Full-color tunable mechanofluorochromism and excitation-dependent emissions of single-arm extended tetraphenylethylenes

Article abstract of DOI:10.1039/c5tc02623g

We propose a new single-arm extension strategy on traditional tetraphenylethylene and successfully develop a new series of full-color (from ～450 nm to ～740 nm) tunable mechanofluorochromic materials. These materials exhibit efficient solid-state emission (quantum yield Φ_f > 10%) and high mechanofluorochromic contrast (wavelength shift from ～50 nm to ～100 nm). More importantly, we discover an unexpected excitation-dependent emission phenomenon of mechanofluorochromic materials and propose to utilize this new excitation-dependent emission behavior of materials to evaluate their mechanical-responsive performances more comprehensively. Finally, the unique feature of abundant emissions of mechanofluorochromic materials by changing the excitation light has shown application potential in dual channel anti-counterfeiting.

Full text of DOI:10.1039/c5tc02623g

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Full-color tunable mechanofluorochromism and
excitation-dependent emissions of single-arm
extended tetraphenylethylenes†
Cite this:DOI: 10.1039/c5tc02623g
Yi Wang, Ivan Zhang, Binhong Yu, Xiaofeng Fang, Xing Su, Yu-Mo Zhang,
Ting Zhang, Bing Yang, Minjie Li* and Sean Xiao-An Zhang*
We propose a new single-arm extension strategy on traditional tetraphenylethylene and successfully develop a
new series of full-color (from B450 nm to B740 nm) tunable mechanofluorochromic materials. These
materials exhibit efficient solid-state emission (quantum yield Ff 4 10%) and high mechanofluorochromic
contrast (wavelength shift from B50 nm to B100 nm). More importantly, we discover an unexpected
excitation-dependent emission phenomenon of mechanofluorochromic materials and propose to utilize this
new excitation-dependent emission behavior of materials to evaluate their mechanical-responsive performances
more comprehensively. Finally, the unique feature of abundant emissions of mechanofluorochromic materials by
changing the excitation light has shown application potential in dual channel anti-counterfeiting.
Received 23rd August 2015,
Accepted 31st October 2015
DOI: 10.1039/c5tc02623g
www.rsc.org/MaterialsC
investigations on the manipulation of mechanofluorochromic
properties, such as the emission color range, quantum yield,
Introduction
Mechanofluorochromic materials are a unique family of smart mechanofluorochromic contrast and reversibility. Most of
materials, which can respond to human-implementary stimuli the present mechanofluorochromic materials are discovered
of mechanical forces (such as grinding, shearing, stretching or serendipitously.46,47 Another crucial problem is that the con-
pressing) and have great application potential in mechano- ventional evaluation standard on mechanofluorochromic
sensors, memory chips and security ink.1,2 In contrast to the materials is very defective, which only uses the emission
photo- and electro-chromic materials which respond to the changes (position or intensity) of the sample ensembles under
stimuli mainly by unimolecular chemical structural transition,3 an arbitrary excitation wavelength to define whether they are
most of the mechanofluorochromic materials work on the mechanical force-responsive or not.5–39 Then the question
change of intermolecular interactions in nature.4 Therefore, is raised why the emissions of non-mechanofluorochromic
the investigation on mechanofluorochromic behavior of materials do not change after their macroscopic states (such
materials is more challenging, which needs to consider not as crystal sizes, crystallinity, surfaces and defects) have changed
only the molecule itself but also its complicated surroundings in under mechanical treatment. Are the emissions of non-
the condensed solid state. Although many families of mechano- mechanofluorochromic materials really inert to the mechanical
fluorochromic materials5–39 have been developed and several forces? With these considerations in mind, our attention
mechanistic explanations (such as stress-induced crystal-to- has been paid to searching for a general strategy to develop
amorphous transition,40,41 crystal-to-crystal transition42,43 and full-color tunable mechanofluorochromic materials with high
chemical changes44,45) have been proposed, there are still some contrast and establishing a more vivid semi-quantitative evalua-
important unresolved fundamental problems that restrict their tion standard on mechanofluorochromic properties to promote
further development and applications. First of all, the general the development of mechanofluorochromic materials.
principles on the design of mechanofluorochromic materials
are quite inadequate, and there are very limited systematic
Results and discussion
State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
A single-arm extension strategy on tetraphenylethylene for
fabricating full-color tunable mechanofluorochromic materials
Changchun, Jilin 130012, China. E-mail: liminjie@jlu.edu.cn,
seanzhang@jlu.edu.cn
† Electronic supplementary information (ESI) available: Detailed experimental
procedures and characterization data for the mechanofluorochromic materials.
