhen,
Guangdong, 518055, P. R. China
*
Corresponding
authors
E-mails:
yanbinscu@126.com
(Bin
Yan),
yangqin@scu.edu.cn (Qin Yang)
ABSTRACT
Three novel bis(tetraphenylethene) (BTPE) derivatives 1-3 were efficiently
synthesized. All the materials exhibited aggregation-induced emission characteristics,
and the dynamic light scattering measurments proved that the average diameter of
the oxidised analogues 2 and 3 was less than that of the thioether precursor 1 due to
the oxidation of the linking S-atom. Linking two tetraphenylethylenes via a sulfur
atom with different oxidation states offered compounds which showed reversible
mechanofluorochromic behavior. The crystal-to-amorphous phase conversion
process was responsible for the mechanofluorochromism. Most notably,
1

sulfone-based BTPE 3 presented polymorphism due to different packing modes, and
the three observable morphologies could be interconverted.
Keywords: Different
oxidation sulfur; Aggregation-induced
emission;
Mechanofluorochromism; Tetraphenylethene; Polymorphism
1
. Introduction
Mechanofluorochromic (MFC) materials, exhibiting distinct fluorescence
changes in the solid-state in response to external force stimuli (grinding, crushing or
rubbing) and returning to the original fluorescence and packing modes by applying
other stimuli (such as heating or solvent fuming), have attracted considerable
attention due to their potential applications in data storage, security inks, mechanical
sensors and optoelectronic devices[1-5]. Generally speaking, MFC materials should
emit solid-state fluorescence. However, conventional organic fluorescent materials
invariably suffer from aggregation caused quenching (ACQ), emitting very weak or
almost no fluorescence in the solid-state. The phenomenon i.e. highly emissive
aggregates or solid-states, by Tang et al., in 2001 has obviated the ACQ problem[6].
In general the fluorescence properties of a material are influenced by various factors,
for
instance,
molecular
packing,
molecular
conformation
and
intramolecular/intermolecular interactions[7-13]. These alterations could be realized
by pressure[14]. At the same time, AIE molecules with loose packing arrangements
are sensitive to pressure stimulation[15]. So, the AIE-active luminogens could be
used as the promising candidates for MFC materials.
2

Up to now, the explanation for mechanofluorochromism is either the
morphology change from crystal state to amorphous state or to a different crystal
state. These signify that the MFC luminogens should have at least two different
stable solid-states and one of the two states could be transformed to another upon the
external pressure. The question is what structures have the resultant
mechanofluorochromic properties. To date, MFC materials based on AIE-active
molecules
for
example
tetraphenylethene
(TPE)
derivatives[16-19],
BODIPYs[20-23], cyanoethylenes[24, 25], and distyrylanthracenes [26], have been
successfully obtained. Obviously, there is still lack of design strategy, and the
development of new MFC luminogens has become a hot topic.
TPE derivatives, as one of the most classic AIE units, have been extensively
explored owing to their facile syntheses and flexible structural modifications[27-29].
In order to obtain materials with new properties based on traditional fluorophores,
many research groups modulate the TPE properties via different linkers connecting
two TPEs[30, 31]. Tangs’ group joined two TPE molecules through a carbazole
group to produce high-emission OLEDs devices[32]. Tang et al synthesized series of
BTPE derivatives through the linkage of luminescent TPE units via tailored alkyl
chains to improve the emission efficiency but retain emission color of TPE-based
AIE luminogens[33]. Wang et al synthesized a new DMCS-TPE fluorescence dye
with twisted donor–acceptor cruciform, which was constructed by two π-conjugated
electron-rich TPE and electron-poor cyanostyryl crossing a central benzene core. The
molecule exhibited unique ICT emission, obvious AIE properties, strong solid-state
3

