Click synthesis, aggregation-induced emission, E/Z isomerization, self-organization, and multiple chromisms of pure stereoisomers of a tetraphenylethene-cored luminogen

Article abstract of DOI:10.1021/ja208883h

It has been difficult to decipher the mechanistic issue whether E/Z isomerization is involved in the aggregation-induced emission (AIE) process of a tetraphenylethene (TPE) derivative, due to the difficulty in the synthesis of its pure E and Z conformers. In this work, pure stereoisomers of a TPE derivative named 1,2-bis{4-[1-(6-phenoxyhexyl)-4-(1,2,3-triazol)yl]phenyl}-1,2- diphenylethene (BPHTATPE) are successfully synthesized. Both isomers show remarkable AIE effect (α_AIE ≥ 322) and high fluorescence quantum yield in the solid state (φ_F 100%). The conformers readily undergo E/Z isomerization upon exposure to a powerful UV light and treatment at a high temperature (>200 °C). Such conformational change, however, is not observed under normal fluorescence spectrum measurement conditions, excluding the involvement of the E/Z isomerization in the AIE process of the TPE-based luminogen. The molecules of (E)-BPHTATPE self-organize into ordered one-dimensional nanostructures such as microfibers and nanorods that show obvious optical waveguide effect. BPHTATPE shows rich chromic effects, including mechano-, piezo-, thermo-, vapo-, and chronochromisms. Its emission peak is bathochromically shifted by simple grinding and pressurization and the spectral change is reversed by fuming with a polar solvent, heating at a high temperature, or storing at room temperature for some time. The multiple chromic processes are all associated with changes in the modes of molecular packing.

Full text of DOI:10.1021/ja208883h

Article
pubs.acs.org/JACS
Click Synthesis, Aggregation-Induced Emission, E/Z Isomerization,
Self-Organization, and Multiple Chromisms of Pure Stereoisomers of
a Tetraphenylethene-Cored Luminogen
Jian Wang,† Ju Mei,† Rongrong Hu,‡ Jing Zhi Sun,*,† Anjun Qin,*,† and Ben Zhong Tang*,†,‡
†MoE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering,
Zhejiang University, Hangzhou 310027, China
‡Department of Chemistry, Institute for Advanced Study, Institute of Molecular Functional Materials, State Key Laboratory of
Molecular Neuroscience, and Division of Biomedical Engineering, The Hong Kong University of Science and Technology, Clear
Water Bay, Kowloon, Hong Kong, China
S
* Supporting Information
ABSTRACT: It has been difficult to decipher the mechanistic
issue whether E/Z isomerization is involved in the aggregation-
induced emission (AIE) process of a tetraphenylethene (TPE)
derivative, due to the difficulty in the synthesis of its pure E and
Z conformers. In this work, pure stereoisomers of a TPE
derivative named 1,2-bis{4-[1-(6-phenoxyhexyl)-4-(1,2,3-
triazol)yl]phenyl}-1,2-diphenylethene (BPHTATPE) are suc-
cessfully synthesized. Both isomers show remarkable AIE effect
(αAIE ≥ 322) and high fluorescence quantum yield in the solid
state (ΦF 100%). The conformers readily undergo E/Z
isomerization upon exposure to a powerful UV light and treatment at a high temperature (>200 °C). Such conformational
change, however, is not observed under normal fluorescence spectrum measurement conditions, excluding the involvement of
the E/Z isomerization in the AIE process of the TPE-based luminogen. The molecules of (E)-BPHTATPE self-organize into
ordered one-dimensional nanostructures such as microfibers and nanorods that show obvious optical waveguide effect.
BPHTATPE shows rich chromic effects, including mechano-, piezo-, thermo-, vapo-, and chronochromisms. Its emission peak is
bathochromically shifted by simple grinding and pressurization and the spectral change is reversed by fuming with a polar solvent,
heating at a high temperature, or storing at room temperature for some time. The multiple chromic processes are all associated
with changes in the modes of molecular packing.
and hexaphenylsilole (HPS), are nonluminescent in the dilute
solutions but become highly emissive in the aggregate state.
Thanks to the novel AIE effect, no elaborate work needs to be
done to interrupt the aggregation processes of the luminogens.
