Activating Versatile Mechanoluminescence in Organic Host–Guest Crystals by Controlling Exciton Transfer

Article abstract of DOI:10.1002/anie.202010166

Mechanoluminescence (ML) materials are attracting increasing interest owing to promising applications in various areas. However, to date, it remains a major challenge to develop a precise and universal route to achieving organic ML materials. Herein, we show that ML can be easily realized in organic piezophotonic host–guest crystals, under conditions in which neither the host nor the guest is ML-active. The experimental and theoretical results reveal that excitons of the host generated by piezoelectricity can be harvested effectively by the guest for light emission, owing to the restraint of intersystem crossing process. Moreover, different host–guest crystals are constructed, wherein the emission color, intensity, color purity, and emission duration of ML can be manipulated. This work deepens our understanding of organic ML generation in piezophotonic host–guest crystals and provides an inspiring principle to design more organic ML materials.

Full text of DOI:10.1002/anie.202010166

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Activating Versatile Mechanoluminescence in Organic Host-
Guest Crystals by Controlling Exciton Transfer
Authors: Wenlang Li, Qiuyi Huang, Zhan Yang, Xiaoyue Zhang,
Dongyu Ma, Juan Zhao, Chao Xu, Zhu Mao, Yi Zhang, and
Zhenguo Chi
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202010166
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10.1002/anie.202010166
Angewandte Chemie International Edition
RESEARCH ARTICLE
Activating Versatile Mechanoluminescence in Organic Host-
Guest Crystals by Controlling Exciton Transfer
Wenlang Li+,[a] Qiuyi Huang+,[a] Zhan Yang,[a] Xiaoyue Zhang,[d] [e] Dongyu Ma,[a] Juan Zhao,*[b] Chao
Xu,*[c] Zhu Mao,[a] Yi Zhang,[a] and Zhenguo Chi*[a]
[a]
W. Li,[+] Q. Huang,[+] Z. Yang, D. Ma, Z. Mao, Prof. Y. Zhang, Prof. Z. Chi
PCFM Lab, GDHPPC Lab, Guangdong Engineering Technology Research Center for High-performance Organic and Polymer Photoelectric Functional
Films, State Key Laboratory of OEMT,
School of Chemistry, Sun Yat-sen University, Guangzhou, 510275, China
E-mail: chizhg@mail.sysu.edu.cn
[b]
[c]
Prof. J. Zhao
School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China
E-mail: zhaoj95@mail.sysu.edu.cn
Dr. X. Chao
Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education,
School of Chemistry, South China Normal University, Guangzhou, 510006, China
E-mail: chaoxu@m.scnu.edu.cn
[d]
[e]
[+]
Prof. X. Zhang
State Key Laboratory of Optoelectronic Materials and Technologies,
School of Physics, Sun Yat-sen University, Guangzhou, 510275, China
Prof. X. Zhang
Center for Physical Mechanics and Biophysics,
School of Physics, Sun Yat-sen University, Guangzhou, 510275, China
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Mechanoluminescence (ML) materials are attracting
increasing interest due to its promising applications in various areas.
However, to date, it remains a major challenge to develop a precise
and universal route to achieving organic ML materials. Herein, we
report the first example that ML can be easily realized in organic
piezophotonic host-guest crystals, under the conditions that neither
the host nor the guest is ML-active. The experimental and theoretical
results reveal that, excitons of the host generated by piezoelectricity
can be harvested effectively by the guest for light emission, due to the
restraint of intersystem crossing process. Moreover, different host-
guest crystals are constructed, wherein the emission colour, intensity,
colour purity and emission duration of ML can be manipulated. This
work deepens our understanding of organic ML generation in
piezophotonic host-guest crystals and provides an inspiring principle
to design more organic ML materials.
focusing on organic ML materials were still neglected due to their
weak brightness and limited research conditions.[10] Until 2015, a
design strategy of combining aggregation-induced emission (AIE)
and ML properties into one molecule was proposed to obtain
bright ML,[11] and since then organic ML materials have drawn
considerable attentions.[12] However, the development of ML
materials still suffers from a lack of predictable design strategies
and well understood excited-state processes. Therefore, a simple
and universal route to precisely designing organic ML materials,
as well as controlling and enriching organic ML performances, is
in high demand.
Doping ions into inorganic piezoelectric materials, such as
ZnS and CaZnOS, is a practical approach to develop inorganic
ML materials.[13] Upon the external force, excitons are generated
in the piezoelectric crystals through defect-induced spatial charge
separations and then transferred to the dopant ions to emit light.
