AIE (AIEE) and mechanofluorochromic performances of TPE-methoxylates: Effects of single molecular conformations

Article abstract of DOI:10.1039/c3ra40734a

Two methoxy-substituted tetraphenylethylene (TPE) derivatives, tetra(4-methoxyphenyl)ethylene (TMOE) and tetra(3,4-dimethoxyphenyl)ethylene (TDMOE), were synthesized by McMurry reaction in high yields. The nearly centrosymmetric and natural propeller shape of TMOE and TDMOE excluded intermolecular effects, such as H or J-aggregation and π-π stacking, on their AIE (AIEE) and mechanofluorochromic performance. The crystal structures of TMOE and TDMOE, and theoretical calculations proved that their emission colours are determined by single molecular conjugation. These molecules were used to investigate pure conformational effects on molecular emissions. The spectral properties of these molecules in five environments of crystal(s), THF solution, THF-water binary solution, solidified THF and amorphous states, were investigated. The crystalline to amorphous phase transition by grinding resulted in good mechanofluorochromic performances with high quantum yields and distinguishable emission change, which was further explored as anti-counterfeiting inks on banknotes.

Full text of DOI:10.1039/c3ra40734a

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AIE (AIEE) and mechanofluorochromic performances of
TPE-methoxylates: effects of single molecular
conformations3
Cite this: RSC Advances, 2013, 3,
7996
Qingkai Qi,a Yifei Liu,*b Xiaofeng Fang,b Yumo Zhang,a Peng Chen,a Yi Wang,a
Bing Yang,a Bin Xu,a Wenjing Tiana and Sean Xiao-An Zhang*a
Two methoxy-substituted tetraphenylethylene (TPE) derivatives, tetra(4-methoxyphenyl)ethylene (TMOE)
and tetra(3,4-dimethoxyphenyl)ethylene (TDMOE), were synthesized by McMurry reaction in high yields.
The nearly centrosymmetric and natural propeller shape of TMOE and TDMOE excluded intermolecular
effects, such as H or J-aggregation and p–p stacking, on their AIE (AIEE) and mechanofluorochromic
performance. The crystal structures of TMOE and TDMOE, and theoretical calculations proved that their
emission colours are determined by single molecular conjugation. These molecules were used to
investigate pure conformational effects on molecular emissions. The spectral properties of these molecules
in five environments of crystal(s), THF solution, THF–water binary solution, solidified THF and amorphous
states, were investigated. The crystalline to amorphous phase transition by grinding resulted in good
mechanofluorochromic performances with high quantum yields and distinguishable emission change,
which was further explored as anti-counterfeiting inks on banknotes.
Received 11th February 2013,
Accepted 15th March 2013
DOI: 10.1039/c3ra40734a
www.rsc.org/advances
materials have vast applications in pressure sensors, rewri-
table media and security ink.5 In previous work, large aromatic
1. Introduction
Tetraphenylethylene (TPE) derivatives are hotly investigated in
recent years for their aggregation induced emission (AIE) or
aggregation induced emission enhancement (AIEE) proper-
ties.1 AIE or AIEE refer to the organic molecules which are
practically non-luminescent or weakly luminescent in solution
but become strongly emissive in aggregated states, because the
non-irradiation transitions from the free rotation of the single
bonds in the conjugation system are forbiden.2 AIE (AIEE)
materials are very important in developing efficient organic
light emitting devices (OLED) and bio-labels.3 Recent studies
show that some TPE derivatives possess solid state mechan-
ofluorochromic performances, which means the materials
have different fluorescent emissions upon mechanical force.4
groups were usually introduced on TPE motif to enable the
tunable mechanofluorochromic properties. It has been
revealed that the mechanofluorochromic performances of
TPE derivatives are related to the crystalline to amorphous
phase transition.6 However, the inherent reasons for emission
shift is hard to elucidate in amorphous state. Changes in
intramolecular planarity,4a,7 as well as intermolecular interac-
tions of p–p stacking,8 and H or J-aggregation9 have all been
assigned as the causes of mechanofluorochromic phenomena.
Whereas, the mechanism is very critical in molecular design,
which determines the mechanofluorochromic performances
of the materials, such as color contrast and quantum
efficiency. Another question is what structures can have
crystalline to amorphous phase transition and the resultant
mechanofluorochromic properties, because it is found that
not all TPE derivatives have the phase transition.6 Resolving
these questions needs more studies on a wider library of
objects, more reasonable designs and characterizations.
