Elucidating the Excited State of Mechanoluminescence in Organic Luminogens with Room-Temperature Phosphorescence

Article abstract of DOI:10.1002/anie.201708119

Two stable, purely organic luminogens exhibit both mechano- (ML) and photoluminescence (PL) with dual fluorescence–phosphorescence emissions at room temperature. Careful analysis of the crystal structures, coupled with theoretical calculations, demonstrate that room-temperature phosphorescence and ML properties are strongly related to molecular packing. In particular, the formation and fracture of molecular dimers with intermolecular charge-transfer properties has a significant effect on intersystem crossing, as well as excited triplet state emissions, in both PL and ML processes.

Full text of DOI:10.1002/anie.201708119

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Accepted Article
Title: An initial attempt to reveal the excited state of
mechanoluminescence from purely organic luminogens with
room temperature phosphorescence
Authors: Zhen Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708119
Angew. Chem. 10.1002/ange.201708119
Link to VoR: http://dx.doi.org/10.1002/anie.201708119
http://dx.doi.org/10.1002/ange.201708119

10.1002/anie.201708119
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An initial attempt to reveal the excited state of
mechanoluminescence from purely organic luminogens with room
temperature phosphorescence
Jie Yang, Xuming Gao, Zongliang Xie, Yanbin Gong, Manman Fang, Qian Peng, Zhenguo
Chi, Zhen Li*
Abstract: For the first time, two stable purely organic luminogens
were reported to exhibit both mechanoluminescence (ML) and
photoluminescence (PL) with dual fluorescence-phosphorescence
emissions at room temperature. Careful analyses of the crystal
structures coupled with theoretical calculations, demonstrated that the
room temperature phosphorescence (RTP) and ML properties are
both heavily related to the molecular packing. Particularly, the
formation and fracture of molecular dimers with intermolecular charge
transfer have great effect on the ISC transitions as well as excited
triplet state emissions, in both PL and ML processes.
Mechanoluminescence (ML), as
a special light-emitting
process induced by mechanical stimulation such as grinding,
rubbing, or shaking, has attracted extensive attention for its
potential applications in the displays, lighting, bioimaging, and
stress sensing.1 Regardless of its long history, with the first report
of sugar by Francis Bacon in 1605, organic ML luminogens
remain limited in number, partially due to the lack of reliable
guidance and unclear inherent mechanism. On the other hand, as
the most investigated light-emitting process, photoluminescence
(PL) has been well understood, which made great contribution to
promote the researches on other kinds of emitting processes,
including electroluminescence (EL) and chemiluminescence
(CL).2 Based on these success stories and the previous reported
ML luminogens, we would like to explore the unknown excited
process of mechanoluminescence with the well understood
photoluminescence (PL) acting as template.
Figure 1 The development process to explore the unknown excited state of
mechanoluminescence with the well understood photoluminescence (PL)
process acting as template.
in principle there should be. In 2016, our group was lucky to obtain
a mechanophosphorescence from DPP-BO at room temperature,
which demonstrated the existence of excited triplet state in the ML
process.4 For the PL emissions of DPP-BO, it was a little pity that
no phosphorescence was observed at room temperature,
although strong and long lifetime phosphorescence could be
achieved under low temperature (i.e. 77 K). The much different
emitting behaviors between PL and ML process at room
temperature left a patch of dark cloud on the understanding of ML
process and internal mechanism. Thus, this urged us to develop
some bright ML luminogens with room temperature
phosphorescence in PL process. Hopefully, through the
comprehensive comparison between their PL and ML behaviors,
more clear understanding on ML could be achieved.