However, the majority of inorganic ML materials have some
drawbacks. The preparation conditions are extremely harsh and
the temperatures generally required are over 1000oC, let alone
the scarcity of metal resources. In addition, due to the low
compatibility, the concentrations of some dopant ions in inorganic
ML materials are quite limited, resulting in poor ML performance.
By contrast, doping luminescent guests into a pure organic
piezoelectric host possesses unique advantages, including lower
cost, better environmental safety and much more facile fabrication
processes.[14] In light of that, a sensitized organic ML (SOML)
system can be achieved, in which ML characteristics depend on
the guests due to the luminescence center is generated in the
guest. Given the existence of so many optional luminescent
guests with distinct luminescence properties, SOML materials can
exhibit excellent ML-property tunability. Furthermore, once the
piezoelectric host is ML-inactive of its own, the colour purity can
also be freely manipulated without the influence of host, which has
Introduction
Smart luminescent materials, which exhibit dramatical changes in
luminescence properties in response to external stimuli such as
light,[1] electricity,[2] temperature,[3] pressure[4] and solvent,[5] are
promising for applications in sensors, encryption, information
storage and photoelectric devices.[6,7] Among of which,
mechanoluminescence (ML) materials, which can emit light
spontaneously upon mechanical force, are quite fascinating due
to their distinctive features of real-time detection and
environmentally-friendly excitation mode.[8] ML materials have
being studied for more than 400 years since it was discovered in
sugar by Francis Bacon in 1605.[9] Despite organic molecules
have numerous inherent merits, such as facile modification, low
toxicity and abundant luminescence characteristics, researches
1
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10.1002/anie.202010166
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RESEARCH ARTICLE
resulting in ML-inactive organic piezoelectric hosts. On the basis
of these design principles, two groups of pure organic carbazole-
based molecules (Group one: 2MCzA, 2ICzA, 2BrCzA and
2ClCzA. Group two: 2BrCzB, 2BrCzBM and 2BrCzBO) were
synthesized (Supporting Information, Figure S1-S18). In Group
one, various substituents showing different electron-withdrawing
abilities were attached to the 3-position and 6-position of the
carbazole moiety, while the hexyl unit acting as the electron-
donating group, was linked to the N-position of the carbazole
moiety. In Group two, diverse substituents with different electron-
donating abilities were attached to the N-position of the carbazole
moiety, while the electron-withdrawing Br atoms were connected
to the 3-position and 6-position of the carbazole moiety.
The molecular dipole moments of these hosts were firstly
investigated to explore the piezoelectricity of these hosts. Figure
2a displays the electrostatic potential (ESP) diagrams of
carbazole and these carbazole derivatives, whose molecular
models (Figures S19, S20) were optimized by density functional
theory (DFT). In comparison to that of carbazole, it can be found
that the negative electron regions in these hosts tend to migrate
from the carbazole groups to the electron-withdrawing groups,
whilst the positive electron regions migrate to the electron-
donating groups. Consequently, in these hosts, both the stronger
electron-withdrawing ability of substituents at 3-position and 6-
position of the carbazole moiety and the stronger electron-
donating ability of substituents at the N-position contribute to
larger molecular dipole moments, which rang greatly from 1.5383
debye to 6.1057 debye in these hosts. For an instance, in
comparison to that of carbazole, the negative electron regions of
2BrCzA migrate to Br atoms, while its positive electron regions
are mainly distributed in the alkyl chain, accounting for nearly a
threefold increase in the molecular dipole moment of 2BrCzA.
Single crystals of these seven carbazole derivatives were
Figure 1. a) Schematic representation and b) proposed mechanism for the
sensitized organic mechanoluminescence of piezophotonic host-guest crystals.
c) Rational design of the organic piezoelectric hosts.
not been realized in ML materials so far. As a result, activating the
ML property in organic piezophotonic host-guest crystals can be
realized, wherein both the host and guest are ML-inactive.
However, such a pure organic ML-inactive piezoelectric molecule,
which can serve as the host in the SOML system, has not been
reported. Therefore, successful development of organic
piezophotonic host-guest crystals for activating organic ML
should be a significant advance in the field of organic ML.
Herein, we report the first example of a series of organic
piezophotonic host-guest crystals based on ML-inactive pure
organic piezoelectric hosts (Figure 1). The piezoelectricity of
these organic hosts, which plays a vital role in providing enough
energy to excite the molecules to the excited state, was observed
by piezoresponse force microscopy (PFM) measurements. In fact,
the whole excited-state process of ML can be divided into two
parts, namely the excitation and emission. However, there is
almost no related experimental research on organic ML from the
perspective of its excitation part. In this text, the excited-state
process of organic ML has been investigated for the first time. In
the presented organic piezophotonic host-guest crystals, ML can
be successfully sensitized through exciton transfer from the
excited piezo host to the ML-inactive luminescent guests.