Most recently, Dong and Tang et al. reported much simpler
ethoxy- and butoxy-substituted TPE, and demonstrated their
morphology-dependent multi-color emissions by different
external stimuli in solid state.10 Due to the natural propeller
shape, intermolecular stacking, such as p–p stacking and H or
J-aggregation, is unlikely to happen in these molecules, which
offered simpler models for AIE (or AIEE) and mechanofluor-
ochromism study. In this article, two even smaller methoxy-
As
a kind of ‘‘smart material’’, mechanofluorochromic
aState Key Lab of Supramolecular Structure and Materials, College of Chemistry,
Jilin University, Changchun, 130012, P. R. China. E-mail: seanzhang@jlu.edu.cn;
Fax: +86-0431-85153812
bCollege of Chemistry, Jilin University, Changchun, 130012, P. R. China.
E-mail: liuyifei@jlu.edu.cn; Fax: +86-0431-85153812
Electronic supplementary information (ESI) available: Characterizations and
3
single cystals of TMOE, TDMOE and TPE. Torsion angle data, theoretical
calculation data, and DSC of TMOE-1 and TMOE-2. Mechanofluorochromic
cycles of TMOE treated by grinding and solvent treatment. PL spectra and
powder X-ray diffraction patterns of TPE crystal before and after grinding.
Analysis of the weak interactions in single crystal structures of TMOE-1, TMOE-
2, TDMOE and TPE. CCDC reference numbers 921515, 921516. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3ra40734a
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substituted TPE derivatives were synthesized, which are
tetra(4-methoxyphenyl)ethylene
(TMOE)
and
tetra(3,4-
dimethoxyphenyl)ethylene (TDMOE). Two different crystal
structures of TMOE with different emission colours were
obtained, which prove that their emissions are determined by
single molecular conformation (or conjugation state).
Therefore, these molecules provide ideal objects to investigate
pure conformational effects on molecular emissions.
Crystal(s), THF solution, THF–water binary solution, solidified
THF and amorphous states were used as five environments to
induce conformational change and emission response. The
crystalline to amorphous phase transition of TMOE and
TDMOE by grinding resulted in good mechanofluorochromic
performance. In the absence of intermolecular effects, they
both have high quantum yields (TMOE >50%, TDMOE >70%)
before and after grinding. Well distinguishable colour contrast
with emission peak shift of 60 nm and 20 nm were obtained
for TMOE and TDMOE, respectively. Moreover, utilizing its
mechanofluorochromic property, TMOE was demonstrated as
an anti-counterfeiting ink for banknotes.
Scheme 1 Synthetic routes for TMOE, TDMOE and TPE.
the yields were about 74% and 86%. Detailed characterizations
of the three compounds are listed in the ESI. Crystals of TPE
3
and TDMOE were both grown from their methanol/dichlor-
omethane solutions. Two different crystals of TMOE (denoted
as TMOE-1 and TMOE-2) were grown from its n-hexane/
dichloromethane and methanol/dichloromethane solution,
respectively.
2. Experimental section
2.1 Materials
Acetonitrile was purchased from Xilong Chemical Reagent.
K2CO3, phenol and H3PO4 were purchased from Beijing
Chemical Plant. ZnCl2, polyphosphoric acid (PPA, P2O5
¢80%) and iodomethane were purchased from Sinopharm
Chemical Reagent. PCl3 was purchased from Tianjin Guangfu
Chemical Reagent. 3,4-Dihydroxy benzoic acid, p-hydroxy
benzoic acid, 1,2-benzenediol, benzophenone, Zn dust, TiCl4
and dry pyridine were all purchased from Aladdin. All the
above chemicals were of analytical grade and used as received
without further purification. Tetrahydrofuran (THF) was
distilled in presence of sodium benzophenone under protec-
tion of dry nitrogen prior to use.
2.3 Instruments
1H NMR spectra were recorded on a 500 MHz Bruker Avance or
a Varian-300 EX spectrometer using CDCl3 or DMSO-d6 as
solvent and tetramethylsilane (TMS) as an internal standard (d
= 0.00 ppm). 13C NMR spectra were recorded on a Bruker
Avance 500 MHz spectrometer using CDCl3 as solvent and
CDCl3 as an internal standard (d = 77.00 ppm). UV-Vis spectra
were measured on a Shimadzu UV-2550 spectrophotometer.