Phenothiazine is a well-known heterocyclic compound with
electron-rich nitrogen and sulfur heteroatoms, which favors the n-
π* transition and good communication between excited singlet
state and triplet state, as partially confirmed by some
phenothiazine-based luminogens with the TADF (thermally
activated delayed fluorescence) property.5 Meanwhile, its non-
planar “butterfly” structure could hinder the possible
intermolecular π-π stacking as well as the non-radiative transition,
to some extent.6 In this communication, the phenothiazine group
was selected as the main building block to construct two purely
organic luminogens of CzS-CH3 and CzS-C2H5 with ultra-simple
molecular structures, in which, the RTP and ML properties have
been successfully integrated. Careful analyses of the crystal
structures, coupled with theoretical calculations, demonstrate that
the RTP and ML properties are both heavily related to the
Thanks to the enthusiasm of scientists, some bright ML
luminogens have been developed by combining the concept of
aggregation-induced emission (AIE) since 2015 (Figure 1).3
However, they mainly concerned on the fluorophores with
emissions from excited singlet state, while the luminogens with
mechanophosphorescence have seldom been reported, although
[a] J. Yang, X. Gao, Y. Gong, M. Fang, Prof. Z. Li
Department of Chemistry
Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials
Wuhan University, Wuhan 430072, China. Fax: (86)27-68756757
E-mail: lizhen@whu.edu.cn or lichemlab@163.com
[b] Z. Xie, Prof. Z. Chi
PCFM Lab, GDHPPC Lab, KLGHEI of Environment and Energy
Chemistry
State Key Laboratory of Optoelectronic Material and Technologies
School of Chemistry and Chemical Engineering
Sun Yat-sen University, Guangzhou510275 (China)
[c] Prof. Q. Peng
Key Laboratory of Organic Solids
Beijing National Laboratory for Molecular Science (BNLMS), Institute
of Chemistry
Chinese Academy of Sciences, 100190 Beijing, China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201708119
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molecular packing, especially for the formation and fracture of
molecular dimers with intermolecular charge transfer in the light-
emitting process. Herein, we present the synthesis, PL and ML
characterizations, crystal structures, and theoretical calculations
of CzS-CH3 and CzS-C2H5 in detail, to understand their unique ML
process and internal mechanism.
Both of CzS-CH3 and CzS-C2H5 could be conveniently
synthesized through only one step (Scheme S1). Their UV-visible
absorption spectra were measured in THF solution, in which,
CzS-C2H5 gave a little violet-shifted absorption at 254, 310 nm in
comparison with CzS-CH3 (256, 312 nm), indicating their minor
different electronic structures (Figure S1). Similar phenomenon
occurred for their PL spectra measured in THF solutions with
deep blue emissions at room temperature (Figure S2). Under a
low temperature of 77K, the PL emissions of CzS-CH3 and CzS-
C2H5 changed to the bright green ones with two emission peaks
around 484, 511 nm (Figure S3), which could last about 1 s even
after stopping the UV-irradiation (Figure 2E). The green
emissions with such long lifetimes could well certify that they were
phosphorescence from the excited triplet state.
CzS-C2H5 crystal presented two much different emission peaks
(426, 497 nm) at room temperature (Figure 2A), and the emission
lifetime of the peak at 497 nm could be up to 4.59 ms (Figure 2B,
Figure S4), exhibiting the typical RTP effect.7 Furthermore, bright
blue emission was observed by scraping its solid samples at room
temperature, showing remarkable ML character. In the ML
spectrum of CzS-C2H5 crystal (Figure 2A, the ML spectrum of as
prepared CzS-C2H5 powder was shown in Figure S5), there were
one main emission peak at about 430 nm and another peak tail in
the long wave region. Through fitting multi-peaks in Origin 8.0,
two emission peaks could be separated: a strong one peaked at
about 430 nm and another weak one at about 490 nm (Figure S5),
which were in good accordance with its fluorescence and
phosphorescence in PL process. Then why CzS-C2H5 gave much
different PL and ML emissions, especially for the
phosphorescence at the longer wavelength?