fluorescence and reversible high contrast MFC nature[34]. At the same time, many
TPE derivatives exhibited polymorphism and MFC behavior [35-37]. In summary, a
slight difference in the chemical structures, such as the steric hindrance or the
electronic variations, can strongly affect the molecular conformations or packing
modes, then impel the activities alteration of the materials.
Recently, we found that through using different oxidation states of sulfur
moieties, the AIE behavior and MFC properties of TPE-based luminogens could be
tuned, especially, sulfone-containing TPE derivative displayed polymorphism
behavior [38]. Inspired by the work of Tang et al., and our previous reports on
sulfone-based fluorescent compounds, in this work, we aimed to know how the
oxidation states of sulfur moieties regulate the properties of symmetric BTPE
derivatives and if the sulfone-containing BTPE possess two or more solid-states with
different emission colors[39-44]. In this regard, we successfully synthesized different
bis(tetraphenylethene) derivatives 1-3 which are linked by a sulfur atom in various
oxidation states (Scheme 1). The structures of compound 1-3 were characterized by
1
13
H NMR, C NMR and HRMS. All of the compounds exhibited AIE behavior and
reversible mechanofluorochromism. The sulfone containing BTPE 3 unexpectedly
presented polymorphism which was manifest by the appearance of green and blue
emission in different packing modes.
Insert Scheme 1
2
. Materials and methods
4

2
.1. Experimental
General: Except that the reaction needed to be performed under a nitrogen
atmosphere, all other reactions were conducted under air. All the chemical reaction
materials were obtained as analytical-grade from commercial suppliers and used as
received without further purification. The solvents were dried and distilled prior to
use by literature methods. The nuclear magnetic resonance (NMR) spectra were
o
recorded at 25 C using CDCl on a Varian Mercury Plus 400 spectrometer at
3
1
13
working frequency 400 MHz for H NMR and 100 MHz for C NMR, respectively.
1
H NMR spectra were reported as followed: chemical shift in ppm (δ) referring to
δTMS = 0.00 ppm, integration, multiplicities (s=singlet, d=doublet, t=triplet,
1
3
m=multiplet), and coupling constant (Hz). C NMR chemical shift values were
recorded in ppm relative to CDCl at 77.16 ppm. The HRMS analysis was obtained
3
by ESI on a GC-TOF mass spectrometer. Analytical TLC plates, Sigma-Aldrich
silica gel 60F200, were viewed using UV light (254 nm). Column chromatography
purification was performed on SDZF silica gel 160. The UV-vis absorption spectra
were carried on a Hitachi U-3310 visible recording spectrophotometer. The
fluorescence spectra of solutions and solids were recorded using a Horiba
FluoroMax-4P. Absolute fluorescence quantum yields were measured on a Horiba
Jobin Yvon Nanolog FL3-2iHR spectrometer combined with a calibrated integrating
sphere. Dynamic light scattering (DLS) measurements were performed by
Malvern-ZEN3600. Scanning Electron Microscopy (SEM) images were taken on
JSM-5900LV. Powder X-ray diffraction patterns were recorded on a Rigaku Ultima
5

IV diffractometer (Rigaku,Japan) using a Cu Kα radiation (λ=1.54 Å, 40 kV, 40 mA).
The differential scanning calorimetry (DSC) trace was obtained on a PerkinElmer
o
-1
DSC8000 at a heating rate of 10 C∙min under a nitrogen atmosphere. The
decomposition temperature was measured by thermogravimetric analysis (TGA)
under a nitrogen atmosphere using a DTG-60 thermogravimetric instrument with a
o
-1
o
heating rate of 10 C∙min from room temperature to 700 C. Cyclic voltammetry
(CV) was measured on a CHI-1040C electrochemical analyzer. The measurements
were determined using a conventional three-electrode configuration consisting of a
+
glassy carbon working electrode, a platinum-disk auxiliary electrode and an Ag/Ag
reference electrode. And the measurements were conducted in a 1.0 mM solution in
dimethyl sulfoxide (DMSO) containing 0.1 mol/L tetrabutylammonium perchlorate
as the supporting electrolyte under a nitrogen atmosphere. The scan rate was 100
mV/s. The ground-state geometrical structures of the three luminogens were
optimized based on density functional theory (DFT) at the B3LYP/6-31G(d) level
with Gaussian 09 program. All the calculations in this work were conducted in the
water and the solvent effects were treated with SMD model.
2
.2. Synthesis
Synthesis of bis(4-(1,2,2-triphenylvinyl)phenyl) sulfane (1): To a 100 mL round
bottom flask, diphenyl sulfide (2 g, 10.74 mmol), NBS (5.8 g, 32.6 mmol), MeCN
o
(
40 mL) and TFA (0.1 mL) were successively added. After stirring 5 h at 50 C, the
reaction mixture was poured into 5% sodium sulfite solution (20 mL), and extracted
6