Photoluminescence (PL) spectra were measured with a RF-
5301PC spectrofluorometer. Powder XRD patterns were
obtained from a PANalytical B.V.Empyrean X-ray diffractomer
with Cu-Ka radiation (l = 1.5418 Å) at 25 uC (scan range: 4–
50u). Single crystal X-Ray analyses were performed on a R-AXIS
RAPID-F X-ray single crystal diffractometer. DSC experiments
were recorded on a NETZSCH DSC 204 instrument at a
scanning rate of 10 K min21. The fluorescence lifetime
measurements were performed on a time-correlated single-
photon counting (TCSPC) system under right-angle sample
geometry using mini-t miniature fluorescence lifetime spectro-
meter (Edinburgh Instruments). A 379 nm picosecond diode
laser (Edinburgh Instruments EPL-375, repetition rate 5 MHz,
64.8ps) was used to excite the samples. Crystalline and
amorphous state PL quantum yields were measured with an
integrating sphere (C-701, Labsphere Inc.) with a 405 nm
Ocean Optics LLS-LED as the excitation source, and the light
was introduced into the integrating sphere through an optical
fiber.
2.2 Synthesis
The synthetic routs of TMOE, TDMOE and TPE are illustrated
in Scheme 1. The diaryl methanones (1, 2, 3, 4) were prepared
according to the literature (see ESI ). TMOE was synthesized by
3
McMurry reaction:11 TiCl4 (0.6 mL, 5.4 mmol) was added
dropwise to chilly (y0 uC) anhydrous tetrahydrofuran (20 mL)
under nitrogen atmosphere. Then Zn dust (0.702 g, 10.8
mmol) and dry pyridine (20 mL, 0.25 mmol) were added to the
mixture immediately. The black suspension was warmed to
room temperature and then refluxed for 2 h. A solution of 2 (2
mmol) in 5 mL THF was added slowly into the above mixture
during refluxing. The reaction was finished until 2 were
completely consumed (monitored by TLC). The mixture was
cooled to room temperature and quenched with aqueous
K2CO3 (5 mL, 10%). The organic layer was separated and the
aqueous suspension was extracted with dichloromethane (4 6
25 mL). The organic phase was dried with anhydrous MgSO4.
The crude products were purified by recrystallization (metha-
nol/dichloromethane) to get crystals with a yield about 85%.
TDMOE and TPE were synthesized with similar protocols and
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Fig. 1 Normalized PL spectra of TMOE-1, TMOE-2 and TDMOE crystals. Inset:
microscopic images of the three crystals under UV irradiation.
3. Results and discussion
Fig. 2 (A) PL (Ex: 328 nm) and (B) UV-vis absorption spectra of TMOE (2 6 1025
M) in THF/water mixtures with different water fractions (fw). Inset: fluorescent
photographs of TMOE in THF and THF/water mixtures (fw = 90%). (C) PL (Ex: 343
nm) and (D) UV-vis absorption spectra of TDMOE (2 6 1025 M) in THF/water
mixtures with different water fractions (fw). Inset: fluorescent photographs of
TDMOE in THF and THF/water mixtures (fw = 90%).
3.1 Crystal structures and emission properties of TMOE and
TDMOE
Two types of single crystals of TMOE with different emission
colours (TMOE-1: 420 nm; TMOE-2: 440 nm) were obtained
(Fig. 1). Analysis of the intermolecular interactions in these
crystals proves that no p–p interaction or any type of H or
J-aggregation exist in either crystals due to the centrosym-
metric and propeller shape (crystal structure of TMOE-1 and
solution of TMOE has virtually no emission because of the free
rotation of carbon–carbon single bonds between the four
periphery phenyls and the ethylene core. Higher fw than 80%
leads to the aggregation of TMOE and the enhancement of its
emission. The fact that aggregation has happened is deduced
from the lifted spectral tails in UV-vis absorption spectra,
which are caused by the Mie or light-scattering effect from
nano-sized particles (Fig. 2B).16 Although it is hard to tell the
change of TMOE conjugation state between in THF solution
and water-induced aggregations from PL spectra, because
TMOE hardly has emission in THF solution, UV-vis absorption
spectra give right the information. The first absorption peak of
TMOE red-shifts about 14 nm from 324 nm to 338 nm when
the fw increases from 0% to 90%, indicating a better
conjugation in the water-induced aggregation.