CCDC 1053197 and 1053198. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5tc02623g
The study begins with the famous non-mechanofluorochromic
tetraphenylethylene (TPE) molecule. It is well-accepted that
the lack of crystal-to-amorphous phase transition during the
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grinding process is responsible for its non-mechanofluorochromism (Fig. 1b), and their XRD patterns do not show obvious changes
because of the quick re-crystallization driven by multiple inter- after mechanical grinding (Fig. 1c). Molecules with increasing
molecular interactions (e.g. C–HÁ Á Áp).48 Here, we propose that molecular anisotropy (LSAR 4 1.10), such as TPE-OCH3, TPE-Phen
the rigid near-spherical geometry of TPE (Fig. 1a left) is another and TPE-Diphen (Fig. 2c), gradually exhibit mechanofluoro-
critical factor that facilitates its fast recrystallization because chromic properties (Fig. 1d) and have partial amorphous
rigid near-spherical molecules can well consume the external phases in their XRD patterns after mechanical grinding
stress by molecular elastic deformation and relatively free (Fig. 1e). Especially for TPE-Diphen (a LSAR of 1.75) (Fig. 2c
sliding or rolling. After the stress is released, molecules can and Fig. S1, ESI†), there is a shift of 14 nm in the emission
quickly restore their preferred conformations and original inter- spectra (lEx = 365 nm) between its unground and ground states
molecular interactions without hindrance from molecular volume (Fig. 1d), which indicates that molecular structural anisotropy
(Video 1, ESI†). Hence, we envision that if the near-spherical can dynamically restrict the recrystallization and keep molecules
geometry of TPE is broken to anisotropic symmetry (Fig. 1a right), in their metastable amorphous phases, which finally causes the
the molecular mobility will be greatly restricted. Consequently, detectable mechanofluorochromism.
external mechanical stress could not well be released and
To further test the universality of the single-arm extension
would induce molecular plastic deformation and phase transi- strategy and study the effects of some molecular inherent proper-
tion in the crystal, which might lead to detectable mechano- ties on mechanofluorochromic performances, another group of
fluorochromism (Video 1, ESI†).
six single-arm extended TPE derivatives (group II) with LSAR
To verify our hypothesis, a general single-arm extension larger than 1.50 is prepared and they have variable electron-
strategy is proposed to break the near-spherical symmetry of withdrawing and electron-donating substituent groups, conjuga-
TPE (Fig. 1a). Firstly, five single-arm substituted TPE derivatives tion lengths and molecular dipoles (Fig. 2a and Fig. S2, ESI†).
(group I) with increasing occupied volume in the substituent
groups were synthesized (Fig. 1a) and the molecular long-to-
short axis ratio (LSAR) (Fig. S1 and S2, ESI†) was used to
quantitatively define the molecular geometrical anisotropy
(Fig. 1a right and 2c). Then the mechanofluorochromic proper-
ties of these molecules are investigated in detail. It is found that
molecules with small anisotropy (LSAR less than 1.10), such as
TPE, TPE-Br and TPE-CH3 (Fig. 2c), do not exhibit mechano-
fluorochromic properties under an excitation light of 365 nm
Fig. 2 (a) Chemical structures of six single-arm extended TPE derivatives
(group II). (b) Fluorescence spectra of the six TPE derivatives in unground
and ground states (the former four samples are excited with a light of
365 nm and the latter two samples are excited with a light of 500 nm).
Fig. 1 (a) Chemical structures of TPE and its five single-arm substituted
derivatives (group I), the molecular short axis and the long axis are defined
as in TPE-Diphen and shown in the ESI.† (b) Fluorescence spectra and (c)
XRD patterns of TPE, TPE-Br and TPE-CH3 in unground and ground states.