Instead, one can take advantage of aggregate formation to
generate efficient solid emitters. The novel AIE effect permits
the use of concentrated solutions of luminogens for sensing
applications. The reliable and sensitive fluorescence turn-on
nature of the AIE probes makes them promising candidates for
applications in field screening, on-site trial, household testing,
etc.4,5 Efficient OLEDs can be readily fabricated using AIE
luminogens as the active layers without complex engineering
control.6
1. INTRODUCTION
Design and creation of efficient light emitters in the solid and
aggregate states are of importance to the development of
advanced optical and biomedical devices, such as organic light-
emitting diodes (OLEDs) and fluorescent biosensors.1
A
formidable challenge in the area is the notorious photophysical
effect of “aggregation-caused quenching” (ACQ): emission of a
conventional luminophore is often weakened or even totally
annihilated when its molecules are aggregated2 due to energy
transfer and the formation of excimers and exciplexes. Various
chemical and physical approaches (e.g. attachment of alicyclic
pendants, encapsulation by amphiphilic surfactants, and
blending with transparent polymers) have been taken to
alleviate the ACQ effect. These attempts, however, have met
with only limited success, because they are basically working
against an intrinsic process, i.e., the natural formation of
aggregates of chromophores in the solid state.3
The useful AIE effect has stimulated much effort to study the
intriguing photophysical process. As a result, a variety of AIE
systems have been developed in the past decade.4−7 Among the
AIE luminogens developed so far, TPE is unique in that it is
synthetically readily accessible (one-step reaction from
We have discovered an unusual photophysical phenomenon
of “aggregation-induced emission” (AIE), which is exactly
opposite to the ACQ process discussed above.4−7 A series of
propeller-shaped molecules, such as tetraphenylethene (TPE)
Received: September 21, 2011
© XXXX American Chemical Society
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inexpensive reagents) and structurally easily manipulable (with
numerous functional groups attachable through simple
reactions).7 Its exact AIE mechanism, however, remains to be
deciphered. The issue involved is whether its AIE process is
associated with the E/Z isomerization (EZI)8 or the restriction
of intramolecular rotation (RIR).9 The EZI hypothesis assumes
that, as TPE is a stilbene derivative, its excited states are
nonradiatively annihilated by the EZI process in a dilute
solution, whereas the decreased probability of EZI in an
aggregate phase gives rise to an increase in the photo-
luminescence (PL) efficiency.8 On the other hand, the RIR
mechanism postulates that the PL of TPE in the solution is
quenched by the dynamic intramolecular rotations of its
multiple phenyl rotors against its ethylene stator, whereas the
aggregate formation effectively suppresses the molecular
motions, blocks the nonradiative energy dissipation channels,
and opens up the radiative decay pathway.4,9
Scheme 1. Synthesis of a BPHTATPE Mixture by “Click”
Reaction and Isolation of Its Pure E and Z Conformers by
Column Separation
a
A correct mechanistic understanding of the AIE process is of
academic value and has practical implications. It will guide our
endeavors of structure design for the development of new AIE
luminogens and assist our efforts to explore their potential
technological applications. Although a wealth of experimental
data in support of the RIR mechanism have been collected,4
little is known about the involvement of the EZI process in an
AIE system. Studying the EZI process requires pure E and Z
isomers of an olefinic luminogen, whose synthesis, however, has
been difficult, as the commonly used titanium-catalyzed
McMurry coupling reaction only produces E/Z mixtures. We
have attempted to use a palladium-catalyzed coupling reaction
to prepare pure isomers.10 The result was better but imperfect:
the product was Z-rich but not 100% pure in conformation.
The great importance of drawing a clear mechanistic picture
has motivated us to further tackle the synthetic difficulty and to
prepare conformationally pure AIE luminogens. To enlarge the
difference between the stereoisomers and to enhance their
macroscopic separability, we designed in this work a triazole-
functionalized TPE derivative named 1,2-bis{4-[1-(6-phenox-
yhexyl)-4-(1,2,3-triazol)yl]phenyl}-1,2-diphenylethene
(BPHTATPE) and accomplished its synthesis through the use
of a copper-catalyzed Click reaction (Scheme 1).11 As expected
and much to our delight, the triazole-functionalization enabled
the macroscopic separation of pure E and Z stereoisomers of
the resultant BPHTATPE mixture by simple silica-gel column
chromatography.
The successful acquisition of the pure stereoisomers of
BPHTATPE permits us to study their conformational changes,
if any, in their photophysical, especially AIE, processes using
standard spectroscopic techniques such as NMR spectroscopy.
An added bonus of the attachment of polar triazole pendants to
the hydrophobic TPE core is the structural amphiphilicity of
the resulting AIE luminogen, which endows its molecules with
self-organizability and morphological tunability. In this paper,
we report the conformational and morphological structures of
BPHTATPE and its luminescent and chromatic properties in
the solid state.
a
BPHTATPE = 1,2-bis{4-[1-(6-phenoxyhexyl)-4-(1,2,3-triazol)yl]-
phenyl}-1,2-diphenylethene, BETPE = 1,2-bis(4-ethynylphenyl)-1,2-
diphenylethene, POHA = 6-(phenoxy)hexyl-azide, and SA = sodium
ascorbate.
of the steric conformers. Keeping this in mind, we thought that
attaching large and polar moieties to a TPE core might bring
about a big enough difference in the shape and polarity of the
resultant TPE derivative to enable macroscopic separation of its
stereoisomers.
Along this line of consideration, we designed the structure of
a functionalized TPE molecule and realized its preparation by
the synthetic route shown in Scheme 1. A mixture of E/Z
stereoisomers of 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene
(BETPE) prepared by McMurry coupling12 was subjected to
the alkyne−azide Click reaction with 6-(phenoxy)hexylazide
(POHA) catalyzed by a mixture of CuSO4/sodium ascorbate
(SA) at room temperature. The reaction proceeded smoothly,
giving an E/Z mixture of BPHTATPE in a high yield (89%).11
The conformer mixture was separated by a routine silica-gel
column using a dichloromethane (DCM)/acetone mixture as
the eluent, affording the pure E and Z isomers as white and
pale-yellow powders, respectively. The success in the isomer
separation by the macroscopic technique of simple column
chromatography proves that our structural design elaborated
above has worked as expected.
The separated stereoisomers were characterized by various
spectroscopic methods, from which satisfactory analysis data
were obtained [Figures S1−S5, Supporting Information (SI)].
1
While the full H NMR spectra of the isomers are given in
Figure S2 in the SI, their partially magnified spectra in the
aromatic region are depicted in Figure 1. The spectral profiles
of the stereoisomers are similar. Closer examination, however,
reveals that many of the resonance peaks of the E isomer are
downfield-shifted from those of its Z counterpart. The most
obvious spectral difference lies in the chemical shift region of
7.04−7.14 ppm. The E isomer resonates at δ 7.09 (Figure 1A),
where the Z isomer is silent (Figure 1B). On the other hand,
the Z isomer exhibits a big resonance peak at δ ≈ 7.06. These
2. RESULTS AND DISCUSSION
Preparation of Pure Stereoisomers. While the McMurry
coupling is a powerful and versatile reaction for the syntheses of
TPE derivatives, the products are commonly mixtures of E and
Z stereoisomers. Their separations by usual purification
processes have been very difficult, if not impossible, due to
the similarities in the molecular shapes and structural polarities
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(Scheme 1), respectively, thereby unambiguously corroborating
their stereochemical structures.