TMOE-2, see ESI ). Because the dihedral angles between the
3
ethylene core and the four peripheral benzene groups in both
crystals were not the same, and it is not easy to compare the
difference in coplanarity as well as conjugation extend directly
(Fig. S7, ESI ). Therefore, bond length alternation (BLA), a
3
critical parameter determining the bandgap and conjugation
of p- delocalized systems, is used for estimation of the
conjugation difference in TMOE-1 and TMOE-2 based on the
exact molecular conformations in the crystals.12 BLA is
calculated to be 0.03843 for TMOE-1, and 0.03830 for TMOE-
2. The smaller BLA value for TMOE-2 suggests the better
molecular coplanarity and conjugation.12,13 Furthermore,
theoretical calculation on the bandgaps of TMOE-1 and
TMOE-2 were performed using the B3LYP/6-31+g(d, p) basis
set.14 It indicated the HOMO–LUMO bandgaps are 4.22 eV for
Introduction of another four methoxy groups on the ortho
position of methoxy of TMOE increases steric effect, resulting
in the restriction of the rotation by the four periphery phenyls.
Thus TDMOE have a weak emission at 494 nm in THF solution
(Fig. 2C). The intensity of emission increases with the fw.
Meanwhile, the emission peak of the aggregated TDMOE blue-
shifts 13 nm from 494 nm to 481 nm when the fw increases
from 0% to 90%. In the same way, the first absorption peak
blue-shifts 6 nm from 343 nm to 337 nm as shown in Fig. 2D.
So, the blueshift of both emission peak and absorption peak
suggests that TDMOE in THF solution have better conjugation
than in the water-induced aggregates. So far, it can be seen
that TMOE and TDMOE have different conformational
response upon poor solvent. Meanwhile, it can be clear seen
that the conjugation state in water-induced aggregations of
both TMOE and TDMOE is better than their crystals by
comparing their emission peaks. The phenomena are very
interesting, but the reason remains unclear, because the
factors that affect the conformations are complex and it is
difficult to make an easy conclusion.
TMOE-1 and 4.17 eV for TMOE-2 (Fig. S8, ESI ). Both the BLA
3
and theoretical calculation results support that better con-
jugation in TMOE-2 is the main reason for the redshift of the
emission compared with TMOE-1. As for TDMOE crystal
(crystal structure of TDMOE, see ESI ), its emission peak is
3
at 460 nm (Fig. 1). There is no p–p interaction or any type of H
or J-aggregation is observed either, indicating the emission of
the crystal is also a single molecular behavior. More evidences
can be found in the following study on their optical properties
in solution.
3.2 Optical properties of TMOE and TDMOE in THF, and THF/
water binary solution: AIE and AIEE performances
THF is a good solvent for both TMOE and TDMOE, and water
is a poor solvent. Adding water to the THF solution of these
two molecules will cause the well-dissolved molecules to
aggregate and result in an enhancement of emission. And this
typical process is called AIE or AIEE properties.15 As seen in
Fig. 2A, when water fraction (fw) is lower than 80%, the
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Fig. 3 Variation of PL spectra with time of TMOE (A) and TDMOE (B) in frozen
THF solution (2 6 1025 M) during their melting.
3.3 Emission properties in solidified solution
Another method to restrict the free rotation of a molecule and
enable the fluorescence is freezing it at low temperature. This
method was used to further investigate the single molecular
conjugation state of TMOE and TDMOE in solutions. Frozen
TMOE and TDMOE were obtained by solidifying their dilute
THF solutions (2 6 1025 M) with liquid nitrogen, and they
show emission peaks at 448 nm and 442 nm, respectively. As
the melting of the frozen THF, the emission peaks red-shift
with the decreasing intensity (Fig. 3). This is consistent with
literature reports that freezing of the fluorophore usually
causes a blueshift in emission,17 because molecular rotation
and vibration are forbidden, and the emission peak red-shifts
with decreasing intensity as the melting of the solid. The
emission peaks of TPE, TMOE-1, TMOE-2, and TDMOE in
various conditions are summarized in Table 1.
Although the emission of frozen single molecular TMOE was
measured at y80 K, its emission peak is still longer than the
crystals of TMOE-1 and TMOE-2. As the emission peak red-
shifts with the melting of THF, it is safe to deduce that the
TMOE solution has longer emission than its crystals, if it could
be measured by steady-state fluorescence spectroscopy. As for
TDMOE, when temperature increases to the melting tempera-
ture of THF, an emission peak at 475 nm is found (indicated
by the arrow in Fig. 3B), which already red-shifts compared
with the emission of its crystals. However, it is still hard to
draw the conclusion that TMOE and TDMOE have better
conjugation in solution than in crystals because of the
existence of complex solvent effects.