(d) Fluorescence spectra and (e) XRD patterns of TPE-OCH3, TPE-Phen
and TPE-Diphen in unground and ground states.
(c) The relationship between the mechanofluorochromic contrast (emission
peak shift) and molecular long-to-short axis ratio of TPE and its group I
derivatives. (d) The relationship between the mechanofluorochromic con-
trast (emission peak shift) and the molecular dipole moment of group II
TPE derivatives (black dots), the molecular long-to-short axis ratio is also
indicated (red dots).
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Table
1
Physical properties of single-arm extended TPE derivatives
(group II) in THF solution and solid states
THF solutiona
Solid state
Unground
Ground
em
max
em
max
em
l
Ffb
tc
l
Ffb
tc
l
max
Ffb
tc
Compounds (nm) (%)
(ns) (nm) (%)
(ns) (nm) (%)
(ns)
TPE-VB
TPE-VBA
TPE-VBN
TPE-VBVBN 555
TPE-VBI 684
487 B0
0.49 453 77.77 2.23 468 45.77 2.49
535
560
0.68 0.15 494 56.88 2.67 514 57.54 2.41
22.55 1.25 498 14.59 1.17 541 19.75 2.62
62.44 1.46 539 23.62 2.20 582 11.00 3.34
1.54 0.38 603 11.19 1.45 678 15.27 2.33
TPE-VBVBI 672 B0
0.41 638 10.19 1.25 707
3.71 2.23
a
b
The concentration of THF solution is 1 Â 10À5 mol LÀ1
.
Fluores-
c
cence quantum yield. The weighted-average lifetime was calculated
by: 1/t = A1/t1 + A2/t2.
Fig. 3 (a) Kubelka–Munk diffuse reflectance absorption spectra, (b)
fluorescence spectra (lEx = 365 nm), (c) powder X-ray diffraction (PXRD)
patterns and (d) DSC curves of TPE-VB in different states.
The optical properties of these six single-arm extended TPE
derivatives in solution and aggregation states can be found
in the ESI† (Fig. S5–S7). The mechanofluorochromic processes
of group II TPE derivatives were investigated in detail with
Kubelka–Munk diffuse reflectance absorption spectrometry,
fluorescence spectrometry, powder X-ray diffraction (PXRD) and
differential scanning calorimetry (DSC) (Fig. 3 and Fig. S8–S12,
ESI†). Encouragingly, these six single-arm extended TPE deriva-
tives (group II) all exhibit good mechanofluorochromic properties
with a maximum emission shift of 15 nm to 75 nm with the
same excitation wavelength, no matter whether the substituent
is electron-withdrawing or electron-donating (Fig. 2b). Take
TPE-VB as an example, after mechanical grinding, its absorption
sideband red-shifts (Fig. 3a) and the corresponding fluorescence
turns to cyan (Em: 468 nm) from blue (Em: 453 nm) (Fig. 3b).
PXRD patterns clearly show that there is a crystal-to-amorphous
phase transition involved in the mechanofluorochromic process
(Fig. 3c) and the recrystallization temperature of the amorphous
ground TPE-VB is around 80 1C (Fig. 3d). The same crystal-to-
Fig. 4 Crystal packing of TPE-VBN (up) and TPE-VBVBN (down) (white
arrows represent the direction of the molecular dipole).
amorphous phase transition has been observed in the mechan- intermolecular interactions is one common contribution to
ofluorochromic processes of other five group II TPE derivatives the blue-shift emissions of group II TPE derivatives, and it is
(Fig. S8–S12, ESI†). Therefore, we believe that the single-arm consistent with other TPE derivatives of the crystal.50 When
extension strategy on TPE is a generally applicable design principle external force is exerted on the well-crystallized samples, the
for enabling TPE derivatives to exhibit mechanofluorochromism crystal phase will transform into an amorphous one, in which
by the crystal-to-amorphous phase transitions.