with DCM (3×40 mL) and the combined organic phase was washed with saturated
aqueous NaCl (3×50 mL), then dried over Na SO and concentrated under a vacuum.
2
4
The resultant residue was purified by chromatography (eluent: petroleum ether (60–
o
9
0 C)) providing bis(4-bromopheny)s sulfane I (2.77 g, yield: 75%). Compound I (5
g, 14.5 mmol) was immediately taken in dry THF (60 mL) and was added into a 250
mL three-necked flask under a nitrogen atmosphere. n-BuLi (29 mL, 2.5 M in
o
hexane, 72.6 mmol) was added dropwise to the solution at -78 C. After stirring for 1
h, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (20.7 mL, 101.5 mmol) was
added. After 1 h, the reaction was raised to room temperature. The suspension was
stirred for 18 h, then poured into saturated aqueous NH Cl (50 mL), extracted with
4
DCM (3×50 mL) and washed with saturated aqueous NaCl (3×50 mL). The
combined organic phase was dried over Na SO and concentrated under reduced
2
4
pressure and then purified by chromatography (eluent: petroleum ether (60–90
o
C)/DCM
=
5:1,
v/v),
providing
bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl) sulfane II (4.33 g, yield:
8%) which was used directly in the next step. Under a nitrogen atmosphere, a
mixture of compound II (2 g, 4.56 mmol), (2-bromoethene-1,1,2-triyl)tribenzene
6
(
6.12 g, 18.26 mmol) and Pd(PPh ) (421 mg, 0.36 mmol), and K CO solution (2 M,
3 4 2 3
4
0 mL) in toluene (100 mL) was stirred at 100 °C for 24 h until the starting materials
disappeared. After cooling to room temperature, the reaction mixture was extracted
with DCM (3×50 mL) and washed with saturated NaCl (3×50 mL). The organic
phase was dried over Na SO and concentrated under a vacuum, and then purified by
2
4
7

o
chromatography (eluent: petroleum ether (60–90 C)/DCM = 3:1, v/v) providing 1
o
1
(
1.78 g, yield: 56%, faint yellow solid, m.p.: 270 C). H NMR (400 MHz, CDCl ,
3
o
13
2
5 C) δ 7.11–7.08(m, 18 H), 7.04–6.98 (m, 16 H), 6.94–6.92 (m, 4 H). C NMR
o
(
100 MHz, CDCl , 25 C) δ 143.67, 143.65, 143.5, 142.7, 141.5, 140.3, 133.6, 132.2,
3
+
1
31.5, 131.4, 130.1, 127.9, 127.8, 127.79, 127.72, 127.7. HRMS (ESI ): calcd for
+
C H S [M + H] 695.2767; found 695.2772.
52
38
o
Synthesis of bis(4-(1,2,2-triphenylvinyl)phenyl)sulfoxide (2): At 0 C, m-CPBA
(85%, 203 mg, 1 mmol) was added into a solution of 1 (1 g, 1.4 mmol) in DCM (50
mL). Stirring for 1 h, the reaction mixture was quenched by saturated NaHCO (50
3
mL), then extracted with DCM (3×50 mL). The combined organic layers were dried
over Na SO and concentrated under a vacuum. Then the residue was purified by
2
4
o
chromatography (eluent: petroleum ether (60–90 C)/DCM = 2:1, v/v) providing 2
o
1
o
(
0.77 g, yield: 75%, white solid, m.p.: 238 C). H NMR (400 MHz, CDCl , 25 C) δ
3
7
.21-7.18 (m, 5 H), 7.06–7.01 (m, 15 H), 6.99–6.92 (m, 14 H), 6.88 (dd, J = 7.8, 1.5
1
3
o
Hz, 4 H). C NMR (100 MHz, CDCl , 25 C) δ 147.1, 143.2, 143.1, 143.06, 143.02,
3
1
42.75, 139.7, 132.2, 131.38, 131.33, 128.0, 127.9, 127.85, 127.0, 126.9, 124.7.
+
+
HRMS (ESI ): calcd for C H SO [M + H] 711.2716; found: 711.2722.
52
38
Synthesis of bis(4-(1,2,2-triphenylvinyl)phenyl)sulfone (3): m-CPBA (85%,
6
29.5 mg, 3.1 mmol) was mixed to the solution of 1 (1 g, 1.4 mmol) in DCM (50 mL)
o
at 0 C. After 4 h, the reaction mixture was poured into saturated NaHCO (50 mL)
3
and extracted with DCM (3×50 mL). The combined organic layers were dried over
Na SO and concentrated under a vacuum, then purified by chromatography (eluent:
2
4
8