Importantly, this strategy is widely applicable to different types of
guests, including pure organic compounds, metal complexes, and
inorganic quantum dots.
cultured successfully by
a
solvent diffusion method in
dichloromethane/ethanol solutions (Table S1). One-dimensional
slab-like crystals of 2BrCzA can be observed, which is similar to
the morphology prediction result by Materials Studio package with
the growth morphology algorithm based on its single-crystal
structure (Figure 2b). The simulation result reveals that, 2BrCzA
grows along the [010] direction with the dominant (200) face.
According to the single-crystal X-ray diffraction (SXRD) analysis,
the single crystal of 2BrCzA exhibits the non-centrosymmetric
polar space group of Pca21. The loose molecular arrangement
without any obvious noncovalent interactions along the a axis and
c axis in the single-crystal structure of 2BrCzA can be observed
(Figure S21). By contrast, along the b axis, a face-to-face packing
mode is adopted and the weak intermolecular π-π interaction with
a distance of 3.493 Å between adjacent carbazole units can be
found, which may be the main driving force for its crystal growth
(Figure 2c). The atomic force microscope (AFM) measurement
based on the single crystal of 2BrCzA further verifies this
viewpoint (Figure 2d and Figure S22). The layered structure
image was obtained with a thickness of 1 nm for each layer, which
is in accordance with the thickness of a molecular layer along the
a axis of the single-crystal structure of 2BrCzA (Figure 2e). In
Group one, except for the block-like crystal of 2ClCzA, similar
face-to-face packing modes can also be discovered in the needle-
like crystals of 2MCzA and 2ICzA (Figure S23 and Figures S25-
S27). Among them, the single crystal of 2ICzA is non-
centrosymmetric while that of 2MCzA is centrosymmetric. The
Results and Discussion
As well known, both the large molecular dipole moments and non-
centrosymmetric crystal structures can contribute greatly to the
piezoelectric property. The former can be achieved by combing
appropriate electron-donating groups with electron-withdrawing
groups,[15] while the latter can be constructed by introducing alkyl
chains or polar units with small sizes into target molecules,[16]
leading to the disorder and asymmetry of molecular arrangements.
Therefore, in addition to various electron-donating groups, the
small-sized electron-withdrawing halogen atoms are also used in
the construction of organic piezoelectric hosts. Besides, halogen
atoms can be utilized to boost the intersystem crossing (ISC)
process from the singlet state to the triplet state through the heavy
atom effect.[17] Thus, excitons generated from the piezoelectric
effect can be trapped by the triplet state and then be quenched,
2
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10.1002/anie.202010166
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RESEARCH ARTICLE
Figure 2. a) Molecular dipole moments of the carbazole and designed organic host molecules. Inset: molecular structures and their ESP diagrams, the potential
energy range is -2 × 10-2 to 2 × 10-2. b) The predicted growth morphology of the single crystal of 2BrCzA. Inset: The microscopy image of 2BrCzA crystals. Scale
bar is 1 mm. c) The molecular arrangement of a face-to-face packing mode in the single-crystal structure of 2BrCzA. d) The AFM image of a 2BrCzA single crystal.
Scale bar is 100 nm. e) The single-crystal structure of 2BrCzA along the b axis. The solvent-accessible void space is visualized by yellow blocks generated with a
probe of 0.9 Å. f) The π-π distances and void ratios of single-crystal structures of 2MCzA, 2ICzA and 2BrCzA. g) Piezoelectric phase and amplitude signals
measured in the crystal surface of 2BrCzA, showing local PFM hysteresis loops. Color code: grey, C; blue, N; brown, Br; white, H.