In consideration of the destruction effect on crystal under
scraping in the ML process, the PL emission of the sample after
grinding should show more close relationship with ML. Thus, the
CzS-C2H5 crystal was ground heavily using a mortar and pestle,
highly-contrast mechanochromism effect could be observed,8 and
its PL emission changed from green to blue (Figure 2C, Figure
S6), accompanying with the destruction of crystal. The changes
of CzS-C2H5 crystal under grinding were further confirmed by
powder X-ray diffraction (PXRD) patterns (Figure 2D). There were
many sharp peaks for the CzS-C2H5 crystal, however much
weakened along with some peaks missing after grinding heavily,
indicating the destruction of crystal to some extent. Through
careful analyses on the PL spectrum of ground CzS-C2H5, it was
found that there was only one main emission peak at about 426
nm and another peak tail in the long wave region, much similar to
that of the ML one. At the same time, the phosphorescence
lifetime shortened to 1.59 ms and the PL quantum yield
decreased from 8.1% to 3.6% after grinding for the weakened
RTP effect, partially demonstrating the crystallization-induced
phosphorescence. Thus, the different PL behaviors in different
states could prove the significant influence of molecular packing
on the emissive property.
Figure 2 (A) The mechanoluminescence (ML) and photoluminescence (PL)
spectra of CzS-C2H5 crystal (Inset: ML image of CzS-C2H5 sample upon
scrapping with a glass rod in daylight); (B) The time-resolved PL-decay curves
for phosphorescence at 497 nm and fluorescence at 426 nm of CzS-C2H5 crystal
at room temperature; (C) the prompt PL spectra and the photos under 365 nm
UV–irradiation of CzS-C2H5 in crystal and after grinding heavily; (D) The PXRD
patterns of CzS-C2H5 in crystal and after grinding heavily; (E) The time-resolved
PL-decay curves for low temperature phosphorescence of CzS-C2H5 in solution
at 77K (Inset: the pictures taking at different times, before and after turning off
the 365 nm UV-lamp).
no delayed component, namely phosphorescence, could be
detected (Figure S4 and Figure S7). Also, no ML emission could
be observed upon scraping. This control experiment could further
demonstrate the great effect of molecular packing on RTP and ML
properties from another side.
As for CzS-CH3, similar phenomena were observed (Figure S8-
S12). It also showed crystallization-induced phosphorescence
and mechanochromism effects (green to blue).9 However, its ML
emission was much weaker and only visual in dark for its lower
PL efficiencies (ΦPL = 3.0% in crystal, 2.1% after grinding heavily).
To explore the possible mechanisms of the emissive property
for CzS-CH3 and CzS-C2H5, we cultured their single crystals,
which corresponded to the C1n1 and Pna21 space groups
respectively, and were both non-centrosymmetric (Table S1). As
shown in Figure 3 and Figure S13, efficient intermolecular
hydrogen bonds could be observed in crystals, while no π-π
interaction was found. This status could contribute much to their
relatively high PL efficiencies.10 Particularly, molecular dimers
were formed with multiple C-H…π and C-H…N bonds.11 With this
packing mode, the efficient intermolecular electronic coupling
effect should exist, thus making significant influence on their light
emissions, including PL and ML. For CzS-CH3, six C-H…π
(ranging from 2.90 to 3.90 Å) and three C-H…N bonds (3.07 to
3.90 Å) were present in the molecular dimers. Interestingly, these
dimers were connected by some relatively weaker hydrogen
bonds, including one C-H…π (2.56 Å) and two C-H…N (3.55 to
3.77 Å). Same phenomena were observed in CzS-C2H5 crystal:
two molecules interacted with each other through six C-H…π
(ranging from 3.00 to 3.90 Å) and two C-H…N bonds (3.34 and
3.36 Å) to form a dimer. Then, each of two CzS-C2H5 dimers were
Furthermore, the CzS-C2H5 was doped in polymethyl
methacrylate (PMMA) with mass fraction about 5% to eliminate
the molecular packing effect as well as restrict the non-radiative
motion. There was just one emission peak at about 440 nm and
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10.1002/anie.201708119
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formed charge transfer states, thus, served as bridge to realize
the good communication between excited singlet state and triplet
state and lead to the efficient phosphorescence in crystals upon
UV-irradiation.11a,11b,13 However, the molecular dimers would be
destructed under scraping, then resulting in the inefficient ISC
transition as well as weak mechanophosphorescence in the ML
process. Thus, the formation and fracture of molecular dimers
with intermolecular charge transfer in the light-emitting process
would be mainly responsible for the big difference between PL
and ML process.