o
petroleum ether (60–90 C)/DCM = 2:1, v/v) providing 3 (0.86 g, yield: 82%, white
o
1
o
solid, m.p.: 249 C). H NMR (400 MHz, CDCl , 25 C) δ 7.59-7.57 (m, 4 H), 7.16–
3
1
3
o
7
1
1
7
.06 (m, 22 H), 7.02–6.93 (m, 12 H). C NMR (100 MHz, CDCl , 25 C) δ 149.3,
3
43.5, 143.01, 142.82, 142.81, 139.2, 139.1, 132.1, 131.4, 131.3, 128.11, 128.07,
+
+
27.9, 127.3, 127.12, 127.09, 127.07. HRMS (ESI ): calcd for C H SO [M + H]
5
2
38
2
27.2665; found: 727.2671.
3
. Results and discussion
3
.1. Synthesis, characterization, and thermal properties of bis(tetraphenylethene)
derivatives 1 – 3
The three desired products were synthesized as shown in Scheme 1. Suzuki
coupling between bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl) sulfur
and (2-bromoethene-1,1,2-triyl)tribenzene gave sulfur-based bis(tetraphenylethene) 1
in an accepted yield (56%). Modulating the amount of the m-CPBA, sulfoxide-TPE 2
and sulfone-TPE 3 were successfully prepared in 75% and 82% yields, respectively.
1
13
All the compounds were characterized by H NMR, C NMR and HRMS (Fig.
S1-S6, ESI†).
The thermal behaviors of compounds 1-3 have been investigated by
thermogravimetric analysis (TGA) under nitrogen conditions. All the luminogens
o
exhibited the high decomposition temperatures (T ) of 360-421 C for 10% weight
d
loss (Fig. S7, ESI†). These results indicated that compounds 1-3 were thermally
stable.
9