molecular internal tension could be well released to make
Then the origin of mechanofluorochromic performances is molecules take relatively planar conformations, which will
studied from molecular structural analysis. Through compar- contribute to the red-shift emission in ground samples. The
ing the fluorescence spectra of group II TPE derivatives in THF red-shifted absorption spectra of ground group II TPE deriva-
solutions and crystal states, we find that emissions in the crystal- tives further confirm that these ground samples have more
state all have a blue-shift compared with those in solution no planar conformations with the increased conjugation (Fig. 3c
matter what the substituent group is (Table 1). It is well accepted and Fig. S8–S12, ESI†).51
that molecular conformation will drastically affect molecular
For the molecules with large molecular dipoles (such as TPE-
fluorescence, in general, planar conformation will result in red- VBN and TPE-VI), they are typical electron donor–p-bridge–
shift fluorescence.49 Here, we believe that the twisted molecular electron acceptor (D–p–A) structured luminogens with obvious
conformation (as shown in Fig. 4) of the TPE moiety by intramolecular charge transfer (ICT) (Fig. S3, ESI†).52–54 It is
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well known that ICT fluorescence is more sensitive to environ- we turn our attention to improving the evaluation standard on
mental polarity than localized state emission. For example, the mechanofluorochromic performances. For fluorescent materials,
ICT fluorescence in solution significantly red-shifts when the the excitation and emission spectra are equally important. But the
polarity of the solvent increases due to the strong solute-solvent current evaluation standard on mechanofluorochromic materials
dipole–dipole interactions.55 The crystal structures of TPE-VBN only focuses on the emission spectra, which is doomed to miss
and TPE-VBVBN clearly show that these strong polar molecules some important underlying information in mechanofluoro-
are stabilized by dipole–dipole interactions in their crystal chromic processes. Taking TPE-VB as an example, if only the
packing (Fig. 4). In crystal, molecules are surrounded by the fluorescence spectra under the same excitation light (365 nm in
solid solvent of themselves. Therefore, for molecules with large our experiment) are considered, it shows excellent mechano-
dipoles (such as TPE-VBN and TPE-VI), they are viewed as in a fluorochromic reversibility after annealing and solvent fuming
polar environment. The generally increased fluorescence times the ground samples (Fig. 3). However, when we study in detail
of ground group II TPE derivatives (Table 1) indicate that the excitation spectra of TPE-VB in different states, it is found
grinding has compacted molecules to be closer to strengthen that the excitation spectra of annealed and solvent fumed
their dipole–dipole interactions (or dipolar coupling), which samples actually cannot recover to the original state (Fig. 5a).
could be seen as ICT molecules are placed in a more polar In addition, the excitation spectrum of the ground sample has a
solvent. Therefore, molecules with large molecular dipoles shoulder peak besides the main peaks (Fig. 5b black curve).
(such as TPE-VBN and TPE-VI) will exhibit significant red- When the ground sample is excited with the two excitation
shifted fluorescence with high mechanofluorochromic contrast peaks (380 nm and 420 nm), the corresponding emission spectra
when they are ground. In contrast, the molecules with small have a large shift of 24 nm (Fig. 5b red and blue curves). When
molecular dipoles (such as TPE-VB and TPE-VBA) would not the excitation light is finely tuned from 300 nm to 460 nm, the
have obvious intramolecular charge transfer (ICT) and dipole– maximum emission peaks of ground TPE-VB have a large
dipole interactions in the crystal. Their stress-induced red-shift difference of 41 nm (Fig. 5d). But for the unground crystalline
fluorescences only come from molecular planarity. As a result, sample, its emission peak does not change with excitation light
mechanofluorochromic contrasts of group II TPE derivatives (Fig. 5c). Further studies show that this unique excitation-
have positive correlation with their molecular dipole moments dependent emission still exists in the annealed and solvent
and molecules with large dipole moments (such as TPE-VBN fumed samples of TPE-VB (Fig. S19, ESI†), which indicates that
and TPE-VBI) will exhibit high mechanofluorochromic contrast
(black dots in Fig. 2d) even their molecular anisotropy is close
(red dots in Fig. 2d). However, further increasing conjugation
length (TPE-VBN vs. TPE-VBVBN and TPE-VI vs. TPE-VBVI) has no
obvious effect on the mechanofluorochromic contrast (Fig. 2d),
but has a positive effect on the red-shift of emission peaks
(Fig. 2b). So we propose that a large molecular dipole is effective
to increase mechanofluorochromic contrast and increasing the
conjugated degree is beneficial to the red-shift of fluorescence
spectra. Bringing together these strategies, the emissions of single-
arm extended TPE derivatives can be finely tuned to cover the full
visible light range with a high contrast of 15 nm to 75 nm (Fig. 2b).