The energy level diagrams and excited state transition
configurations of isolated molecules and dimers, were calculated
to explore how the molecular dimer affecting ISC transition
(Figure S18-S20, Table S2-S5).14 As shown in Figure S18, there
was just one minor ISC channel for the isolated CzS-CH3, while
one main and two minor channels could be observed in the
isolated CzS-C2H5, indicating its more efficient ISC transitions for
the little different molecular structure. When it turned to the
molecular dimers, much increased number of ISC channels were
obtained for the efficient intermolecular interactions and formed
intermolecular charge transfer states. For CzS-CH3 dimer, one
main and four minor channels were found. In the meantime, the
ISC channels increased to one main and six minor ones for the
CzS-C2H5 dimer. On the other hand, the decreased energy gaps
between S1 and T1 states, Δ(S1-T1), could also be found from the
isolated molecules to dimers―0.68 to 0.57 eV for CzS-CH3 and
0.66 to 0.58 eV for CzS-C2H5 (Figure S21-S22). The decreased
energy gaps of Δ(S1-T1) in the dimers could surely contribute
much to the spin-orbit coupling between S1 and T1 states.
Accordingly, the increased ISC channels between S1 and Tn
states, as well as the small energy gaps of Δ(S1-T1) could both
improve the RTP efficiency.14 Thus, these calculated results could
well certify the great promotion of RTP effect for the formed
molecular dimers, further proving the important role of molecular
packing on their light-emitting process.
Figure 3 The entire (left) and local (right) packing modes of the CzS-C2H5
crystal (The local packing picture, namely dimer, was selected from the part in
cycle of corresponding entire one).
linked with three C-H…π (ranging from 2.68 to 3.93 Å) and one
C-H…N bonds (3.76 Å) or one C-H…π (3.10 Å) and two C-H…N
bonds (3.24 and 3.42 Å). Finally, these strong intermolecular
interactions in crystals of CzS-CH3 and CzS-C2H5 could largely
reduce the possible energy loss via non-radiative relaxation
channels by decreasing the molecular slippage, under the
stimulus of mechanical force, just as demonstrated in our previous
work.3d,4
In order to further evaluate the strength of intermolecular
interactions, their powder X-ray diffraction (PXRD) spectra in
different states were measured (Figure S14-S15). As easily seen,
the as-prepared samples exhibited non-negligible peaks, which
partially disclosed their good crystallinity. On the other hand, their
crystals presented the good mechanical stability, that is, they
could retain sharp and high diffraction peaks after grinding lightly.
Only upon heavy grinding, could these crystals be destructed to
some extent without obvious PXRD peaks. These results proved
the strong intermolecular interactions present in crystals, which
could contribute much to PL and ML properties by restricting the
non-radiative transitions in these two processes.
To explore the origin of the big difference between ML and PL
spectra, and look more closely into the influence of molecular
packing, TD-DFT calculations were carried out on the isolated
molecules and relevant dimers (derived from their single-crystal
structures). First, the theoretic analyses on their S0→S1
transitions were carried out.12 As shown in Figure S16, the
isolated molecules of CzS-CH3 and CzS-C2H5 were both π-π*
transitions because of the involved HOMO, LUMO and LUMO+1
electron clouds spreading on the phenothiazine moiety. This
would not favor the ISC transition, neither for the RTP effect.
Besides, the oscillator strength (f) of CzS-C2H3 (0.0011) was
much larger than that of CzS-CH3 (0.0002), and this should be
mainly responsible for its higher PL efficiency (8.1%) than that of
CzS-CH3 (3.0%). Accordingly, increasing the overlap between
HOMO and LUMO and expanding the π conjugation could lead to
the enhanced oscillator strength (f), then improve the PL as well
as ML efficiencies.12 Interestingly, obvious CT transitions were
revealed in the dimers for both of them from an occupied orbital
of one molecule to the vacant orbital of another. Also, the red
shifted absorptions from solution to aggregation state could well
prove the formation of intermolecular CT states for CzS-CH3 and
CzS-C2H5 aggregates (Figure S17).11 Then the intermolecular
charge transfer states could result in an enhanced ISC process.