Fig. 4 PL spectra (A) and XRD patterns (B) of TMOE-1: pristine, ground and
annealed sample (150 uC for 1 min). PL spectra (C) and XRD patterns (D) of
TDMOE: pristine, ground and annealed sample (150 uC for 1 min). Insets: real
object illustration of the process.
3.4 Emission shift from crystalline to amorphous state upon
grinding: mechanofluorochromic performances
The strong fluorescence of TMOE-1 pristine crystals can
change from blue (Em: 420 nm) to cyan (Em: 480 nm) by
grinding. While, the emission peak of TMOE-2 crystal change
from 440 nm to 487 nm after grinding. Similarly, the emission
of TDMOE pristine crystal at 460 nm red-shifts to 480 nm after
grinding. The fluorescence can almost go back to the original
again upon thermal treatment (Fig. 4, Fig. S9, ESI ). XRD
3
measurements show that the mechanofluorochromic proper-
ties are directly caused by crystalline to amorphous phase
transition. Upon grinding, the crystalline structure convert to
amorphous phase as indicated from the disappearance of XRD
patterns. The reversible mechanofluorochromic behaviors can
be repeated for many cycles. Deserving to be mentioned, the
emission peaks of ground samples of TMOE-1 and TMOE-2
were not exactly the same, furthermore, the amorphous
samples of TMOE-1 and TMOE-2 converted to TMOE-1 crystals
and TMOE-2 crystals respectively upon heat-annealing, which
indicating that the mechnofluorochromic performance of
TMOE is related to the original crystal structure.
The change in aggregation state of TMOE-1 and TDMOE was
further confirmed by DSC analysis (Fig. 5). There are two
adjacent endothermic peaks at 176 uC and 187 uC for TMOE-1
pristine crystals. The former refers to the crystal phase
conversion10 from TOME-1 to TMOE-2 (Fig. S10, ESI ), and
3
Table 1 Summary of emission peaks of TPE, TMOE-1, TMOE-2, and TDMOE in
the latter corresponds to melting point. After grinding, an
exothermic peak at 55 uC appeared, indicating that the ground
sample is in a metastable amorphous state and can recrys-
tallize in the solid state by heating. The phenomenon is
consistent with literature reports.18 However, the former
endothermic peak became broader and shifts to 159 uC,
suggesting the regenerated crystal structure induced by heat
annealing is not as good as the original TMOE-1 crystallized in
solution. After the amorphous TOME-1 was annealed at 150 uC
for 1 min, the exothermic peak for the amorphous to TMOE-1
various conditions
TPE
TMOE-1
—
TMOE-2
—
TDMOE
Solutiona (RT)
—
494
488
442
460
Aggregateb (RT)
Solidified solutionc (83 K)
Crystal (RT)
463
449
447
492
448
420
440
a
b
THF solution (2 6 1025 M). Aggregates generated in THF/water
mixtures (fw = 90%). THF solution (2 6 1025 M) frozen by liquid
c
nitrogen.
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amorphous states shouldn’t have significant change in
quantum yields. Assuredly, integration sphere measurements
prove these samples all have comparable high quantum yields,
TMOE-1: 54%, TMOE-2: 60%, ground TMOE: 67%; TDMOE:
74%; ground TDMOE: 78%. Therefore, they all have potential
applications in solid state stimuli-responsive materials with
good optical response.
It is known that TPE does not have mechanofluorochromic
properties, because it can not undergo a crystalline to
amorphous phase transition (Fig. S12, ESI ). Although only
3
Fig. 5 DSC curves of TMOE-1 (A) and TDMOE (B): pristine, ground and annealed
sample (150 uC for 1 min).