single-crystal structure of 2BrCzA exhibits the longest π-π
distance, accompanied by a solvent-accessible void ratio of 0.8%,
whilst the π-π distances are 3.338 Å (void ratio of 0.2%) and 3.452
Å (void ratio of 0) in the single-crystal structures of 2MCzA and
2ICzA, respectively (Figures 2e, 2f and Figure S24). The larger
π-π distance and solvent-accessible void ratio indicate the
molecular arrangement of the 2BrCzA crystal is much looser than
that of the others. In terms of Group two, the crystal space group
of 2BrCzBO is Fdd2 (non-centrosymmetric), while that of others
are P21/n and P-1 (centrosymmetric). Due to steric hindrance of
the methoxy unit, single-crystal structure of 2BrCzBO possesses
the longest π-π distance of 3.617 Å between two adjacent
carbazole units and the largest void ratio of 0.9% (Figures S28-
S31). Crystals with a looser molecular arrangement are more
prone to be deformed and fractured upon mechanical stimulus,
than that of the traditional inorganic piezoelectric material (ZnO,
d33 = 12 pC·N−1).[19]
When the crystalline samples of the above-mentioned three
piezoelectric molecules were scraped using a spatula, none of
them exhibit the ML phenomenon at room temperature due to the
heavy atom effect (Table S5). That is to say, they are all ML-
inactive at room temperature. To unveil the influence of the heavy
atom effect on the excited-state process, the nature transition
orbitals (NTOs) calculations of 2BrCzA, 2ICzA and 2BrCzBO
were carried out and analyzed (Figure 3a and Figures S34, S35
and Table S6). The bromine component of the hole NTO in the
lowest singlet excited state (S1) of 2BrCzA is 12.70%, and that in
low-lying triplet excited states T2 and T1 are 3.35% and 10.36%,
respectively, while the particle NTOs are mainly distributed in the
carbazole unit. Such proportions of n→π* in S1 and T2 lead to a
which is favorable for generating a powerful electric field. Thereby, larger spin-orbit coupling (SOC) constant (ξS1T2 = 2.77 cm−1),
2BrCzA, 2ICzA and 2BrCzBO are quite promising for serving as
organic piezoelectric hosts in piezophotonic host-guest crystals.
To further confirm the piezoelectricity of 2BrCzA, 2ICzA and
indicating that the efficient ISC can take place from the singlet
state to the triplet state and then excitons are quenched. The
existence of triplet states in 2BrCzA, 2ICzA and 2BrCzBO
molecules can also be confirmed by the experiment of singlet
2BrCzBO,
piezoresponse
force
microscopy
(PFM)
measurements were performed based on their crystals (Figure 2g
and Figures S32, S33). Intriguingly, their crystals all exhibit
piezoelectric hysteresis and butterfly-shaped loops, revealing the
characteristic piezo-response and distinct polarization switching
behaviors.[18] It is worth noting that, this is the first time that the
piezoelectricity of carbazole derivatives is observed by
experiment. Furthermore, due to the larger molecular dipole
moment and the looser molecular arrangement, the 2BrCzA
crystal exhibits the highest calculated piezoelectric coefficient (d32
= 31.9 pC·N−1) (Table 1 and Tables S2-S4), which is much higher
Table 1. Calculated piezoelectric coefficients of 2BrCzA.
dij / pC·N−1
1
2
3
4
5
6
1
2
3
0
0
0
0
0
0
0
-27.0
0
-13.4
0
0
0
0
0
20.8
-31.9
-6.8
3
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10.1002/anie.202010166
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RESEARCH ARTICLE
can be recoverable easily for infinite times by heating to restore
the large-size crystals in situ.
To uncover the mechanism of this SOML phenomenon,
besides R, another three organic luminophores with different
energy gaps were chosen as guests and synthesized, denoted as
B, G and Y (Figures 4a, 4b and Figure S39). These organic
piezophotonic host-guest crystals, named 2BrCzA-B, 2BrCzA-G,
2BrCzA-Y and 2BrCzA-R, were prepared by adding 1% mass
ratio of B, G, Y and R into 2BrCzA crystalline matrices through the
melt-casting method, respectively. The guests with the small
amount of doping were dispersed uniformly in the piezoelectric
matrices (Figure 4d). As showed in Figure 4c, no distinct impurity
peaks appear, indicating that the molecular assembly of the
original crystal has not been changed with introduction of the
dopants. As expected, multicolour ML are achieved, whose
profiles are identified with that of their photoluminescence (PL)
spectra (Figures 4d, 4e and Figure S40). That is to say, the
process of ML is similar to that of PL. The spectral overlaps
between the emission spectrum of the host and the absorption
spectra of the guests indicate the possible energy transfer of
resonant excitation from the host to the guests (Figure S41).[22]
The energy transfer is further proven by the lifetime decay profiles
of 2BrCzA crystals with and without dopants (Figures 4f, 4g and
Figures S42, S43). For instance, the significant change in
fluorescence lifetime of 2BrCzA crystals in the absence and
presence of R as monitored at 400 nm (from 17.96 ns to 0.98 ns)
reveals the occurrence of nonradiative energy transfer.
Meanwhile, a decrease in phosphorescence lifetime of 2BrCzA
crystals as monitored at 550 nm indicates the restraint of the ISC
process. In other word, as for 2BrCzA-R, excitons of the host can
transfer to the singlet state of R instead of the triplet state of
2BrCzA, due to a faster energy transfer rate (9.65 × 108 s-1) than
an ISC rate of the host (< 0.56 × 106 s-1) (Table S7), and then
relax to the ground state producing red emission. Moreover, when
another guest (named U) with a higher singlet state energy level
than that of the host was used, the resulting host-guest crystal
2BrCzA-U is ML-inactive at room temperature due to the
forbidden exciton transfer from 2BrCzA to U, while the
phosphorescence lifetime can be monitored at 550 nm (Figure
S44). This result verifies that the guest with suitable singlet state
is of importance to the occurrence of energy transfer.
