Alkyl Chain Introduction: In Situ Solar-Renewable Colorful Organic Mechanoluminescence Materials

Article abstract of DOI:10.1002/anie.201806861

Mechanoluminescence (ML) materials are environmentally friendly and emit light by utilizing mechanical energy. This has been utilized in light sources, displays, bioimaging, and advanced sensors. Organic ML materials are strongly limited to application by in situ unrepeatable ML. Now, in situ solar-renewable organic ML materials can be formed by introducing a soft alkyl chain into an ML unit. For the first time, the ML from these polycrystalline thin films can be iteratively produced by simply recrystallizing the fractured crystal in situ after a contactless exposure to sunlight within a short time (≤60 s). Additionally, their ML color and lifetime can be also easily tuned by doping with organic luminescent dyes. Therefore, large-area sandwich-type organic ML devices can be fabricated, which can be repeatedly used in a colorful piezo-display, visual handwriting monitor, and sensitive optical sensor, showing a lowest pressure threshold for ML of about 5 kPa.

Full text of DOI:10.1002/anie.201806861

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Alkyl Chain Introduction: In-Situ Solar-Renewable Organic
Colorful Mechanoluminescence Materials
Authors: Wenlang Li, Qiuyi Huang, Zhu Mao, Qi Li, Long Jiang,
Zongliang Xie, Rui Xu, Zhiyong Yang, Juan Zhao, Tao Yu, Yi
Zhang, Matthew. P. Aldred, 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.201806861
Angew. Chem. 10.1002/ange.201806861
Link to VoR: http://dx.doi.org/10.1002/anie.201806861
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10.1002/anie.201806861
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Alkyl Chain Introduction: In-Situ Solar-Renewable Organic
Colorful Mechanoluminescence Materials**
Wenlang Li,† Qiuyi Huang,† Zhu Mao, Qi Li, Long Jiang, Zongliang Xie, Rui Xu, Zhiyong Yang,* Juan
Zhao, Tao Yu, Yi Zhang,* Matthew P. Aldred and Zhenguo Chi*
Abstract: Mechanoluminescence (ML) materials are
environmentally friendly and emit light by utilizing mechanical
energy, which have been utilized in light source, display,
bioimaging and advanced sensors. Organic ML materials have
attracted much attention because of convenient synthesis and
various ML properties. However, they are strongly limited to
application by in-situ unrepeatable ML. Here, a novel strategy of
in-situ solar-renewable organic ML materials was shown, by
introducing “soft” alkyl chain into an ML unit. For the first time, the
ML from these polycrystalline thin film can be iteratively produced,
by simply in-situ recrystallizing the fractured crystal after a
contactless exposure to sunlight in a short time (≤ 60s).
Additionally, their ML color and lifetime can be also easily tuned
by doping with organic luminescent dyes. Therefore, large-area
sandwich-type organic ML devices can be firstly fabricated, which
can be repeatedly used as colorful piezo-display, visual
handwriting monitor and sensitive optical sensor, showing a
lowest pressure threshold for ML ≈ 5k Pa. This “soft” alkyl chains
introduction strategy provides a promising approach to promote
the applications of organic ML materials.
systems.[4] However, most of materials exhibit unrepeated ML and
difficult to be utilized in these real-world applications. Only few
inorganic materials are elastic-ML systems, such as zinc sulfide
doped with manganese (ZnS:Mn) composites. They exhibit
repeatable bright ML during their elastic deformation response to
mechanical stimuli, in which neither fracture nor plastic
deformation is required.[5, 6] However, these inorganic systems
with efficient ML are generally prepared at very high temperature,
which limits their applications. In contrast, organic ML materials
show convenience of preparation and various ML properties.
Purely organic ML materials have attracted much attention
recently, because of simple organic compounds with bright ML in
daylight have been reported constantly in recent years.[7-9]
Nevertheless, all of these organic materials show unrepeated ML
properties, which ML active crystals cannot be reproduced in-
situ.[10] It is required very carefully recrystallization of the fractured
samples from their solutions or hot melts to renew their crystals
and activate them for a second ML round. Therefore, in order to
repeat ML in their devices for practical applications, the key
challenge is to design a reliable system that allows in-situ renewal
of the organic crystals, especially by a contactless approach.