On the other hand, the increased dipole moments from 2.43 and
2.40 debye in isolated CzS-CH3 and CzS-C2H5 to 3.74 and 3.45
debye in the relevant dimers, could further certify the formation of
intermolecular charge transfer states in the molecular dimers. The
As revealed by our experimental results coupled with the theory
calculations, the molecular packing played a significant role in the
light-emitting process, including PL and ML (Figure 4). In crystal,
the formed molecular dimers through the strong intermolecular
hydrogen bonds, could lead to the intermolecular charge transfer
(CT) states, thus serving as bridge to realize the good
communication between the excited singlet and triplet states. On
one hand, it could increase the ISC channels for the orbital
Figure 4 A proposed mechanism for PL and ML process in CzS-CH3 and CzS-
C2H5―in the isolated molecules with π-π* transition from S0 to S1 state, ISC
transition would be hard to happen, while the formed CT transition in dimers
could serve as bridge to realize the good communication between excited
singlet state and triplet state, thus realizing the RTP effect; As for the ML
process, the destructed dimers upon scraping would lead to the inefficient ISC
transition as well as the weak mechanophosphorescence in ML process.
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degeneracy of two molecules;14a on the other hand, the energy Acknowledgements
gaps between S1 and T1 states could be largely shortened for the
strong electron coupling effect in dimers.14b Thus, these two
We are grateful to the National Science Foundation of China (no.
21325416, 51573140) for financial support.
factors could both facilitate their RTP effect of crystals in PL
process. If their crystals were ground heavily, the molecular dimer
would be destroyed, then leading to the weak RTP property and
mechanochromism effect. Furthermore, when they were doped
into PMMA without the intermolecular coupling effect, no RTP
effect could be achieved. On the other hand, the destructed
dimers under scraping would lead to the inefficient ISC transition
as well as the weak mechanophosphorescence in ML process.
In comparison with previous cases, the unique point of CzS-
CH3 and CzS-C2H5 is the both existing RTP and ML effects. The
RTP property demonstrated the presence of ISC and the
corresponding stable T1 state. However, the similarity of their ML
and PL (after grinding heavily) spectra indicated that the excited
energy of single molecules mainly accounted for the ML behavior
because of the fracture effect under scraping in ML process,
although the excited level of RTP mainly depended on the
molecular dimer (the crystal samples with molecular dimers show
stronger RTP than the grinding samples with cracked molecular
dimers). Further on, the excited energy of single molecules
transferred to adjacently uncracked molecular dimers partly, thus
leading to the strong mechanofluorescence and weak
mechanophosphorescence in CzS-CH3 and CzS-C2H5. Actually,
for most other ML luminogens, their ML spectra were also much
similar to the PL emissions after grinding, certifying the accuracy
of our understanding on the other hand.5 As for DPP-BO with dual
mechanofluorescnce and mechanophosphorescence in our
previous report, no RTP effect was found.4 Its excited energy
should be also formed on the single molecules at the initial state.
At the same time, some stronger coupled units for DPP-BO could
be achieved under scrapping which favored efficient ISC
transitions. Then, part of the excited energy on isolated molecules
could transfer to the strong couple ones and lead to the dual
mechanofluorescence and mechanophosphorescence.
Keywords:
phosphorescence, multiple stimulus response
Mechanoluminescence,
room
temperature
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TOC
Jie Yang, Xuming Gao, Zongliang Xie, Yanbin
Gong, Manman Fang, Qian Peng, Zhenguo Chi
and Zhen Li*
An initial attempt to reveal the excited state
of mechanoluminescence from purely
organic luminogens with room temperature
phosphorescence
For the first time, two stable purely organic luminogens were reported to exhibit both mechanoluminescence (ML) and
photoluminescence (PL) with dual fluorescence-phosphorescence emissions at room temperature. With the help of well
understood PL process acting as template, the unknown excited state of ML were revealed preliminarily.
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