Remarkable mechanofluorochromism and low efficiency-off electroluminescence from a fully aromatic D-A cruciform emitter

Article abstract of DOI:10.1016/j.dyepig.2019.108170

Two-dimensional aryl center-crossed cruciforms generally exhibit the strongly twisted conformation and the spatial separation of frontier molecular orbitals, which can render the emitters destructible packing structures and high solid-state fluorescence e

Full text of DOI:10.1016/j.dyepig.2019.108170

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Remarkable mechanofluorochromism and low efficiency-off electroluminescence from
a fully aromatic D-A cruciform emitter
Xin Xu, Lei Xu, Qikun Sun, Lingtai Yue, Yaguang Wang, Guangshui Yu, Haichang
Zhang, Shanfeng Xue, Wenjun Yang
PII:
S0143-7208(19)32814-1
DOI:
https://doi.org/10.1016/j.dyepig.2019.108170
DYPI 108170
Reference:
To appear in: Dyes and Pigments
Received Date: 2 December 2019
Revised Date: 18 December 2019
Accepted Date: 23 December 2019
Please cite this article as: Xu X, Xu L, Sun Q, Yue L, Wang Y, Yu G, Zhang H, Xue S, Yang W,
Remarkable mechanofluorochromism and low efficiency-off electroluminescence from a fully aromatic D-
A cruciform emitter, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2019.108170.
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Graphical Abstract

Remarkable mechanofluorochromism and low efficiency-off electroluminescence
from a fully aromatic D-A cruciform emitter
Xin Xu,ǂ Lei Xu,ǂ Qikun Sun, Lingtai Yue, Yaguang Wang, Guangshui Yu, Haichang
Zhang, Shanfeng Xue,* Wenjun Yang*
Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province
(QUST),
School of Polymer Science & Engineering, Qingdao University of Science &
Technology, 53-Zhengzhou Road, Qingdao 266042, P. R. China.
Xin Xu ǂ and Lei Xuǂ contributed equally.
* Corresponding author: Shanfeng Xue, Email: sfxue@qust.edu.cn
* Corresponding author: Wenjun Yang, Email: ywjph2004@qust.edu.cn
Abstract:
Two-dimensional aryl center-crossed cruciforms generally exhibit the strongly twisted
conformation and the spatial separation of frontier molecular orbitals, which can
render the emitters destructible packing structures and high solid-state fluorescence
efficiency. In the current work, we design and synthesize a benzene-centered
cruciform emitter (DOT) with two adjacent phenyloxadiazoles and two adjacent
triphenylamines starting from commercially available chemicals. Single crystal
analysis indicates that DOT molecules loosely pack together in a twisted
conformation and weak inter-molecular interactions. Upon mechanical grinding, the
crystalline DOT is changed into an amorphous state, accompanying with a remarkable
(56 nm) mechanofluorochromism (MFC). In view of the crossed donor and acceptor
structure and the strong solid-state fluorescence, the non-doped electroluminescence
(EL) devices with the structures ITO/HATCN/NPB or TAPC/DOT/TPBi/LiF/Al are

fabricated. The devices efficiently emit 500 nm green EL with the low turn on voltage
of 2.7 V and the low efficiency roll-off. Thus, we have provided a feasible synthetic
strategy for various isomers and analogues of fully aromatic benzene-centered D‒A
cruciforms and demonstrated a new class of candidates for MFC and EL materials.
Keywords: benzene-center cruciform; two-dimensional conjugated dye;
electroluminescence; mechanofluorochromism; efficiency off
1. Introduction
Organic light-emitting materials are attracting much attention owing to their
promising applications in light-emitting diodes, optical recording and
anti-counterfeiting, etc.[1‒11] Many conjugated organic molecules with diverse
conformations and configurations have been designed and synthesized for
obtaining multifunctional materials and improving optoelectronic properties.
Among them, linear or one-dimensional conjugated organic molecules are ever
the mainstay of advanced materials for optical and optoelectronic applications.
In contrast, high dimensional organic molecules including conjugated organic
framework and arylene-centered branched oligomers have started to draw
research interest in recent years, which has expedited the appearance of some
benzene-, anthracene-, benzobisoxazole-, pyrazine-centered cruciforms.[3, 12‒
15] These new configuration molecules have greatly widened the scope of
material candidates in chemosensors, molecular switches, nonlinear optics,