Additionally, colorful ML is essential for the real-world applications
as well. Therefore, it becomes a fundamental challenge for
organic ML materials that design of a material system which can
easily in-situ form and renew large-size polycrystalline samples
with colorful ML properties, by non-contact approach.
Mechanoluminescence (ML), also known as triboluminescence,
fractoluminescence and piezoluminescence, is the emission of
light from fracture or deformation of a solid due to mechanical
stimuli.[1] ML is quite distinctive compared to other types of
luminescence because external electrical or optical excitation are
not required to generate the light-emission.[2] This gives ML
materials attractive environmentally friendly properties, which
make them promising candidates for lighting, display, and
bioimaging by utilizing natural mechanical energy source.[3] In
addition, ML materials can be also utilized in advanced sensors
to enable wireless, in-situ, real-time and distributed (WIRD)
damage, stress, and impact sensing in civil and aerospace
To achieve such a perfect ML system, tuning the intermolecular
interactions and stackings in its crystal are thus the most
important elements for these ML systems. It is well known that
“soft” alkyl chains are widely utilized in the design of self-assembly
12]
systems and liquid crystals.[11,
This is due to their strong
tendency for self-assembly and cluster formation (e.g.,
glycerophospholipids bilayers[13] in cell membrane, and
interdigitation in poly(3-hexylthiophene-2,5-diyl), P3HT[14]). On
the other hand, they also brought the systems new properties of
low phase-transition temperatures (e.g., low glass transition
temperature) and improved compatibility with various materials,
because of the weak intermolecular interactions in the alkyl chain
zone.[15] Inspired by this and as shown in Figure 1, a “soft” alkyl
chain with appropriate length was introduced into an N-carbazole
ML unit. Profiting from these alkyl chains, these easily
synthesized materials actually possess loose crystals, sensitive
deep-blue ML, strong crystallization ability, and low phase-
transition temperatures. Most amazingly, their fractured crystals
can be readily renewed to large-size crystals simply by in-situ
recrystallization after exposed to sunlight in a very short time (≤
60s), resulting in novel contactless solar-renewable ML systems.
Additionally, the ML color and lifetime of these organic systems
can be easily tuned via. energy transfer mechanism by doping
with small amounts of organic luminescent dyes. Therefore, a
[*]
Dr. W. Li, Dr. Q. Huang, Dr. Z. Mao, Dr. Q. Li, Prof. L. Jiang, Dr. Z. Xie,
Dr. R. Xu, Prof. Z. Yang, Dr. J. Zhao, Dr. T. Yu, Prof. S. Liu, Prof. Y.
Zhang, Dr. M. P. Aldred, Prof. J. Xu, 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:
yangzhy29@mail.sysu.edu.cn;
ceszy@mail.sysu.edu.cn;
chizhg@mail.sysu.edu.cn
[†]
Dr. W. Li and Dr. Q. Huang contributed equally to this work.
This work was financial supported from NSFC (51733010,21672267,
51603232 and 61605253), Guangdong Natural Science Funds for
Distinguished Young Scholar (2017B030306012), Science and
Technology Planning Project of Guangdong (2015B090913003), and
the Fundamental Research Funds for the Central Universities.
Supporting information for this article is given via a link at the end of the
document.
[**]
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10.1002/anie.201806861
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strategy for the design of in-situ solar-renewable organic colorful
ML material systems was sought. For the first time, solar-
renewable sandwich-type devices can be conveniently fabricated
with their polycrystalline large-area thin films for different
applications, such as colorful piezo-display, visual handwriting
dynamic monitor, and sensitive optical sensors for
for Cz-alkyl-6 crystalline solid, when gently shaking the sample in
a round-bottomed flask (Figure 2b and Video S1).