3
.2. AIE characteristics
To explore the optical behaviors of the luminogens 1-3, the UV-vis absorption
spectra of materials in the THF/H O mixtures with different water fractions (f ) were
2
w
firstly recorded (Fig. S8, ESI†). In pure THF, the luminogens 1-3 exhibited the
similar bands peaking at around 325 nm, attributed to the π–π* transitions. The
changes in electronic nature from the donor (S) to acceptor (SO or SO) didn’t
2
influence the absorption of these luminogens. At the same time, as the f increased to
w
7
0% level-off tailing phenomenon due to the Mie scattering effect was observed in
the visible region and the maximum absorption peaks were red shifted, indicating the
formation of nano-aggregation particles. The nano-sized aggregates were proved by
dynamic light scattering (DLS) and scanning electron microscopy (SEM)
measurements. The average nanoparticle sizes of compounds 1-3 were 181 nm, 150
nm, and 150 nm in aqueous solution containing 5% (v/v) THF, respectively (Fig. 1).
And the aggregations of 1-3 were virtually confirmed in n-hexane (poor solvent)
containing 5% (v/v) DCM (good solvent). And the average size of particles 1 was 67
nm, which was larger than that of 2 (40 nm) and 3 (48 nm) (Fig. S9, ESI†). SEM
results were well certified the nano-level aggregates of the dyes in the THF/H O
2
mixture with f = 95% (Fig. 1 inset). Next, the emission spectra of luminogens 1-3
w
(
10 μM) in THF/water mixtures with different f at room temperature were displayed
w
(
Fig. 2). In pure THF solution (good solvent) or THF/H O mixtures with the f ≤70%,
2
w
due to the free rotation of the benzene rings, all compounds showed no obvious
fluorescence emission. The absolute fluorescence quantum yields (Φ ) of
F
1
0

luminogens 1-3 in pure THF solution were 0.14%, 0.13%, and 0.08%, respectively
Fig. S10, ESI†). Increasing the f to 80%, the emission of the compounds 1 and 2
(
w
just became a little noticeable with a maximum (λmax) at 487 nm for 1, 484 nm for 2,
respectively, and intensity of the emission bands remarkably increased, accompanied
by continuing addition H O to THF (Fig. 2). Only when f increased to 90%,
2
w
compound 3 exhibited a new strong green emission band at 489 nm, and the
fluorescence quantum yields all increased (Fig. S10, ESI†). There was no obvious
difference in AIE turn-on barrier for the materials, related to their weak solubility
induced by high molecular weight. Finally, the emission was enhanced by many-fold
with further increase of water content to 95%, approximately 2080-fold for 1
(
1370-fold for 2, 960-fold for 3) compared to pure THF, meaning that the oxidation
states of sulfur atom could regulate the AIE properties of the sulfur-based BTPE (Fig.
). These dramatic increase in luminescence indicated that more and more
2
nanoparticles were aggregated at the higher ratio of water in solvent. At the same
time, the fluorescence quantum yields of luminogens 1-3 were up to 19.02%,
1
0.56%, and 6.93%, respectively (Fig. S10, ESI†).
Insert Figure 1
Insert Figure 2
3
.3. Reversible mechanofluorochromic behavior of compounds 1–2
Subsequently, we further studied the solid-state fluorescence properties of
compounds 1 and 2. Quenching the melting state of 1 in liquid nitrogen, an
1
1

amorphous state emitting at 505 nm was prepared (Fig. S11, ESI†). Unfortunately,
the crystal of 1 could not be cultivated. We could only obtain the solid-state of 1 as
prepared. The fluorescence color of 1 was blue-green under UV irradiation at 365
nm in the solid-state, and the emission was blue-shifted to 463 nm (Φ = 19%) with
F
respect to amorphous solid (Fig. 3). Many AIE molecules exhibited
mechanochromism. Solid 1 was ground with a pestle to know if the emission of
S-BTPE 1 could be respond to the pressure stimulus. The ground samples exhibited a
new emission peak at 481 nm, being red-shifted by 18 nm compared to the pristine
powder. Furthermore, the green-emitting sample could be recovered back to its
original blue-green fluorescence (λem = 465 nm) using methanol vapor to stimulate
the ground sample. As a result, compound 1 showed obvious and reversible
mechanofluorochromic properties. Additionally, powder X-ray diffraction (PXRD)
experiments were conducted upon different treatments to validate the mechanism of
mechanofluorochromism. Multiple sharp and intense diffraction peaks appeared in
the X-ray diffractogram of pristine powder of 1, suggesting it had a crystalline
structure (Fig. 3C). In contrast, most of sharp diffraction peaks vanished upon
grinding, which distinctly indicated that grinding destroyed the well-ordered packing
and led to a metastable amorphous phase with poorly organized structure.
Interestingly, the ground sample showed recovered corresponding diffraction peaks
to those of the pristine solid again after fumed by the vapor of methanol. Therefore,
the PXRD measurements suggested that the mechanofluorochromic mechanism of 1
was actually a crystal-to-amorphous phase conversion process and the
1
2