By annealing or solvent fuming these ground samples, their
emission peaks can recover (or partially recover) under the same
excitation wavelengths (Fig. 3 and Fig. S8–S12, ESI†). In addition,
owing to the existence of the TPE moiety, these six single-arm
extended TPE derivatives well conquer the aggregation-caused
emission quenching (ACQ) effect. Except TPE-VBVI (emissions
in red and near-infrared region), the other five compounds all
show relatively high fluorescence quantum yields (Ff 4 10%)
no matter grinding them or not (Table 1). These unique features
(such as easy-fabrication, full-color tunable, high mechanofluoro-
chromic contrast and good quantum yield) of these six single-
Fig. 5 (a) Excitation spectra of TPE-VB at different emission wavelengths in
arm extended TPE derivatives will certainly endow them with
wide application potential.
different states. (b) Excitation spectrum of ground TPE-VB at an emission
wavelength of 468 nm (black curve) and emission spectra of ground TPE-VB
with the excitation light at 380 nm (red curve) and 420 nm (blue curve),
respectively. (c) Emission spectra of unground TPE-VB with different excita-
tion wavelengths (inset: the normalized emission spectra). (d) Emission
spectra of ground TPE-VB with different excitation wavelengths (inset: the
normalized emission spectra). (e) Illustration of the irreversible influence of
mechanical stress on mechanofluorochromic materials.
Excitation-dependent emissions of tetraphenylethylene and its
derivatives
After the successful design and development of full-color
tunable and high contrast mechanofluorochromic materials,
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stress-treated samples actually cannot restore to their original phenomenon for ground samples (Fig. 5e). Unfortunately,
states by traditional annealing and solvent fuming. Extended traditional annealing and fuming treatments could only induce
studies further assure us that this newly discovered excitation- partial recrystallization of ground samples and residual amor-
dependent emission phenomenon in the ground samples is not phous molecules from the surfaces, interfaces and defects still
a single case. Besides TPE-VB, TPE and its other derivatives in show excitation-dependent emission behavior, which indicates
our experiments all exhibit excitation-dependent emission that the effects of stress on molecular conformations are
features in the ground, annealed and solvent fumed samples destructive and not fully recoverable under annealing or solvent
(Fig. S13–S24, ESI†). We conjecture that these unrecovered fuming treatments owing to the low mobility of molecules in
parts of the optical properties should come from the diversified the confined solid state (Fig. 5e). Therefore, the excitation-
molecular conformations in grinding-derived surfaces, inter- dependent emission experiment can more directly reflect the
faces and defects.
effects of external stress on molecular conformations and will
To confirm our assumption, we systematically investigate be a more comprehensive method to evaluate the mechanical-
the excitation-dependent emission properties of conformation- responsive properties of fluorescent materials. By this method, we
changeable TPE derivatives and conformation-fixed planar can find that the previously defined non-mechanofluorochromic
molecules (Anthracene and Rhodamine 6G) in both solutions TPE is actually mechanofluorochromic when it is excited with
and solid polymer films. In THF solutions, the emissions of different wavelengths of excitation light (Fig. S13, ESI†). In contrast,
these two distinct types of molecules both obey Kasha’s rule the previously defined reversible mechanofluorochromic materials
(Fig. S25–S32, ESI†), namely, the emission wavelengths of their (e.g. TPE-VB) could be re-considered as not fully reversible (Fig. S19,
fluorescence are independent of the excitation light. This is ESI†). What is more, the excitation-dependent emission experiment
because molecules could freely rotate in solutions and they take can also enhance mechanofluorochromic contrast by selecting
the optimized conformations. However, significant differences different wavelengths of excitation light. Taking TPE-VB as an
appear in optical inert PMMA films. While the emission peaks example, the formerly obtained mechanofluorochromic wavelength
of Anthracene or Rhodamine 6G remain independent of shift (15 nm) with a single excitation wavelength (lEx = 365 nm)
the excitation light due to their fixed molecular conformations can be enhanced to 56 nm by changing the excitation wave-
(Fig. S33 and S34, ESI†), TPE and its derivatives amazingly show lengths (Fig. S19 and S47, ESI†), in this way, the maximum
excitation-dependent emission behavior presumably attributed contrast of the six single-arm extended TPE derivatives can
to their diversified molecular conformations, which are confined ranges from B50 nm to B100 nm (Fig. S47, ESI†). Therefore,
in the PMMA film during the solvent evaporation (Fig. S35–S46, by integrating our discovery of excitation-dependent emission
ESI†). These exciting results explicitly indicate that excitation with the conventional strategies on molecular structure modifi-
spectra or excitation-dependent emission spectra can be applied cation (e.g. changing alkyl lengths),56–60 applicable high contrast
to represent more detailed physical states of the ensemble mechanofluorochromic materials are expected.