In addition to these various pure organic guests, to verify the
universality of our approach and enrich the performances of
SOML systems, metal complexes and inorganic quantum dots
were also employed to construct organic piezophotonic host-
guest crystals (Figure 5a). When two iridium complexes (named
CB and CY, Fig. 5b) with high PL quantum yields are utilized to
fabricate piezophotonic host-guest crystals 2BrCzA-CB and
2BrCzA-CY, shining blue and yellow ML are clearly observed
under the force even at daylight (Figure S45), respectively.
Furthermore, when inorganic CdSe quantum dots featured with
extremely narrow emission serve as the guest in the
piezophotonic host-guest, ultra-high color purity of ML with a full
width at half-maximum (FWHM) of 23 nm is achieved in the crystal
2BrCzA-CdSe (Figure 5c). Remarkably, as for another
piezophotonic host-guest crystal 2BrCzA-O by using 1,8-
naphthalic anhydride (named O) as the guest, bright yellow
persistent ML can be observed by naked eyes (Figure 5d). The
ML emission duration can last for about one second, and such a
long-lived ML phenomenon, which is quite rare, has been
successfully realized in our organic piezophotonic host-guest
Figure 3. a) Calculated energy levels and related spin-orbit coupling constants
of 2BrCzA based on the single-crystal geometry (left). The natural transition
orbitals (NTOs) of 2BrCzA for S1, T2 and T1 (bottom: hole; top: particle), with the
corresponding proportions and the bromine components in NTOs (right). b)
Illustration of the fabrication and regeneration processes of piezophotonic host-
guest crystals with the SOML property. c) The ML spectra of organic
piezoelectric host crystals and piezophotonic host-guest crystals named
2BrCzA-R, 2ICzA-R, 2BrCzBO-R, respectively, prepared by doping with 1%
mass ratio of guest R.
oxygen generation measurements using 9,10-anthracenediyl-
bis(methylene)dimalonic acid (ABDA) as an indicator in THF/H2O
(10/90; v/v) solutions.[20] Under irradiation, the characteristic
absorbance (359, 378 and 399 nm) of ABDA obviously decrease
over time, indicating the formation of triplet excited states indued
by effective ISC processes upon photoexcitation of 2BrCzA,
2ICzA and 2BrCzBO molecules (Figure S36). This result is further
supported by the fact that obvious green ML can be observed in
these three compounds at 77 K due to the suppression of non-
radiative pathways in triplet states (Figure S37). Their ML profiles
are similar to corresponding phosphorescence spectra, implying
the emissions come from triplet excited states.
Sensitization plays a vital role in tuning excited-state
properties of molecules and improving performances of traditional
materials in fields of light-harvesting systems, solar cells and
organic light-emitting diodes.[21] Considering that the above
organic piezoelectric crystals exhibit a significant character of fast
ISC, they are suitable for developing organic piezophotonic host-
guest crystals. When a small amount of luminescent dopant is
added into the organic piezoelectric host, energy transfer
between host and guest occurs, which benefits to restrain the ISC
process of host and harvest the excitons by guest, and then ML
can be sensitized at room temperature. A ML-inactive red
luminescent dye (denoted as R, Figure 4b) was used firstly as the
guest in the energy transfer system. After separately doping 1%
mass ratio of R into the crystalline samples of 2BrCzA, 2ICzA and
2BrCzBO via a melt-casting method, remarkable ML properties of
these three host-guest crystals can be activated at room
temperature (Figures 3b, 3c and Figure S38). Very bright red ML
is observed clearly by naked eyes at room temperature upon
mechanical force, with emission maxima at 618 nm, 615 nm and
598 nm, respectively. When the piezophotonic host-guest crystals
become amorphization or particles with smaller sizes, the ML
performance will fade gradually. Owing to the relative low melting
points of the three organic piezoelectric hosts, their ML properties
4
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10.1002/anie.202010166
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RESEARCH ARTICLE
Figure 4. a) Diagram of the sensitization process for different piezophotonic host-guest crystals. b) The molecular structure of guests. c) XRD spectra of 2BrCzA
crystals and host-guest crystals, as well as the simulated XRD spectrum from the single-crystal structure of 2BrCzA. d) The ML photos (left) and fluorescent
microscopy images (right) of different host-guest crystals. Scale bar: 1 mm. e) The ML spectra of different piezophotonic host-guest crystals. Inset: The CIE
coordinate diagram of ML for different piezophotonic host-guest crystals. f) Lifetime decay profiles of 2BrCzA crystals with and without 1% R measured at 400 nm
under ambient conditions. IRF: Instrument response function. g) Lifetime decay profiles of 2BrCzA crystals with and without 1% R measured at 550 nm under
ambient conditions.
crystals. The appearance of persistent ML is inseparable from the
contribution of the host, given the fact that the guest O possesses
neither ML nor persistent luminescence properties (Figure S46).