mechanochromism of 1 was reversible. Differential scanning calorimetry (DSC)
o
measurements showed that the melt points of compounds 1 was 270 C.
Unfortunately, before grinding, after grinding or fuming with methanol, 1 showed no
any peaks in the DSC curves before melting, meaning that 1 couldn’t show
multicolored emissions only by heating (Fig. S12, ESI†).
Insert Figure 3
Next, the photophysical properties of 2 were investigated. Firstly, the melted
state of 2 was quenched in liquid nitrogen to acquire the amorphous powder emitting
at 500 nm (Fig. S11, ESI†). Fortunately, due to the introduction of the S=O moiety in
2
, the solubility of 2 was better than that of 1, then the solid of 2 was assembled by
recrystallization from the solution of MeOH/Cl CH (v: v = 5:1). The fluorescence
2
2
emission color of solid 2 was blue-green under irradiation at 365 nm in the solid-state
λem = 457 nm, ΦF 16.4%, Fig. 4). As for the thioether 1, the sulfoxide 2 also showed
(
=
mechanochromic fluorescence behavior. Grinding 2 in a mortar, a new green emission
was boosted (λ = 477 nm). Also, when methanol vapor was used to fume the ground
em
sample, blue-green fluorescence might be recovered (λem = 457 nm, Fig. 4A and 4B).
To gain the origin of sample 2 in the mechanical process, PXRD patterns were
measured (Fig. 4C). As dye 1, with the treatments of grinding and fuming the solid 2,
diffuse peaks were observed in the ground solid and peaks well coincided with those
of as prepared solid in the fumed solid, revealing a reversible mechanofluorochromic
property and a reversible morphology transition between the crystalline and
amorphous states. As dye 1, there was no obvious peaks in the DSC curves of 2 in
1
3

different solid-states (Fig. S12, ESI†).
Insert Figure 4
3
.4. Polymorphism-dependent emission for sulfone-based BTPE 3
Next, to gain a better understanding the effect of sulfone moiety in the
polymorphism, we continued to study the photophysical properties of 3 in the solid-
state. The amorphous solid gained by quenching the melted 3 in liquid nitrogen
emitted at 513 nm (Fig. S11, ESI†). By recrystallization from the
dichloromethane-methanol mixture, transparent crystal 3C emitted at 464 nm as
presented in Fig. 5A. Unfortunately, the crystal was not stable and deteriorated
o
o
during the single crystal test at 25 C. When it was heated at 50 C for 30 min or at
o
3
0 C for 3 h, another solid conformation 3G was obtained (λ = 488 nm; Ф =
em
F
3
2%), predicting a large number of solvent molecules in the crystal 3C (Fig. 5A).
Actually, the molar ratio of 3 versus the DCM in the crystal of 3C was 1:2 confirmed
1
by H NMR (Fig. S13, ESI†). Fortunately, upon stimulation with DCM vapor, green
3
G could return to the original colorless crystalline 3C. This might be largely due to
the solid dissolution and recrystallization when fuming 3G. The PXRD patterns
showed that 3C and 3G were in different crystalline lattices (Fig. 5B and 5C).
However, hardly grinding sample 3G with a pestle, the fluorescence color didn’t
change (Fig. S14, ESI†). Some DCM was also present in green solid 3G (Fig. S13,
ESI†), whether DCM could volatilize via annealing at high temperature. The
differential scanning calorimetry (DSC) thermogram of 3G showed an additional
1
4