field effect transistors, and photovoltaic and electroluminescence devices.
While we have synthesized and investigated the asymmetrical π-center
2,6,9,10-tetra(arylvinyl)anthracene derivatives showing unique aggregation-
enhanced one- and two-photon absorption and emission behaviors, we are now
interested in symmetrical π-center benzene-cored cruciforms with fully
aromatic donor and acceptor branches. It should be noted that the benzene-
centered cruciforms with unsaturated double and triple bonds as conjugation
bridge to link electron donors and acceptors have been synthesized and
investigated widely,[16‒20] including those with pyridine and dibutylamine as
the end groups and sensing some positive ions.[21‒27]
However, the fully aromatic benzene-centered conjugated cruciforms are
rarely synthesized and investigated, to the best of our knowledge. Recently, Xu
et al. synthesized a series of benzene-centered derivatives with two carbazoles
and two diphenylphosphine oxide as the branches to serve as the blue thermally
activated delayed fluorescence-emitting host to demonstrate the superiority of
blue TADF-emitting host for high-efficient white light-emitting diodes.[28‒32]
Intrinsically, these molecules are not the real benzene-centered conjugated
cruciforms due to the sp3-hybrid P atom. Samuel and Zysman-Colman et al.
reported that the benzene-centered cruciforms with oxadizole and carbazole as
branches are the thermally activated delayed fluorescence emitters.[33, 34] The
doped organic light-emitting diodes can show the efficient blue
electroluminescence but the high turn-on voltages of 3.85‒4.60 V and the sharp

efficiency roll-off. We consider that conjugation length, molecular
configuration, and donor and acceptor nature can seriously influence molecular
conformation, intramolecular charge transfer, and aggregation state structure to
tune the optoelectronic properties. Thus, a benzene-centered cruciform emitter
with two adjacent phenyloxadiazoles and two adjacent triphenylamines as the
branches, namely DOT (Scheme 1), is designed and readily synthesized. The
results show that crystalline DOT can be ground into an amorphous state,
accompanying with a remarkable (56 nm) mechanofluorochromism, and the
non-doped light-emitting diodes with DOT as emitting layer efficiently emit
green electroluminescence with the low turn on voltage of 2.7 V and the low
efficiency roll-off.
Scheme 1. The structure and synthetic route for the fully aromatic
benzene-centered cruciform DOT.
2. Experimental section

2.1 Materials
All solvents and reagents were of analytical grade and commercially
available and used as received without further purification, unless otherwise
claimed. 1,2-Dibromo-4,5-dimethylbenzene, 5-phenyl-1H-tetrazole, and (4-(di-
phenylamino)phenyl)boronic acid were from Energy Chem. Co. Ltd., China.
2.2 Measurements
13
1H and C NMR spectra were recorded on a Bruker-AC500 (500 MHz)
spectrometer with CDCl3 as the solvent. Elemental analysis was performed on a
Perkin-Elmer 2400. Photoluminescence spectra were recorded with a Hitachi
F-4600 spectrophotometer, and the peak wavelength of the lowest energy
absorption band was used as the excitation wavelength Powder wide angle
X-ray diffraction (PXRD) was performed on a Powder X-ray Diffractometer
(INCA Energy, Oxford Instruments), operating at 3 kW. Differential scanning
calorimetry (DSC) experiments were carried out on a Netzsch DSC 204F1 at a
o
heating rate of 10 C min–1. Thermal gravimetric analysis (TGA) was
undertaken on a Perkin-Elmer thermal analysis system at a heating rate of 10 oC
min–1 and a nitrogen flow rate of 80 mL min–1. UV-vis absorption spectra were
recorded on a Hitachi U-4100 spectrophotometer. Solution and solid-state
fluorescence efficiency was determined with FLS980 Spectrometer equipping a
integrating sphere. Mechanochromic luminescence and recovering experiments:
a quantity of DOT solid was placed on a glass plate and simply ground by a
metal spade. After the fluorescence color and spectrum was recorded, the