AIE Phenomena. After obtaining the pure E and Z isomers
of BPHTATPE, we studied their photophysical processes. As
can be seen from Figure S6 (SI), the dilute solution of the E
isomer in THF shows an absorption maximum (λmax) at ∼332
nm. When water is mixed with THF, the absorption spectrum
of the isomer remains practically unchanged until the amount
of water in the aqueous mixture reaches a volume fraction (fw)
larger than 60 vol %. In the mixture with a high water content
(fw > 60%), the λmax of the E isomer is blue-shifted to 319 nm.
Meanwhile, the spectrum starts to show a tail in the long
wavelength region. Similar spectral changes are observed in the
case of the Z isomer (Figure S7, SI).
In the THF/water mixtures with high water content (fw
>
60%), the BPHTATPE molecules must have aggregated, due to
their immiscibility with the polar media. The blue-shifts in the
λmax values of the E and Z stereoisomers (Δλmax > 10 nm)
indicate that their molecules take a more twisted conformation
in the aggregates, in agreement with the conclusion from a
recent theoretical study.13 The spectral tails in the long
wavelength region, on the other hand, are caused by the Mie or
light-scattering effect,4−7 implying that the BPHTATPE
aggregates are nanoparticles in dimensionality (vide infra).
Variations of the PL spectra of (E)-BPHTATPE with fw in
the THF/water mixtures are shown in panel A of Figure 2.
Figure 1. 1H NMR spectra of (A) (E)-BPHTATPE, (B) (Z)-
BPHTATPE, and (C, D) (E)-BPHTATPE after irradiation by a UV
light of 365 nm for (C) 11 and (D) 70 min.
differences make it possible to monitor conformation changes
of the isomers by NMR spectroscopy.
In addition to the spectroscopic techniques, we looked for
other means to substantiate our structural assignments. We
tried to grow single crystals of the isomers, but our effort was in
vain. BETPE (a starting material) could crystallize, but the
resultant crystals were mixtures of its E and Z isomers. We
carefully inspected the crystals and found that they contained
two distinct forms: smaller needles and bigger chunks. We
picked up some of the needles and chunks from the cocrystal
mixtures and analyzed their structures by X-ray diffraction
(XRD). The XRD crystallography data reveal that the needles
and chunks are the single crystals of E (CCDC 834092) and Z
(CCDC 834091) stereoisomers of BETPE, respectively.12 We
carefully separated and collected sufficient amounts of the E
and Z stereoisomers by hand and used them as starting
materials to carry out model reactions using the same Click
procedures (Scheme 2). The products obtained from the model
reactions using BETPE with E and Z conformations gave NMR
spectral data identical to those of the E and Z isomers separated
from the BPHTATPE mixture by column chromatography
Figure 2. (A) Examples of PL spectra of (E)-BPHTATPE in the
THF/water mixtures with different fractions of water (fw); λex = 332
nm, [BPHTATPE] = 10 μM. (B) Changes in the fluorescence
quantum yields (ΦF) of (E)- and (Z)-BPHTATPE with variations in
fw in the aqueous mixtures; ΦF estimated using quinine sulfate in 0.05
M H2SO4 (ΦF = 54.6%) as standard. (Inset) Fluorescence images of E
and Z isomers of BPHTATPE in THF (fw = 0%) and aqueous
mixtures (fw = 60−90%).
Scheme 2. Syntheses of Pure Stereoisomers of BPHTATPE
from Corresponding Pure Stereoisomers of BETPE by Click
Reactions
When excited by a UV light of 332 nm, the signals collected
from the solution of the isomer are weak, and the plot of these
signals vs wavelength is practically a flat line, indicating that the
isomer is virtually nonfluorescent when it is molecularly
dissolved in its good solvent. Not until the fw is increased to
>60 vol % has the spectrum started to change. The decreased
solubility of the isomer in the aqueous mixture with high water
content induces aggregate formation, which turns the
fluorescence on. In other words, the emission is induced by
aggregation, or the isomer is AIE active. When the fw is further
increased, the emission is continuously intensified, accompany-
ing with a small red-shift in the spectrum. Similar AIE behavior
is observed in the case of the Z isomer (Figure S7B, SI).
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The formation of the luminogen aggregates in the aqueous
mixtures with high water content was confirmed by dynamic
light scattering (DLS) measurements (Figures S8−S11 in the
SI). As can be seen from the data shown in Figures S8 and S9
(SI) for (E)-BPHTATPE, large aggregates with an average
diameter (d) of ∼1400 nm are formed in the aqueous mixture
with fw = 70%. The size of the aggregates is decreased with an
increase in the water content, as previously observed in many
other AIE systems.4−7 At fw = 90%, the size of the aggregates is
decrease to d ≈ 81 nm. Similarly, the molecules of (Z)-
BPHTATPE are aggregated in the aqueous mixtures with fw >
70% (Figures S8 and S10 [SI]). In comparison to the size of
the aggregates of the E isomer, the average size of the
aggregates of the Z isomer formed at fw = 70% is much smaller
(d ≈ 527 nm), although the sizes of the particles in the
mixtures with higher fw values are similar.