Therefore, various kinds of luminogens can be chosen as guests
for SOML systems, which can not only enable excellent ML
performances with the nature characteristics of the guests, but
also endow ML materials with new potentials.
which should promote the development of more advanced
organic ML materials with excellent functions in the future.
Conclusion
In summary, we developed a sensitized organic ML (SOML)
system to activating mechanoluminescence by constructing
organic piezophotonic host-guest crystals. A series of carbazole
derivatives with rational design were synthesized and worked as
ML-inactive organic piezoelectric hosts. With incorporating a
small amount of ML-inactive luminogen guests into the ML-
inactive hosts, remarkable ML properties can be activated in the
organic piezoelectric host-guest crystals at room temperature.
The studies on the SOML process revealed that the mechanism
of such an extraordinary SOML phenomenon was related to
efficient exciton transfer between the host and guest. More
importantly, the ML-inactive organic piezoelectric hosts can be
combined with different kinds of guests such as organic molecules,
metal complexes and inorganic quantum dots, exhibiting the
universality of our approach to activating ML. With our SOML
system, the ML performances including emission colour, intensity,
colour purity, and emission duration can be manipulated. We
believe that this work not only provides compelling clues for the
design of organic ML materials with versatile performances, but
also deepens our understanding on the mechanism of organic ML,
Figure 5. a) The guest universality to construct SOML systems. b) ML photos
of 2BrCzA-CB (top) and 2BrCzA-CY (bottom) prepared by doping 1% CB and
1% CY, respectively. Inset: molecular structures of CB and CY. c) The ML
spectrum of 2BrCzA-CdSe prepared by doping 1% CdSe. d) The ML spectra of
2BrCzA-O prepared by doping with 1% O. Inset: corresponding ML images.
5
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RESEARCH ARTICLE
Y. Zhang, Z. Chi, Angew. Chem. Int. Ed. 2018, 57, 6449-6453; Angew.
Chem. 2018, 130, 6559-6563.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (NSFC: 51733010, 61605253,
21672267, 51973239), Science and Technology Planning Project
of Guangdong (2015B090913003) and the Fundamental
Research Funds for the Central Universities.
[13] a) X. Wang, H. Zhang, R. Yu, L. Dong, D. Peng, A. Zhang, Y. Zhang, H.
Liu, C. Pan, Z. L. Wang, Adv. Mater. 2015, 27, 2324-2331; b) J.-C. Zhang,
L.-Z. Zhao, Y.-Z. Long, H.-D. Zhang, B. Sun, W.-P. Han, X. Yan, X. Wang,
Chem. Mater. 2015, 27, 7481-7489.
[14] R. Kabe, C. Adachi, Nature 2017, 550, 384-387.
[15] J.-i. Nishida, H. Ohura, Y. Kita, H. Hasegawa, T. Kawase, N. Yakada, H.
Sato, Y. Sei, Y. Yamashita, J. Org. Chem. 2016, 81, 433-441.
[16] a) W. Li, Q. Huang, Z. Mao, Q. Li, L. Jiang, Z. Xie, R. Xu, Z. Yang, J. Zhao,
T. Yu, Y. Zhang, M. P. Aldred, Z. Chi, Angew. Chem. Int. Ed. 2018, 57,
12727-12732; Angew. Chem. 2018, 130, 12909-12914; b) X. Wang, H.
Ma, M. Gu, C. Lin, N. Gan, Z. Xie, H. Wang, L. Bian, L. Fu, S. Cai, Z. Chi,
W. Yao, Z. An, H. Shi, W. Huang, Chem. Mater. 2019, 31, 5584-5591; c)
B. Xu, W. Li, J. He, S. Wu, Q. Zhu, Z. Yang, Y.-C. Wu, Y. Zhang, C. Jin,
P.-Y. Lu, Z. Chi, S. Liu, J. Xu, M. R. Bryce, Chem. Sci. 2016, 7, 5307-
5312.