With the help of these alkyl chain zone and loose packing in its
crystal, Cz-alkyl-6 possesses both very low temperatures for
glass transition (Tg = -50.8 oC) and crystallization (Tc = -16.5 oC),
resulting in a strong tendency for crystallization at room
temperature (Figure 2e).[18] As a result, polycrystalline samples of
Cz-alkyl-6 can be prepared simply by crystallization from its
Figure 1. Schematic diagram about the strategy of “alkyl chain introduction”:
after introducing an alkyl chain into an ML unit, weak interactions zone (alkyl
chain zone) and loose stacking will form in the crystal. As a result, these crystals
possessed good material compatibility and thus can show colorful ML by doping
with different organic dyes. More interestingly, these crystals exhibited novel
solar-renewable ML, by in-situ recrystallizing the fractured crystal after a
contactless exposure to sunlight in a short time.
finger touching or gas flow. These sensors showed a reported
lowest pressure threshold for ML (≈ 5k Pa), which is much smaller
than that of typical inorganic ZnS (≈ 600k Pa).[6]
In this strategy, the carbazolyl unit was chosen as the ML unit
because some of its derivatives exhibit efficient ML.[16] Alkyl
chains with different lengths were introduced at the carbazole N-
position via Hofmann alkylation reactions, affording a series of
carbazole derivatives (Cz-alkyl-N, N = 2 to 8, where N = the
number of carbon atoms in the alkyl chain, Scheme S1 and Figure
S1a-b). Except for the liquid sample Cz-alkyl-8, crystals of these
carbazole derivatives were grown to check the ML properties.
Unfortunately, only Cz-alkyl-6 and Cz-alkyl-7 displayed bright ML
in daylight. The Cz-alkyl-6 crystal is non-centrosymmetric while
the crystal of Cz-alkyl-7 is centrosymmetric. The mechanism for
their ML is likely to electron bombardment during crystal fracture.[5,
Figure 2. Molecular stacking in Cz-alkyl-6 single crystal in different views: (a) a-
axis; (f) c-axis; and (g) b-axis. The blue parts are carbazole groups, while the
red parts are alkyl chains. The green dash circle indicates the ankyl chain zone.
(b) ML photos of Cz-alkyl-6 sample obtained by slightly mechanical stimuli:
scratching and shaking (above in daylight, below in dark). (c) Large-size
polycrystalline Cz-alkyl-6 samples (letters shape): the letter held in hand. (d) PL
and ML spectra of Cz-alkyl-6 sample. The chemical structure of Cz-alkyl-6 is
inset. (e) Differential scanning calorimetry (DSC) curve of Cz-alkyl-6 sample,
showing temperatures of glass transition (Tg), crystallization (Tc) and melting
(Tm), as indicated.
17]
As expected, the alkyl chains in these two compounds
aggregate and stack together in the crystalline state (Figures 2a,
2f, 2g, S2a, S2f, and S2g). This generates alkyl chains zone within
the crystalline structure, with very weak intermolecular
interactions (Van der Waals force only) in the zone (Figure 2f). In
addition, six molecules aggregate together in Cz-alkyl-6 crystal to
form a conical funnel and then insert each other to form a column,
in which the alkyl chains assemble in the cusp whilst the
carbazolyl units are crowded at the bottom (Figures 2a, 2f and 2g).
While the heptyl chains of Cz-alkyl-7 aggregate assemble in a
spiral fashion along the carbazolyl columns (Figure S2a, S2f and
S2g). It is noted that both the alkyl chain zone and the loose
packing in crystal are favorable for sensitive ML, because facile
crystal destruction leads to a low pressure threshold for ML
(Figure 2b). Actually, extremely sensitive ML can be achieved for
their crystals, showing deep-blue emission in daylight (~375 nm,
Figures 2b, 2d, S2c, and S2e). Bright ML can even be observed
hot melt or various solutions. It produced crystalline samples with
identical ML (Figure S3a) and X-ray diffraction (XRD, Figure S3b)
spectral features for both methods. During the crystallization
process of Cz-alkyl-6, strong emission is also observed
(crystalloluminescence,[19] CL, Figure S4). Therefore, large-size
polycrystalline samples with special shapes can be readily
prepared (letter A, Figure 2c). It is even more exciting that large-
area polycrystalline thin films can be easily obtained as well
(Figure 6d).