The formation of large aggregates in the fw = 70% aqueous
mixture is probably associated with a dynamic crystallization or
organization process of the luminogen.7b,14a The larger d value
of the aggregates of the E conformer than that of its Z
counterpart indicates that the former is easier to crystallize or
organize than the latter, in good agreement with the fact that
the former is more readily isolated by the column separation
process, although it is difficult to grow single crystals of high
quality from the isomers. With an increase in fw, the solvating
power of the aqueous mixture is rapidly decreased. This makes
the crystallization or organization process difficult to proceed
and favors the formation of amorphous aggregates of smaller
sizes.
The PL spectrum of (E)-BPHTATPE changes with time in
the fw = 70% aqueous mixture; it is intensified initially and
weakened at later stages (Figure S12 in the SI). At fw = 90%,
however, almost no change in the PL spectrum was observed.
The Z isomer shows similar behavior (Figure S13, SI). These
PL spectral data support the discussion above. In the fw = 70%
mixture, the crystallization or organization process propagates
with time. Accompanying the process, the PL is enhanced, as
bigger crystals or assemblies are known to luminesce more
efficiently.4 Precipitation, however, occurs at some point when
the crystals or assemblies grow to some critical sizes, which
explains why the emission is weakened at the later stage. On the
other hand, the fine amorphous particles do not grow with time
at fw = 90%,4 which accounts for the time independence of the
PL spectra in this mixture (Figures S12C and S13C [SI]). Since
the PL spectrum changes with time in some aqueous mixtures
(e.g., at fw = 70%), in this work all the spectra were taken
immediately after the samples were prepared.
blue light in the THF/water mixture with fw = 70%. The PL
color is red-shifted to bluish-green when the water content in
the aqueous mixture is increased to 90%. The absolute PL
efficiencies of the two isomers in the solid state are measured to
be 100% by the integrating sphere technique.17 Repeated
measurements over a long period (2 h) give the similarly high
ΦF values with little fluctuation (Figure S15, SI), verifying the
reliability of the efficiency data and indicating that both of the
stereoisomers are highly efficient emitters in the solid state and
are photochemically stable under the integrating-sphere
measurement conditions.
Not only the pure isomers but also their mixtures are AIE
active. The UV, PL, and DLS data for an equimolar mixture of
the E and Z isomers are shown in Figures S8, S11, S14, S16 and
S17 in the SI as examples. The mixture is nonemissive with an
insignificantly small ΦF when dissolved in THF. Its emission is
greatly enhanced when the water content in the THF/water
mixture is increased to >60%. Its αAIE value at fw = 90% is 323,
well comparable to those of its counterparts of pure
stereoisomers. Similarly, the color of its emission is changed
from blue to bluish-green when the water content is increased
from 70% to 90%, as can be understood from the photographs
shown in the inset of Figure S17 in the SI.
EZI vs RIR. The difference in the NMR spectra of the pure E
and Z stereoisomers (cf., panels A and B in Figure 1) permits
spectroscopic study of their EZI processes. As (E)-BPHTATPE
is easier to obtain from the column separation of the reaction
mixture, its EZI process is investigated in detail in this work.
Irradiation of the E isomer with a UV lamp with a high power
(1.10 mW/cm2) readily yields the Z isomer (Table S1, SI), as
evidenced by the photoinduced spectral changes (Figures S18
and S19 [SI]). After the E isomer is exposed to the irradiation
for 11 min, new NMR signals, although weak in intensity,
appear at δ 7.237, 7.059, and 6.855 (Figure 1C), where the Z
stereoisomer resonates (cf., Figure 1B). The NMR signals are
intensified when the UV irradiation is prolonged to 70 min,
along with the appearance of new resonance peaks at δ 7.645,
6.930, and 6.900 (Figure 1D). As shown in Figure 3A, the Z
fraction in the photogenerated E/Z mixture is rapidly increased
to 35% in a nearly linear fashion in the first 50 min. Afterward,
the EZI rate slows down. At 150 min, the fraction of the Z
The AIE feature of (E)-BPHTATPE is further verified by the
trajectory of change in its fluorescence quantum yield (ΦF) in
the THF/water mixtures (Figure 2B). The ΦF values of its
solutions in the mixtures with fw ≤ 60% are negligibly small
(down to 0.12%). In the mixture with fw > 60%, aggregates
commence to form. As a result of aggregate formation, the ΦF
value is swiftly increased. At fw = 90%, the ΦF value is boosted
to 38.6%. Similar results are obtained in the case of the Z
isomer. An αAIE effect is defined as the ratio of the PL
efficiencies of an AIE luminogen in the aggregate (ΦF,a) and
solution (ΦF,s) states.15 The αAIE values for the E and Z isomers
are found to be 322 and 334, respectively. It is remarkable that
these values are much higher than that for HPS (αAIE = 220),
an iconic AIE luminogen.16
Figure 3. Variation in the Z fraction of BPHTATPE with time during
the course of (A) irradiation of a chloroform-d solution of its E
conformer by a UV light of 365 nm at room temperature or (B)
heating of the solid powder of its E conformer at 203 °C under
nitrogen.
As can be seen from the photographs shown in the inset of
Figure 2B, both the E and Z isomers of BPHTATPE emit a
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isomer reaches 50.5%. The fractions calculated using the
integrated areas of different resonance peaks give similar data
within experimental errors (Table S1, SI).
lamp in the spectrofluorometer for 30 min at an excitation
wavelength (λex) of 332 nm. The H NMR spectrum of the
1
irradiated sample is identical to that of the original one (Figure
S23A, SI), indicating that the EZI process has not occurred.
Even when the λex is changed to a shorter wavelength carrying a
higher energy (254 nm), still no spectral change is detected.
Similarly, no EZI process is observed after the Z conformer has
been exposed to the irradiation by the xenon lamp in the
spectrofluorometer (Figure S23C, SI).