o
endothermic transition peak at 100 C before melting, corresponding to a possible
phase transition from 3G (Fig. S15, ESI†). Indeed, a blue luminogen 3W (λ = 465
em
o
nm; Ф = 35%) was obtained upon heating 3G at 100 C for 0.5 h. 3W was white but
F
not transparent (Fig. 5A), and no solvent was found in the powder (Fig. S3, ESI†).
Although the maximum emission of 3W was the same as that of 3C, the packing
modes of them were so different (Fig. 5C). These signified that compound 3 had
three polymorphs with recognizable contrast, exhibiting polymorphism-dependent
luminescence properties. However, 3W did not display any endothermic peak before
melting in the DSC curve, inferring that the phase of 3W might be the most
thermodynamic stable state in the three polymorphs and 3W couldn’t revert to the
original 3G only via heating. Similarly, when 3W was fumed with various solvents,
no phase transformation occurred. 3C could be recovered again by recrystallization
from the mixed solution of DCM and methanol (v/v = 1:4), (Fig. 5A). So, the
polymorphs of solid 3 could be interconverted through simple external stimuli to
realize the emission switching. Furthermore, unlike 3G, 3W showed reversible
mechanochromic luminescence behavior. After grinding 3W with a pestle, a new
emission peak was observed at 486 nm and the fluorescence color changed from
blue-green to green. Compared with the original powder, the ground state showed a
redshift by 22 nm. Besides, when the ground sample was fumed with methanol vapor,
the green light-emitting sample could return to the original blue-green fluorescence
(Fig. 6A and 6B). And like compounds 1 and 2, the reason for the
mechanofluorochromic behavior of 3W was also a crystal-to-amorphous phase
1
5

conversion process (Fig. 6C). Using DSC measurements to investigate the thermal
o
properties of 3w after grinding, an obvious broad endothermic peak at ~ 132 C
appeared (Fig. S15, ESI†). A blue-green emission was recovered after annealing
o
ground sample of 3W at 132 C for 0.5 h. The PL spectra and XRD patterns of the
heated luminogen meant ground 3W could transform into original solid by heating.
Insert Figure 5
Insert Figure 6
3
.5. Theoretical calculations
Theoretical calculations were performed to investigate the electronic properties
of the three isomers using density functional theory (DFT) at the B3LYP/6-31G(d,p)
level. The optimized structures and the orbital distributions of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
energy levels are shown in Fig. 7. For compound 1, the electron density in HOMO is
localized mainly on the PhSPh group, and while the LUMO is mainly distributed
over one of TPE moiety. However, for compounds 2 and 3, due to the introduction of
electron-withdrawing groups sulfoxide and sulfone, the electron density distributions
of HUMO for compounds 2 and 3 are mainly located in the phenyl groups which do
not connect to sulfoxide or sulfone, and the LUMO distributions are mainly located
in the PhSOPh or PhSO Ph groups.
2
Insert Figure 7
3
.7. Electrochemical Behavior
1
6

To evaluate the practical HOMO and LUMO energy levels, the electrochemical
properties of 1-3 were investigated by cyclic voltammetry (CV) (Fig. S16, ESI†).
ox
onset
The values of E
were represented by the onset oxidation potentials. The onset
ox
oxidation potentials of (E
) of 1-3 occurred at 0.26, 0.72 and 0.70 eV,
onset
respectively. Thus, the HOMO energy levels were -4.66, -5.12 and -5.10 eV,
respectively, according to the empirical equation [HOMO = -(Eox + 4.40) eV].
Moreover, the energy band gaps (E ) of 1-3 were 3.22, 3.27, 3.17 eV, respectively,
g
which were determined from the absorption edges of the UV-vis absorption spectra.
The LUMO energy levels of 1-3 were -1.44, -1.85 and -1.93 eV, respectively, from
the equation of LUMO = (HOMO + E ) eV. In 2-3, when sulfur was oxidized to
g
sulfoxide or sulfone, the decreased both HOMO and LUMO energy levels were
observed. The introduction of electron-withdrawing groups enhanced the electron
affinities. The energy levels determined electrochemically were in accordance with
the values calculated through DFT method.
4
. Conclusions
In this work, three new bis(tetraphenylethene) derivatives 1–3 linked by an S
atom in various oxidation states. Their photophysical properties in solutions and
solids were discussed. The luminogens showed very weak emission in THF solution,
but highly efficient luminescence in THF/H O mixtures with f ≥80%. These results
2
w
demonstrated that all the compounds 1-3 exhibited aggregation-induced emission
characteristics. Grinding the TPE-based luminogens 1-3, the materials exhibited
1
7