The strong crystallization ability of Cz-alkyl-6 at room
temperature also means its amorphous state cannot be obtained
simply by grinding. From XRD analysis, the diffraction peaks are
almost identical for the original and the ground samples (Figure
S3c). This determined that no morphological changes take place
during mechanical stimuli. Therefore, the ML from the Cz-alkyl-6
crystals can be continuously produced for more than a hundred
grinding rounds (Figure 3a). In other words, Cz-alky-6 already
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show repeated ML in a certain extent. This shows a big
improvement over the prior-art organic ML materials that can only
survive a mere several grinding rounds, as their crystals are
transferred to ML-inactive amorphous samples after mechanical
stimuli.[5] However, the ML will ultimately reduce in efficiency due
to the production of smaller size Cz-alkyl-6 crystals from the
excessive grinding rounds. These small-size crystals are more
difficult to be pressed or ground under standard mechanical
stimuli.[2] This is a serious limitation of organic ML materials for
their applications. Therefore, repairing or renewing the fractured
sample back into larger-size crystals becomes an important issue.
close to the temperature of human body (35.9 °C, Figure S2d),
Cz-alkyl-7 crystals show high potential for biological applications,
such as organic artificial skins with ML property.
Another challenge for the application of organic ML materials is
to tune the emission color and lifetime of their ML. However, there
is no design strategy about this yet. Doping technology is widely
used for tuning the energy levels of organic materials via energy
transfer from the host to guest.[20] Fortunately, sensitive and solar-
renewable organic systems with various ML properties can be
firstly achieved based on these new materials, by using a guest-
host approach. Highly emissive dyes with different emission
colors were synthesized, and classed as dye B (blue),
Figure 4. (a) ML photos in daylight and (b) dual-peak ML spectra of Cz-alkyl-6
doping systems: sample doped with dye B, dye G, dye Y, and dye R (form left
to right). (c) Delayed PL and ML spectra and (d) time-resolved ML decay curve
of Cz-alkyl-6 sample doped with dye G at 77 K. The inset photos (in dark) show
the phosphorescent ML at different delayed times. All doping concentrations for
these dyes are 1% (weight/weight, dye/Cz-alkyl-6).
Figure 3. (a) ML intensities of Cz-alkyl-6 sample obtained by only continuous
ground for 100 cycles. (b) ML spectra of device with polycrystalline Cz-alkyl-6
sample before and after solar-renewal (Figure 6d). (c) ML intensities of Cz-alkyl-
6 crystals after different cycles of solar-renewal (every ML measurement was
taken after 5 cycles intervals). (d) Contactless solar-renewal process of Cz-
alkyl-6 crystals by sunlight irradiation from a solar simulator.
dye G (green), dye Y (yellow) and dye R (red) (Scheme S2-3,
Figure S7, and Table S1). These dyes are doped into the host
(Cz-alkyl-6) crystal, forming ML systems with dual-peak spectra
of host (~375 nm) and guest (dye), as shown in Figure 4b. Their
ML emission color can be varied by changing the doping dye and
its doping concentration (from 0.001% to 1%, weight/weight,
dye/Cz-alkyl-6), which results in different ratio of the dual ML
peaks. Therefore, ML covering the whole visible region from blue
to red (Figures 4a-b, S8-12, and Videos S2-3) and even white
(0.001%Y, Figures S8c and S11a) can be achieved in these
doping systems. Additionally, green phosphorescent ML from
1%G with emission lifetime as long as 24 milliseconds can be also
obtained (77K, Figure 4c-d and Video S4). The measured higher
energy levels of singlet and triplet excited states for host (Cz-alkyl-
6, Figures 5d and S13-14), as well as the spectral overlap
between the emission of host and the absorbance of doped dyes
(Figure 5a), reveal the possibility of energy transfer in the systems.