The AIE phenomena of virtually all the AIE luminogens, in
particular the TPE derivatives, have been observed under the
PL spectral measurement conditions or under the excitation of
a xenon lamp of low power at room temperature for a short
while (commonly less than 1 min).4−7 As stated above, the EZI
processes of the E and Z isomers of BPHTATPE do not occur
even after they have been irradiated for a much longer time (30
min). Clearly, the EZI process is not involved in the AIE
process. The low power of the xenon lamp cannot break down
the CC bond, which is the critical initial step for the EZI
process. The emission of BPHTATPE in the solution state is
thus not quenched by the EZI process but the photoactivated
intramolecular rotations of the aromatic rotors. The drastically
boosted light emission in the aggregate state is caused by the
restricted motions of the aromatic rotors. In other words, the
RIR process is the mechanistic cause for the AIE activity of the
TPE-based luminogen.
It is known that the EZI process of an olefin can be caused
not only by UV irradiation but also by thermal treatment.18−20
The BPHTATPE isomers are resistant to thermal decom-
position. The temperatures for 5% weight losses (Td’s) for the
E and Z isomers are 374 and 273 °C (Figure S20, SI), while the
melting points (Tm’s) for the former and the latter are 202 and
163 °C, respectively. These thermal analysis data suggest that
the E conformer takes a more regular molecular packing
structure in the solid state. The wide gaps between the Td and
Tm values (ΔT = 110−172 °C) of the isomers allow the studies
of their thermally induced EZI processes without worrying
about the interference from the thermolysis process.
1
No change in the H NMR spectrum is detected after the E
isomer has been heated to and kept at 180 °C under nitrogen
for 30 min (Figure S21, SI). However, when it is heated at 203
°C, a temperature above its Tm, the EZI process takes place, as
revealed by the NMR data (Figure 4B as well as Figure S22 and
Self-Organization. During the study of the AIE process of
(E)-BPHTATPE in the aqueous medium, white flocculent
precipitates were found to appear in the THF/water mixture
with fw = 70% (vide ante). This implies the formation of some
sort of organized structure.21 We thus used the scanning
electron microscope (SEM) to examine the morphological
structure of the white precipitates. As can be seen from Figure
5A, the precipitates are one-dimensional microribbons with
diameters down to a few hundred nanometers and lengths up
to several tens of micrometers.
1
Figure 4. H NMR spectra of (A) (E)-BPHTATPE (pure isomer),
(B) (E)-BPHTATPE after heating at 203 °C for 90 min under
nitrogen, (C) (Z)-BPHTATPE (pure isomer), and (D) (Z)-
BPHTATPE after heating at 203 °C for 30 min under nitrogen.
Table S1 in the SI). The Z fraction is increased with an increase
in the heating time (Figure 3B), although the rate is slower
than that of its photoinduced EZI process. There is no
1
detectable change in the H NMR spectrum of the Z isomer
when it has been heated to 165 °C and kept in the molten state
for 30 min (Figure S21B, SI), indicating that the Tm is not high
enough to activate the stereochemical reaction of this isomer.
The EZI process, however, is triggered when the Z isomer is
heated at a higher temperature of 203 °C (Figure 4D).
The data presented above clearly show that the EZI process
of BPHTATPE can be induced by the irradiation with a UV
lamp of a high power (1.10 mW/cm2) or the thermal treatment
at a high temperature (203 °C). In the measurement of a PL
spectrum, however, a spectrofluorometer is operating at much
lower power (∼52 μW/cm2) and temperature (∼20 °C). Does
the EZI process occur under such mild conditions? To answer
this question, a chloroform-d solution of the E conformer of
BPHTATPE is put under continuous irradiation of the xenon
Figure 5. SEM microphotographs of the self-organized structures of
(E)-BPHTATPE formed in the THF/water mixtures (A) at fw = 70%
instantly, (B) at fw = 60% after standing for 1 week, and (C) the
precipitate obtained after dropwise addition of hexane into a
concentrated solution of (E)-BPHTATPE in chloroform. (D)
Fluorescent image of the microfibers taken under a fluorescence
microscope with a 337-nm excitation (sample from panel B).
E
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Many larger microfibers are formed when a THF solution of
(E)-BPHTATPE is diluted by a THF/water mixture with fw =
60% and then kept standing under ambient conditions for 7
days (Figure 5B). The microfibers are visible to naked eyes, ∼2
μm in diameter and several hundreds of micrometers in length.
The microfibers may be used as building blocks for the
construction of complex microstructures. Ordered assemblies
are also formed when hexane, a poor solvent of the E isomer, is
added dropwise into its concentrated solution in chloroform
under slow stirring. The formed white precipitates are
nanolumps with diameters of about 250 nm and lengths of
tens of micrometers, as revealed by the SEM analysis (Figure
5C). This is evidently a rapid process for the fabrication of
organic microstructures.
The organized microstructures of (E)-BPHTATPE, e.g., the
microfibers, emit blue fluorescence upon excitation with a UV
beam (Figure 5D). Glaring light emissions are seen at the ends
of the microfibers (highlighted by the red circles in the figure),
indicating that an optical waveguide effect is operating in the
light transmission process of the microfibers. The waveguide
effect is usually observed in an optically anisotropic system,
such as crystalline assembly. The formation of microcrystals is
thus the likely driving force for the (E)-BPHTATPE molecules
to self-organize. The molecules of its Z isomer, on the other
hand, can hardly form any microcrystalline structures, because
of their irregular conformation.
Figure 6. (A) Photographs of as-prepared (left) and ground (right)
samples of (E)-BPHTATPE taken under room lighting (upper) and
UV illumination (lower); G = grinding; H = heating (at 120 °C for 1
min). (B) Mechano- and thermochromic processes of (Z)-
BPHTATPE, with the photographs arranged in the same order as in
panel A. (C) PL spectra of (left) E- and (right) Z-conformers of
BPHTATPE before and after grinding (λex = 332 nm).