Keywords: mechanoluminescence • host-guest systems •
molecular packing • intersystem crossing • energy transfer
[1] a) Q. Qi, C. Li, X. Liu, S. Jiang, Z. Xu, R. Lee, M. Zhu, B. Xu, W. Tian, J.
Am. Chem. Soc. 2017, 139, 16036-16039. b) L. Gu, H. Shi, M. Gu, K. Ling,
H. Ma, S. Cai, L. Song, C. Ma, H. Li, G. Xing, X. Hang, J. Li, Y. Gao, W.
Yao, Z. Shuai, Z. An, X. Liu, W. Huang, Angew. Chem. Int. Ed. 2018, 57,
8425-8431; Angew. Chem. 2018, 130, 8561-8567.
[2] H. Sun, S. Liu, W. Lin, K. Y. Zhang, W. Lv, X. Huang, F. Huo, H. Yang, G.
Jenkins, Q. Zhao, W. Huang, Nat. Commun. 2014, 5, 3601.
[17] O. Bolton, K. Lee, H.-J. Kim, K. Y. Lin, J. Kim, Nat. Chem. 2011, 3, 205-
210.
[3] a) Z. Zhang, Y.-S. Wu, K.-C. Tang, C.-L. Chen, J.-W. Ho, J. Su, H. Tian,
P.-T. Chou, J. Am. Chem. Soc. 2015, 137, 8509-8520; b) Z.-Q. Yao, J. Xu,
B. Zou, Z. Hu, K. Wang, Y.-J. Yuan, Y.-P. Chen, R. Feng, J.-B, Xiong, J.
Hao, X.-H. Bu, Angew. Chem. Int. Ed. 2019, 58, 5614-5618; Angew.
Chem. 2019, 131, 5670-5674.
[18] a) Y.-M. You, W.-Q. Liao, D. Zhao, H.-Y. Ye, Y. Zhang, Q. Zhou, X. Niu,
J. Wang, P.-F. Li, D.-W. Fu, Z. Wang, S. Gao, K. Yang, J.-M. Liu, J. Li, Y.
Yan, R.-G. Xiong, Science 2017, 357, 306-309; b) F. Wang, F. Wang, X.
Wang, S. Wang, J. Jiang, Q. Liu, X. Hao, L. Han, J. Wang, C. Pan, H. Liu,
Y. Sang, Nano Energy 2019, 63, 103861.
[4] a) Y. Liu, Q. Zeng, B. Zou, Y. Liu, B. Xu, W. Tian, Angew. Chem. Int. Ed.
2018, 57, 15670-15674; Angew. Chem. 2018, 130, 15896-15900; b) C.-X.
Chen, Z.-W. Wei, Y.-N. Fan, P.-Y. Su, Y.-Y. Ai, Q.-F. Qiu, K. Wu, S.-Y.
Yin, M. Pan, C.-Y. Su, Chem 2018, 4, 2658-2669; c) Z. Sun, Q. Zang, Q.
Luo, C. Lv, F. Cao, Q. Song, R. Zhao, Y. Zhang, W.-Y. Wong, Chem.
Commun. 2019, 55, 4735-4738.
[19] a) I. B. Kobiakov, Solid State Commun. 1980, 35, 305; b) K. Tao, W. Hu,
B. Xue, D. Chovan, N. Brown, L. J. W. Shimon, O. Maraba, Y. Cao, S. A.
M. Tofail, D. Thompson, J. Li, R. Yang, E. Gazit, Adv. Mater. 2019, 31,
1807481.
[20] a) S. Xu, Y. Yuan, X. Cai, C.-J. Zhang, F. Hu, J. Liang, G. Zhang, D. Zhang,
B. Liu, Chem. Sci. 2015, 6, 5824-5830; b) W. Zhao, Z. He, Q. Peng, J. W.
Y. Lam, H. Ma, Z. Qiu, Y. Chen, Z. Zhao, Z. Shuai, Y. Dong, B. Z. Tang,
Nat. Commun. 2018, 9, 3044.
[5] a) Q. Li, H. Zhu, F. Huang, J. Am. Chem. Soc. 2019, 141, 13290-13294;
b) R.-W. Huang, Y.-S. Wei, X.-Y. Dong, X.-H. Wu, C.-X. Du, S.-Q. Zang,
T. C. W. Mak, Nat. Chem. 2017, 9, 689.
[21] a) M.-J. Sun, Y.-W. Zhong, J. Yao, Angew. Chem. Int. Ed. 2018, 57, 7820-
7825; Angew. Chem. 2018, 130, 7946-7951; b) Y. Hao, W. Yang, L. Zhang,
R. Jiang, E. Mijangos, Y. Saygili, L. Hammarström, A. Hagfeldt, G.
Boschloo, Nat. Commun. 2016, 7, 13934; c) X. Song, D. Zhang, Y. Lu, C.