In addition, the energy transfer has been also proved by the
prolonged lifetime of guest (dye Y) and the decreased lifetime of
host (Cz-alkyl-6) in 1%Y doping system (Figure 5b-c, other
experimental data see Figure S15a).
Fortunately, Cz-alkyl-6 exhibits not only a low Tc but also a low
melting point temperature (Tm, 68.8 oC, Figure 2e), due to the long
hexyl chain substitution. To our delight, it was found that both Cz-
alkyl-6 and Cz-alkyl-7 are fascinating “solar-renewable” organic
ML materials (Figures 3d and S5). The fractured sample was
exposed to artificial sunlight under a solar simulator (1 minute for
Cz-alkyl-6 and 10 seconds for Cz-alkyl-7, respectively). This
exposure creates some heat to melt the small-size crystalline
samples, benefited from their low Tm. As a result, their small-size
crystals can be readily and rapidly renewed into larger original-
type crystals for efficient ML again, by simply allowing the melted
sample to in-situ recrystallize after exposure. Theoretically, this
renewable process can be reproduced for infinite times, which
endows these organic materials repeatable ML for real-world
applications. Actually, there are only small changes in the ML
intensity after renewal for 50 cycles (Cz-alkyl-6, Figure 3b-c). The
common renewable processes for previous organic ML materials
are generally ex-situ and rigorous, including sample dissolution
followed by solvent removal, or sample melted on a heater
followed by carefully cooling recrystallization. What is exciting that
such solar-renewable process demonstrates a novel convenient
in-situ method to regenerate ML active organic crystal under
contactless and mild conditions (Figure S6), allowing for ML
device applications with infinite ML emission. Moreover, with a Tm
Benefited from the low Tm of Cz-alkyl-6, this doping system can
be prepared by slightly heating up (~70 oC) the guest-host mixture
then cooling down to room temperature, without the aid of any
solvents (Figure S16).[5] Similar to the previous results regarding
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organic semiconductors with long alkyl substituents,[15] Cz-alkyl-6
exhibits very good compatibility with organic materials even in a
crystalline state. Thus large-size polycrystalline samples with
different shapes can be easily prepared as well (letters, Figure
S17a). The XRD spectra of these doping systems resemble that
of pure Cz-alkyl-6, even at high dopant concentration (1%Y,
Figure S17b). Additionally, these doping systems are very stable
and phase separation are not observed. Their photoluminescence
(PL) spectral profile are stable over a one-week time-frame
(Figure S17c). Interestingly, these doping systems show different
CL colors during crystallization spanning the whole visible
spectrum, thus providing a new strategy for emission tuning of CL
as well (Figure 5e).
obtained from Cz-alkyl-6 samples doped with 1% different dyes (M: dye R; L:
no dye; C: dye Y; H: dye G; I: dye B, respectively). (d) Lager-area polycrystalline
thin film of Cz-alkyl-6. (e) ML device of Cz-alkyl-6:1% dye G system for finger
touching optical sensor (Video S7). (f) ML device of Cz-alkyl-6:1% bye G system
for gas flow sensor (Video S8), the pressure threshold for ML turn on in this
device is measured to be about 5k Pa.
So, excellent colorful organic ML can be firstly obtained simply
by guest-host systems based on Cz-alkyl-6. It provides an
efficient approach for color tuning of organic ML materials. More
excitingly, these doping systems possess excellent ML properties
like the pure host Cz-alkyl-6, especially in-situ solar-renewable
(Figure S18) and sensitive ML. Thus, for the first time, preliminary
large-area sandwich-type organic ML devices can be fabricated
readily, by placing the ML sample between a quartz and a soft
polymer layer (Figure 6a). Profited from the solar-renewable
bright ML of these samples, these devices can be repeatedly
utilized as visual handwriting dynamical monitor (pure Cz-alkyl-6
sample, Video S5), or clearly colorful piezo-display of letters
(doping samples, Figure 6b-c and Video S6). Amazingly, these
organic samples exhibited extremely sensitive ML even in their
sandwich-type devices. For example, the device of sample doped
with 1% dye G can emit obvious green light when lightly touched
with fingers (Figure 6e and Video S7) or blown with a gentle
stream of nitrogen (N2) gas (Figures 6f, S19 and Video S8). From
this gas blowing ML experiment, a ML pressure threshold for
organic materials was measured for the first time. It is about 5k
Pa on the sample surface for ML turn-on (≈ pressure produced by
a table tennis upon a piece of printing paper). This is also the
reported lowest one, even compared with the typical inorganic ML
material (ZnS, ≈ 600k Pa). Therefore, these organic systems with
excellent ML properties are novel promising candidates for
practical ML applications.