Chromic Processes. Organic materials with tunable light-
emitting behaviors have attracted much interest due to their
potential applications as sensors, memories, security inks, logic-
gate units, etc.22,23 While luminescence from an emitter can be
modulated by changing its molecular structure through
chemical reactions or altering its molecular packing through
external stimuli, the latter approach is advantageous in terms of
simple procedures and engineering operability. A propeller
shape is one of the most distinct structural features of an AIE
luminogen. The packing structure of such molecules is
susceptible to external perturbation, leading to a chromic
response.24 Since the molecule of BPHTATPE is also propeller
shaped, it is expected to be chromically active. Indeed, it shows
multiple chromic effects, including mechano-, piezo-, thermo-,
vapo-, and chronochromisms.
The as-synthesized (E)-BPHTATPE is an off-white solid
with a blue emission (Figure 6A). It changes to a pale-yellow
powder with a bluish-green emission after grinding, showing a
mechanochromic effect. Treating the ground sample at 120 °C
for 1 min transforms it back to the off-white solid with the blue
emission (noting that 120 °C is the lowest temperature at
which a rapid thermochromic process occurs; see Figure S24 in
the SI). The mechanochromism of the E isomer is further
verified by the change in its PL spectrum. The emission peak of
its as-obtained solid is located at 447 nm (Figure 6C). The
corresponding CIE coordinate of the emitted light (x = 0.15, y
= 0.12) is close to that of pure blue light, as defined by the
National Television Standards Committee.25 The isomer is thus
an excellent blue emitter. Its emission peak is red-shifted to 477
nm by grinding. This suggests a morphological transition from
crystalline to amorphous phase: an amorphous powder of an
AIE luminogen often emits a redder light than its crystalline
counterpart because its molecules usually take a more twisted
conformation in the crystalline state than in the amorphous
state.4−7,13,24
emission (460 nm) is redder than that of its E conformer (447
nm), implying that the solid is largely amorphous in nature, as
discussed above.4−7,14 Mechanical grinding of the solid causes
little change in its physical appearance or emission color. As can
be seen from Figure 6C, the grinding shifts its emission peak
from 460 to 470 nm, with a change of merely 10 nm. This is
reasonable because grinding a mainly amorphous solid should
not make a great change in the morphologic structure and,
hence, the fluorescence spectrum.
The different responses of the E and Z isomers to the
grinding offer an opportunity to tune the extent of the chromic
effect by changing the composition of the mixture of the two.
Taking the mixtures with E/Z ratios of 8:2, 5:5, and 2:8 as
examples, their emission peaks are shifted to 476, 474, and 472
nm by grinding, which respectively return to 452, 455, and 459
nm by heating (Figure S25, SI). The associated spectral shifts
are 24, 19, 13 nm, respectively, clearly demonstrating that the
bathochromic shift is readily tunable by the isomeric ratio.
Powder X-ray diffraction (XRD) pattern of the as-prepared
solid of (E)-BPHTATPE shows intense and sharp reflection
peaks (Figure 7A) due to its self-organized microcrystalline
structure. After grinding, the XRD profile becomes broader
with weaker reflection peaks (Figure 7B), indicating that the
ordered assembly has been randomized to a largely amorphous
mass. After heating at 120 °C, the sharp peaks in the XRD
pattern are recovered (Figure 7C), manifesting that the ground
sample has recrystallized into microcrystalline assembly upon
the thermal treatment at the high temperature. The XRD data
thus attest that the mechano- and thermochromisms are indeed
associated with the morphology transformations between the
crystalline and amorphous phases.
The XRD pattern of the as-obtained solid of the Z isomer is
composed of a few blunt “peaks” (Figure 7E), meaning that it is
mainly amorphous in nature or its crystallites are extremely
small in size. The grinding almost completely amorphizes the
solid, as reflected by the weak and broad XRD pattern of the
ground sample. Heating at the high temperature again recovers
The as-synthesized (Z)-BPHTATPE is a pale-yellow solid
with a bluish-green emission (Figure 6B). The color of its
F
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The isomer shows a novel chronochromic phenomenon: its
emission spectrum changes with time.24,26 The emission peak
of its ground sample is hypsochromically shifted by 8 nm after
it has been kept under ambient conditions for 2 h (Figure 9A).
Figure 7. XRD patterns of as-prepared powders of (A) E-BPHTATPE
and (E) Z-BPHTATPE and their samples after (B, F) grinding, (C, G)
heating (at 120 °C for 1 min), and (D) pressurization (at 30 MPa for 1
min; for E isomer only). G = grinding, H = heating, and P =
pressurization.
Figure 9. (A) Hypsochromic shift of PL spectrum of ground (E)-
BPHTATPE with elapse of time at room temperature; λex = 332 nm.
(B) Plot of emission peak vs standing time. (Inset) Semilog
relationship between peak wavelength and elapse of time.
the morphological structure (Figure 7G). The XRD patterns of
the E and Z isomers (cf., panels A and E in Figure 7) clearly
demonstrate the differences in their crystallization capability,
which account for their obviously different chromic behaviors.
Because of the more pronounced chromic effects of the E
isomer, its chromic processes are studied in greater detail. Its
emissions can be reversibly switched between two states (447
and 477 nm) by the grinding−heating cycle with almost no
fatigue (Figure 8). Besides grinding, pressurization can also
The spectrum continues moving to shorter wavelengths as time
elapses. The chronochromic process is very rapid at the
beginning and levels off at the end, while in the middle region,
the change in the PL peak follows a semilog relationship with
time (Figure 9B). The chronochromism hints that the ground
sample is in a metastable state, which transforms back to the
thermodynamically stable crystalline state quickly at 120 °C
and slowly at room temperature. The spontaneous recovery of
morphological structure makes the AIE luminogen a unique
self-healing optical material.