Yin, L. Duan, Adv. Mater. 2019, 31, 1901923.
[6] a) W. Li, Q. Huang, Z. Mao, J. Zhao, H. Wu, J. Chen, Z. Yang, Y. Li, Z.
Yang, Y. Zhang, M. P. Aldred, Z. Chi, Angew. Chem. Int. Ed. 2020, 59,
3739-3745; Angew. Chem. 2020, 132, 3768-3774; b) L. Bai, P. Bose, Q.
Gao, Y. Li, R. Ganguly, Y. Zhao, J. Am. Chem. Soc. 2017, 139, 436-441;
c) Y. Su, Y. Zhang, Z. Wang, W. Gao, P. Jia, D. Zhang, C. Yang, Y. Li, Y.
Zhao, Angew. Chem. Int. Ed. 2020, 59, 9967-9971; Angew. Chem. 2020,
132, 10053-10057; d) K. Isayama, N. Aizawa, J. Y. Kim, T. Yasuda,
Angew. Chem. Int. Ed. 2018, 57, 11982-11986; Angew. Chem. 2018, 130,
12158-12162; e) Y. Zhang, H. Yang, H. Ma, G. Bian, Q. Zang, J. Sun, C.
Zhang, Z. An, W.-Y. Wong, Angew. Chem. Int. Ed. 2019, 58, 8773-8778;
Angew. Chem. 2019, 131, 8865-8870.
[22] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, New
York, 2006, pp. 443-472.
[7] a) Y. Sagara, S. Yamane, M. Mitani, C. Weder, T. Kato, Adv. Mater. 2016,
28, 1073-1095; b) M. Kato, H. Ito, M. Hasegawa, K. Ishii, Chem. Eur. J.
2019, 25, 5105-5112; c) Z. Zhao, H. Zhang, J. W. Y. Lam, B. Z. Tang,
Angew. Chem. Int. Ed. 2020, 59, 9888-9907; Angew. Chem. 2020, 132,
9972-9993.
[8] a) J.-C. Zhang, X. Wang, G. Marriott, C.-N. Xu, Prog. Mater. Sci. 2019,
103, 678-742; b) Y. Xie, Z. Li, Chem 2018, 4, 943-971.
[9] D. O. Olawale, O. O. I. Okoli, R. S. Fontenot, W. A. Hollerman,
Triboluminescence: Theory, Synthesis and Application, Springer,
Heidelberg, 2016, p. 3.
[10] P. Jha, B. P. Chandra, Luminescence 2014, 29, 977-993.
[11] a) B. Xu, J. He, Y. Mu, Q. Zhu, S. Wu, Y. Wang, Y. Zhang, C. Jin, C. Lo,
Z. Chi, A. Lien, S. Liu, J. Xu, Chem. Sci. 2015, 6, 3236-3241; b) C. Wang,
B. Xu, M. Li, Z. Chi, Y. Xie, Q. Li, Z. Li, Mater. Horiz. 2016, 3, 220-225.
[12] a) C. Wang, Y. Yuan, C. Ren, Q. Liao, J. Wang, Z. Chai, Q. Li, Z. Li, Matter
2020, 2, 181-193; b) J. Wang, Z. Chai, J. Wang, C. Wang, M. Han, Q. Liao,
A. Huang, P. Lin, C. Li, Q. Li, Z. Li, Angew. Chem. Int. Ed. 2019, 58,
17297-17302; Angew. Chem. 2019, 131, 17457-17462; c) X. Zhang, L.
Du, W. Zhao, Z. Zhao, Y. Xiong, X. He, P. F. Gao, P. Alam, C. Wang, Z.
Li, J. Leng, J. Liu, C. Zhou, J. W. Y. Lam, D. L. Phillips, G. Zhang, B. Z.
Tang, Nat. Commun. 2019, 10, 5161; d) L. Zhan, Z. Chen, S. Gong, Y.
Xiang, F. Ni, X. Zeng, G. Xie, C. Yang, Angew. Chem. Int. Ed. 2019, 58,
17651-17655; Angew. Chem. 2019, 131, 17815-17819; e) J.-A. Li, J. Zhou,
Z. Mao, Z. Xie, Z. Yang, B. Xu, C. Liu, X. Chen, D. Ren, H. Pan, G. Shi,
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10.1002/anie.202010166
Angewandte Chemie International Edition
RESEARCH ARTICLE
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The versatile mechanoluminescence can be activated in organic piezophotonic host-guest crystals through controlling exciton
transfer, under the conditions that neither the host nor the guest has the mechanoluminescence property.
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