red-shifted emissions. Fuming the ground samples with MeOH, the emission colors
recovered to those of original solids. Electron-withdrawing group SO endowed
2
sulfone-BTPE 3 possessed three different phases due to different packing modes, and
one of which exhibited mechanofluorochromism upon grinding.
Acknowledgments
This work was supported by China Postdoctoral Science Foundation (2017M623032)
and the National Natural Science Foundation of China (21876119, 22001182). We
gratefully thank Professor Peng Wu (Analytical & Testing Centre, Sichuan University)
for the help with photophysical measurements.
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Table of Contents
1
2
. Scheme 1. Synthesis routes of compounds 1-3.
. Figure 1. Size distribution curves of (A) 1, (B) 2 and (C) 3 in the THF-water
mixture (f = 95%). Luminogens concentration = 10 μM. Inset: SEM images of the
W
nanoaggregates of 1-3 in the THF-water mixture (f = 95%).
W
3
. Figure 2. FL spectra of luminogens (A) 1, (B) 2 and (C) 3 in THF-H O mixtures
2
with various water contents (f ). (D) Plots of relative FL intensity (I/I ) versus f ,
W
0
W
where I is the FL intensity in pure THF solution. Concentration = 10 μM. λ = 340
0
ex
nm. Slit: 1.5/1.5 nm. Inset: Photographs of luminogens 1–3 in THF/H O mixtures
2
taken under illumination with 365 nm UV lamp with f ranging from 0% to 95%.
W
4
. Figure 3. (A) Photos upon 365 nm UV illumination, (B) PL spectra (λ = 380 nm)
ex
and (C) PXRD patterns of a) pristine, b) ground, c) MeOH fumed powders of 1.
. Figure 4. (A) Fluorescent photographs under 365 nm illumination, (B) solid-state
5
PL spectra, (C) PXRD diffractograms of a) as prepared, b) ground, and c) fumed
solids of 2.
24

6
. Figure 5. (A) Photographs of 3C, 3G and 3W under natural light. Conditions: I,
o
o
heating at 50 C for 30 min; II, fuming with DCM vapor; III, heating at 100 C for 0.5
h; IV, recrystallization from DCM/methanol solution. (B) Solid-state PL spectra (λ =
ex
3
80 nm) of 3C, 3G and 3W. (C) PXRD curvs of 3C, 3G and 3W. Insets in (A): The
pictures of 3C, 3G and 3W under UV light.
7
. Figure 6. (A) Photographic images upon 365 nm UV illumination, (B) Solid-state
PL spectra, and (C) PXRD patterns of a) pristine, b) ground, c) fumed, and d) heated
solids of 3W.
8
. Figure 7. Molecular orbitals of the HOMO and LUMO energy levels of 1–3 in
water calculated using the B3LYP/6-31G(d) basis sets.
Scheme 1. Synthesis routes of compounds 1-3.
2
5

Figure 1. Size distribution curves of (A) 1, (B) 2 and (C) 3 in THF-water mixture (fW
95%). Luminogens concentration = 10 μM. Inset: SEM images of the
nanoaggregates of 1-3 in the THF-water mixture (f = 95%).
=
W
2
6

Figure 2. FL spectra of luminogens (A) 1, (B) 2 and (C) 3 in THF-H O mixtures with
2
various water contents (f ). (D) Plots of relative FL intensity (I/I ) versus f , where I
0
W
0
W
is the FL intensity in pure THF solution. Concentration = 10 μM. λ = 340 nm. Slit:
ex
1
.5/1.5 nm. Inset: Photographs of luminogens 1–3 in THF/H O mixtures taken under
2
illumination with 365 nm UV lamp with f ranging from 0% to 95%.
W
27