(E)-BPHTATPE further exhibits a solvent-dependent
vapochromic effect. Its ground sample is sensitive to volatile
polar solvents, such as chloroform, DCM, and THF. After
exposure to the vapor of chloroform for as short as 1 min, the
bluish-green emission of the ground sample of the E isomer is
quickly reverted back to the blue emission of its crystals, owing
to the fuming-induced crystallization (Figure 10A). When
exposed to acetone, the vapochromism process becomes
slower, with the blue emission being recovered in ∼3 min.
Figure 8. Reversible switching of emission of (E)-BPHTATPE by
repeated grinding−heating (red line) or pressurization−heating (blue
line) cycle; heating: 120 °C, 1 min; pressurization: 30 MPa, 1 min; λex
= 332 nm.
cause a red-shift in the PL spectrum of the isomer, although the
shift is small (∼8 nm; Figure S26, SI). The small change in the
XRD pattern induced by the pressurization (Figure 7D)
explains why the piezochromic effect is moderate in extent. The
reversibility of the piezochromism is again superb (Figure 8).
The great difference in the extent of the mechano- and
piezochromic effects tells that, in comparison to compression
(pressurization), shearing (grinding) is a more powerful force
to bring about a larger change in the morphological structure
and hence the emission spectrum.
Figure 10. Fluorescence images of (A) a sphere of ground (E)-
BPHTATPE with its right side fumed with chloroform vapor for 1
min, (B) a spray coating of the E isomer on a filter paper, then the
letters of “ZJU” were written on the coating paper with a spatula, and
subsequently the coating paper was treated by heating or vapor fuming
(letters of “ZJU” becoming invisible or erased).
G
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On the other hand, the ground sample is strongly resistant to a
nonpolar solvent (e.g., hexane); fuming with the vapor of such
a solvent for 10 min brings about no obvious change in the
emission color. The distinct responses of the ground sample to
the different fuming processes make it a promising chemo-
sensor for discriminating solvents with different polarities.
It is envisioned that judicious utilization of the multiple
chromic effects of (E)-BPHTATPE will lead to an array of
technological applications. An example of such applications is
demonstrated in Figure 10. When the powder of the isomer is
sprayed on a filter paper as a thin film, it emits a blue light upon
excitation with a hand-held UV lamp (Figure 10B). Letters of
“ZJU” are written by gently scratching the film with a spatula,
and due to the writing-induced mechanochromism the letters
emit a bluish-green light. Heating at 120 °C or fuming with a
polar solvent erases the letters by converting the light emission
back to the background emission. No apparent change in the
filter paper is visible under normal room lighting conditions in
the whole process. The letters are invisible under room lighting
but become visible with UV illumination. The isomer thus may
be used as a security ink, and its reversible chromisms may be
utilized to construct rewritable information storage systems.
advanced photonic devices employing the microfibers as the
building blocks.
Both the conformers undergo multiple chromisms, with the
E isomer showing more marked chromic effects, again owing to
its better organizability or crystallinity. A simple grinding gives
rise to a large bathochromic shift in its emission color
(mechanochromism). Pressurization can also cause a red-shift
in its emission color (piezochromism), though to a lesser
extent. Heating of the ground or pressurized sample at a
moderate temperature for a short while (120 °C, 1 min) can
recover its emission color (thermochromism) with excellent
reversibility. PL restoration of the ground sample proceeds with
a semilog dependence on time at room temperature
(chronochromism), exhibiting a novel optical self-healing
effect. Fuming of the ground samples with the vapors of
polar solvents can readily turn their emission colors back
(vapochromism).
The multiple chromisms are associated with the distinctive
structural feature of the AIE luminogens. The packing of their
molecules is often loose, due to their propeller shape. Such
molecular packing can be readily tuned, which in turn easily
activates the phase transition or morphological transformation.
The chromic responses of the AIE luminogens to the multiple
external stimuli make the molecules promising candidates for
advanced “smart” materials, which may find a wide variety of
high-tech applications in miniature photonic devices, chemical
sensors, biological probes, security inks, logic gates, optical
displays, information storage, etc.
3. CONCLUSIONS
Synthesis of stereopure conformers of TPE derivatives has been
a difficult task. In this work, through rational structure design,
we have successfully synthesized a pair of triazole-functionalized
TPE derivatives, i.e., (E)- and (Z)-BPHTATPE, by the copper-
catalyzed Click reaction and separated their pure conformers by
the commonly used column chromatography technique. Their
precise stereostructures are duly verified by the spectral
characterizations and by the model reactions using stereopure
BETPE isomers as the precursors, whose absolute conforma-
tions have been determined by the crystallographic analysis.
Both of the E and Z isomers show pronounced AIE effects, with
αAIE ≥ 320 and ΦF = 100% in the solid state.
The acquisition of the pure stereoisomers has enabled us to
study their EZI processes without ambiguity and gain useful
insights into the AIE mechanisms of TPE-based luminogens.
Our study reveals that the EZI process readily occurs when a
BPHTATPE isomer is irradiated with a high flux UV lamp or
heated at a high temperature. Under the conditions of normal
PL spectral measurements, however, no EZI process can be
detected even after the isomer has been put under irradiation
by the xenon lamp in a standard spectrofluorometer for as long
as 30 min. On the basis of the data collected in this work and
the data obtained from our previous investigations,4−7 we can
conclude that the RIR process is the main cause for the novel
AIE process of a TPE-based luminogen. The clear picture of the
AIE mechanism will facilitate the structural design of new AIE
luminogens and the exploration of their potential high-tech
applications.27,28
4. EXPERIMENTAL SECTION
Materials and Instrumentation. Chemicals, reagents, and
solvents used in this work were all purchased from Acros, Alfa,
and Aldrich, unless otherwise specified. THF was purified by
the simple distillation from sodium benzophenone ketyl in an
atmosphere of dry nitrogen immediately prior to use. Other
solvents were purified by the standard procedures.