sample in the solid state to discriminate semi-quantitatively
the diversifications of the molecular conformations.
Application potential of the excitation-dependent emission
phenomenon in dual channel anti-counterfeiting
According to the detailed studies on the excitation spectra
and excitation-dependent emission spectra of the fluorophores The abundant emissions of TPE and its derivatives by changing
during the mechanofluorochromic process, the mechanism of the excitation light unambiguously endow them with some new
mechanofluorochromism can be described more clearly. For application potential in multi-channel responses. Here, TPE-VB
the unground samples, they usually have a bulky crystal struc- is chosen as an example to demonstrate its application in dual-
ture with ignorable surfaces, interfaces and defects, in which channel anti-counterfeiting. While light of 365 nm can excite all
molecules have thermodynamically favorable directional align- the samples in the crystal, ground and recovered states with
ment with several alike molecular conformations. As a result, fluorescence switching between 453 nm and 468 nm (Fig. 3
they exhibit excitation-independent emission (Fig. 5e and and 6b), light of 460 nm could only excite the ground sample
Fig. S47, ESI†). Undoubtedly, mechanical grinding could destroy with emission at 509 nm (Fig. 6a and b), which indicates that
the ordered crystal packing and change molecular conforma- TPE-VB is also an off-to-on fluorescence switch. TPE-VB was
tions. But with the simple manual grinding it is very difficult to printed on a practice banknote as two Chinese characters of
make the original large and well-shape crystals transform into ‘‘5 yuan’’ and then annealed at 100 1C for several minutes for
amorphous phases completely. Here, we prefer to believe that crystallization on the banknote. The two Chinese characters
our observed ground amorphous samples from XRD measure- could not be observed under ambient light because TPE-VB
ments (Fig. 3 and Fig. S8–S12, ESI†) consist of an amount does not have obvious absorption in the visible light range;
of small microcrystals covered by multiple-layer amorphous however, it shows blue fluorescence under an excitation light of
molecules, in which various surfaces, interfaces and defects are 365 nm and non-fluorescence under an excitation light of 460 nm
non-ignorable. In contrast to a constrained environment in (Fig. 6b and c). This process could be thought of as encryption
microcrystals, molecules in amorphous surfaces, interfaces and for banknotes. When we rub the two Chinese characters, their
defects have a larger degree of freedom, and stress will induce encrypted information could be read by its dual fluorescence
them to form diversified molecular conformations, which signals of blue-to-cyan transition and green light turning on
finally give rise to the unique excitation-dependent emission under the excitation light of 365 nm and 460 nm, respectively.
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wavelengths of excitation light, as well as enabling the multi-
channel responses. Finally, we demonstrate the application
potential of the mechanofluorochromic materials in dual-
channel anti-counterfeiting. Results from our experiments would
not only deepen the understanding of mechanofluorochromism,
but also promote developing new application potential of
mechanofluorochromic materials.
Acknowledgements
We thank National Science Foundation of China (No. 51373068),
the program of Chang Jiang Scholars and Innovative Research
Team in University (No. IRT101713018) and Graduate Innovation
Fund of Jilin University (No. 2015017) for financial support.
Notes and references
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