small methoxy groups are substituted on TPE, more flexible
…
…
p
intermolecular interactions of C–H
O
and C–H
are
introduced (Fig. S13, ESI ). The flexible interactions make
3
the sliding and deformation of the crystal structure easier,
crystalline phase transition disappeared at 55 uC, and the
endothermic peaks responsible for TMOE-1 to TMOE-2 phase
transition and melting points resembled with the ground
sample, indicating heat annealing at 150 uC for 1 min is still
insufficient to generate good TMOE-1 crystals, that’s why the
fluorescence can not go back to the original 420 nm. If higher
temperature was applied, it can be conjectured that crystal
structure of TMOE-2 could be generated. As for TDMOE
crystals, there is only one endothermic peak in DSC curve
assigned to the melting point. The ground sample of TDMOE
crystallized at 81 uC with correspondence to the exothermic
peak as indicated by the arrow in Fig. 5B. Similarly, after heat
annealing of the ground TDMOE at 150 uC for 1 min, the
exothermic peak responsible for the amorphous to crystal
transition disappears with a slightly lower melting points
(Fig. 5B). The above results proved the crystalline to
amorphous phase transition is the direct cause of the
mechanofluorochromic behavior. And the more planar con-
formation and resultant better conjugation of single molecules
in the amorphous state is the inherent reason for the
fluorescence shift. Wetting with organic solvents is another
way to transform the ground samples to crystals (Fig. S11,
facilitating the transition from crystalline to amorphous
…
phase. In contrast, TPE crystals with only rigid C–H
p
interactions between benzyl groups exhibit fragile character
under external pressure or stimuli. Therefore, increment of
relative ‘‘soft interaction’’ between the molecules is thought to
be the essential factor for TPE derivatives which possess the
mechanofluorochromic performance.10 Moreover, the weak
interactions were thought to stabilize the metastable state of
more planar conformation under pressure.
3.5 Application in anti-counterfeiting label in paper money
Fluorescent dyes have been incorporated onto paper money
and used as anti-counterfeiting labels widely. Apparently,
reversible mechanofluorochromic materials can further
increase the security. TMOE, which has more obvious
fluorescence change than TDMOE, was selected as anti-
counterfeiting ink. Chinese ‘‘5 yuan’’ was printed onto a
practice banknote to demonstrate the application (Fig. 7). The
fluorescence of the characters have cyan fluorescence as soon
as the THF solution of TMOE was sprayed on the paper
because of the fast volatilization of solvent, and the molecular
packing was in the meta-stable amorphous state in this
situation. The molecules went to a more stable crystalline
state after annealing treatment by IR light or heating on a hot
plate, and the fluorescence change to blue. Selective grinding
ESI ). However, heat annealing is more practical when
3
applying these molecules on daily anti-counterfeiting applica-
tions.
Fluorescence decay experiments show that the lifetimes of
all the crystals and their amorphous counterparts are all at a
level of several nanoseconds. The very small changes in
lifetime before and after grinding indicate that the molecular
environments do not have significant change, and this also
excludes the possibility of intermolecular p–p stacking after
grinding (Fig. 6). Therefore, samples in crystalline and
Fig. 7 Illustration of TMOE as an anti-counterfeiting ink on a 5-yuan RMB
practice note printed with Chinese characters of ‘‘5-yuan’’. Images are (a) the
note immediately after ‘‘5-yuan’’ was printed, (b) after annealed; (c) when 5
was ground (d) after annealed again under UV light irradiation. (e) and (f) are
images of (b) and (c) under visible light, respectively.
Fig. 6 Time-resolved emission decay curves of (A) TMOE-1, TMOE-2 and ground
TMOE-1; (B) TDMOE crystal and ground TDMOE.
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the character ‘‘5’’ can cause an obvious color contrast with the
unground ‘‘yuan’’. There is no obvious change in the visible
light before and after grinding as well as annealing because
they don’t have absorption in visible light range. Repeated
annealing and grinding can be used to induce the responsive
fluorescent color change and increase the complexity of the
anti-counterfeiting performance compared with common
fluorescent dyes. The reversible mechanofluorochromic pro-
cess can be reproduced for many times. The results indicate
this kind of material has practical applications in high security
anti-counterfeiting inks.
4. Conclusions
Two methoxy-substituted TPE derivatives, TMOE and TDMOE,
have been synthesized. Crystal structures and theoretical
calculations both prove that their emissions are determined
by single molecular conformation (or conjugation state)
without intermolecular effects, such as H or J-aggregation
and p–p stacking. Pure conformational effect on the molecular
emissions has been studied in five environments of crystal(s),
THF solution, THF–water binary solution, solidified THF and
amorphous states. The response to these chemical environ-
ments enables TMOE and TDMOE to show AIE (AIEE) and
mechanofluorochromic properties. In the absence of inter-
molecular effects, TMOE and TDMOE have comparable high
quantum yields in crystalline and amorphous states. TMOE,
which has better color contrast, has been used for anti-
counterfeiting on banknotes, showing the practical applica-
tions of these materials in security inks.
Acknowledgements
We would like to thank Dr Minjie Li for fruitful discussions
and generous support. We are grateful to the National Science
Foundation of China (21072025) for financial support.
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