NMR spectra were measured on a DMX-500 spectrometer
(Brulcer) in chloroform-d using TMS (δ = 0) as the internal
standard. IR spectra were taken on a Vector 22 spectrometer
(Bruker). Elemental analysis was performed on a Thermo-
Finnigan Flash EA1112. MALDI-TOF mass spectra were taken
on a Waters GCT Premier GC-TOFMA mass spectrometer.
Absorption spectra were measured on a Varian CARY 100 Bio
spectrophotometer. PL spectra were recorded on a Perkin-
Elmer LS 55 spectrofluorometer. Relative ΦF values were
estimated by using quinine sulfate in 0.05 M sulfuric acid (ΦF =
54.6%) as standard. The absorbance of the solution was kept at
∼0.05 to avoid internal filter effect. Absolute ΦF values were
determined by a calibrated integrating sphere (see the SI for
details).17
DLS measurements were conducted on a Brookhaven 90
Plus Particle Size Analyzer. Thermal stability was evaluated by
taking thermograms on a Perkin-Elmer TGA 7 at 20 °C/min
under nitrogen. Tm was measured on SGW X-4 micro melting
point apparatus. EZI process was studied using a Spectroline
UV lamp. Self-organization microstructure was checked by a
SIRION-100 (FEI) SEM. Fluorescence micrographs were taken
on a Carl-Zeiss LSM 10 META confocal laser scanning
microscope. Powder XRD patterns were recorded on X’pert
PRO, PANalytical, with Cu Kα radiation operating at 40 kV
and 40 mA.
The pure isomers have permitted the examination of their
structure−morphology−function relationships. The symmetric
and asymmetric shapes of the E and Z isomers significantly
affect their molecular packing, with the former having much
better organizability. The Z isomer exhibits poor assembling
and crystallization capability, while the molecules of its E cousin
can self-organize into ordered microstructures, such as
microfibers and nanorods. Glaring light emmissions from the
two ends of the microfibers occur upon photoexcitation. The
optical waveguiding effect may allow the construction of
For the AIE measurement, a stock solution of a luminogen in
THF (0.1 mM) was prepared. Aliquots of this stock solution
H
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were transferred into volumetric flasks (10 mL), into which
appropriate volumes of THF and water were added dropwise
under vigorous stirring to give 10 μM solutions with different
water contents (fw = 0−90 vol %). UV and PL spectra were
measured immediately after the solutions were prepared.
Synthesis and Characterization. The synthetic route to
BPHTATPE is shown in Scheme 1. BETPE and POHA were
prepared according to our previously published procedures,5a
with all their characterization data matching the literature data.
The model reactions (Scheme 2) were carried out in a similar
manner. The detailed procedures for the preparation and
separation of the E and Z isomers of BPHTATPE as well as
their characterization data are given below.
BPHTATPE with time, change in its ΦF with fw in the
aqueous mixtures, NMR spectra of the BPHTATPE exposed to
UV and xenon lamps and heated to high temperatures, its TGA
thermograms, and mechano-, thermo-, and piezochromisms of
its isomers and mixtures. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
sunjz@zju.edu.cn (J.Z.S.); qinaj@zju.edu.cn (A.Q.); tangbenz@
ust.hk (B.Z.T.)
■
Notes
The authors declare no competing financial interest.
Into a 50 mL Schlenk tube were added BETPE (0.380 g, 1.0
mmol), POHA (0.658 g, 3.0 mmol), and THF (10 mL). After
complete dissolution of the starting materials, 2 mL of water
and freshly prepared aqueous solutions of SA (1 M, 200 μL, 10
mol %) and CuSO4 (1 M, 100 μL, 5 mol %) were added into
the tube under vigorous stirring. The color of the solution
turned to brown, orange, and then yellow. The reaction mixture
was stirred at room temperature overnight. After the precipitate
was filtered and THF was evaporated, the residue was extracted
by DCM three times, washed with saturated ammonium
chloride solution, brine, and water, and dried over magnesium
sulfate. After filtration and solvent evaporation, the crude
product was purified by a silica gel column using a DCM/
acetone mixture (100:1 by vol) as the eluent. White (E isomer)
and pale-yellow (Z isomer) powders were obtained in a total
yield of 89.0% (0.729 g), with the E/Z ratio varied from batch
to batch.
ACKNOWLEDGMENTS
■
This work was partly supported by the Natural National
Science Foundation of China (21074113, 50873086, 20974028,
and 20974098), the Ministry of Science and Technology of
China (2009CB623605), the Research Grants Council of Hong
Kong (603509, HKUST2/CRF/10, and N_HKUST620/11),
and the University Grants Committee of Hong Kong (AoE/P-
03/08). B.Z.T. thanks the support from Cao Guangbiao
Foundation of Zhejiang University. We thank Prof. Yuguang
Ma and Dr. Ping Lu for the measurement of the solid-state
quantum yields.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Text describing the procedures for measuring the absolute ΦF
in the solid state. Figures showing the FTIR, NMR, HRMS,
UV, and PL spectra of the pure isomers of BPHTATPE and
their mixtures, variations in the sizes and size distributions of
their aggregates with fw in aqueous mixtures, time dependences
of their PL spectra, variations in the solid-state ΦF of
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