In summary, substitution of long alkyl chain into organic ML
materials brings alkyl chain zone (weak intermolecular interaction
zone) and loose packing to their crystals. This endows the crystal
easily to be destroyed and re-formed. In other words, the crystal
possesses strong crystallization ability and low phase transition
temperatures (including Tg, Tc, and Tm). Therefore, these organic
materials exhibited partially repeated ML, as their amorphous
samples are difficult to obtained by ground. More amazingly, their
crystals can be renewed for next round ML emission, simply by
in-situ recrystallizing their melts after exposed to standard
sunlight in a short time (≤ 60s). That is, the crystal shows in-situ
solar-renewable ML, which is a totally repeated ML operated by a
contactless approach. Furthermore, the long alkyl chain also
gives the organic ML crystal excellent compatibility with different
kinds of organic dyes. Thus highly emissive fluorescent or
phosphorescent organic materials with different colors can be
doped into these ML crystals. The resulted stable doping crystals
show interesting dual-peak ML emission, due to the energy
transfer from the host crystal to the guest doped dye. Various ML
properties, including diverse colors and lifetime of ML, can be
achieved in these doping systems. The above advantages of long
alkyl chain substituted ML materials could provide a promising
route for the application of organic ML materials, showing solar-
renewable and colorful ML. Furthermore, large-area sandwich-
type ML devices, which can be renewed in-situ by non-contact
solar exposure, have been firstly fabricated conveniently and
repeatedly used as visual handwriting dynamical monitor, colorful
piezo-display, and sensitive optical sensor for finger touching or
Figure 5. (a) Solid state absorbance of different dyes; PL and ML spectra of Cz-
alkyl-6 (lines with shadow). Time-resolved emission decay curves of Cz-alkyl-
6:dye Y doping systems: (b) dye Y (1%) excited at 350 or 455 nm; (c) Cz-alkyl-
6 in systems with different doped concentrations of dye Y. Instrument Response
Function (IRF) is also provided for comparison. (d) Schematic diagram of energy
transfer between host (Cz-alkyl-6) and the guest (doped dye), the energy values
were calculated from their spectra (Table S1). (e) Photos of CL from the above
Cz-alkyl-6 doping systems in Figure 4a.
Figure 6. (a) Stereogram (up) and cross-section view (below) of the sandwich-
type ML device. (b) Schematic diagram of piezo-display (letter V) obtained from
ML device using a metal mold (Video S6). (c) Colorful piezo-display (letters)
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Keywords: solar-renewable • colorful • mechanoluminescence •
alkyl chain • in-situ
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10.1002/anie.201806861
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
In-Situ Solar-renewable organic
colorful mechanoluminescence
Wenlang Li,† Qiuyi Huang,† Zhu Mao,
Qi Li, Long Jiang, Zongliang Xie, Rui
Xu, Zhiyong Yang,* Juan Zhao, Tao
Yu, Yi Zhang,* Matthew P. Aldred
and Zhenguo Chi*
materials
are
induced
by
introduction of “soft” alkyl chains.
For the first time, colorful ML from a
polycrystalline thin film can be
totally interatively repeated for
preliminary sandwich-type ML
device application, by simply in-situ
recrystallinzing the fractured crystal
after a non-contact exposure to
sunlight in less than one minute.
Page No. – Page No.
Alkyl Chain Introduction: In-Situ
Solar-Renewable Organic Colorful
Mechanoluminescence Materials**
This article is protected by copyright. All rights reserved.