o
ground sample was put into an 180 C oven and annealed for 5 min, and then
kept for 1 min at room temperature before the measurements. The fuming
experiment is to place the ground sample above the dichloromethane level and
kept for 10 min at room temperature.
2.3 Device fabrication
Indium-tin oxide (ITO) coated glass with a sheet resistance of 15–20 Ω per
square was used as the substrate. The substrate was prepatterned by photo-
lithography to give an effective device size of 0.09 cm2. It was then cleaned in
an ultrasonic bath with acetone, detergent, deionized water, and isopropanol in
a sequence, and dried in an oven. On the top of that, organic layers,
dipyrazino[2,3-f.2’,3’-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN, 5
nm),
N,N’-dinaphthyl-N,N’-diphenylbenzidine (NPB, 40 nm) or 1,1-bis[4-
[N.N’-di(p-tolyl)amino]phenyl]cyclohexane (TAPC, 40 nm), emitting layer
(DOT, 20 nm), and 1,3,5-tri(phenyl-2-benzimidazolyl)benzene (TPBi, 40 nm)
were vacuum deposited in a sequence. Finally, a 1.0 nm thick LiF film and a
100 nm thick Al film were evaporated with a shadow mask to form the top
electrode, at a base pressure of 3 × 10–4 Pa. The thickness of the evaporated
cathodes was monitored using a quartz crystal thickness/ratio monitor (Model:
STM-100/MF, Sycon). The effective luminous area of the device is 0.09 cm2.
Electroluminescence (EL) spectra were measured by a PR650 spectra scan
spectrometer, and luminance–current density–voltage characteristics were
recorded simultaneously with the measurement of the EL spectra by combining

the spectrometer with a Keithley model 2400 programmable voltage–current
source. Measurements were carried out at room temperature ambient
conditions.
2.4 Synthesis
4,5-Dibromophthalic acid.
In a 500 mL of two-neck flask, 1,2-dibromo-4,5-dimethylbenzene (10.2 g)
o
and water (250 mL) were added. The mixture was heated to 100 C and then
KMnO4 powder (24 g) was successively added in 10 portions within 2 h
o
under stirring. After stirring for 8 h at 100 C, the mixture was cooled to room
temperature and sodium hydrogen sulfite (24 g) was added and stirred for 2 h.
Further, KOH (16 g) was added and stirred for 2 h. The insoluble solid was
filtrated and the filtrate was dropwise added sulfuric acid to precipitate the
product. The white solid was collected and washed with water. The dried
product (2.9 g, yield 24%) was used for next step without further
characterization and purification.
5,5'-(4,5-Dibromo-1,2-phenylene)bis(2-phenyl-1,3,4-oxadi-azole).
4 mL of anhydrous DMF was added dropwise into a 100 mL of one-neck
flask containing 4,5-dibromophthalic acid (2.6 g) and SOCl2 (20 mL). After
o
stirring for 8 h at 80 C, the volatile liquids were removed by distillation. The
residue (4,5-dibromophthaloyldichloride) in the flask was added pyridine (10
o
mL) and 5-phenyl-1H-tetrazole (2.6 g) and then stirred at 90 C for 24 h. After
removing the pyridine by rotating evaporation, ethanol was added to precipitate

the crude product. After separation by a column chromatography on silica gel
using ethyl acetate/dichloromethane (1/10) as the eluent, the pure product was
1
obtained (071 g, 24%). H NMR (500 MHz): 8.40 (s, 2H), 7.93‒7.89 (m, 4H),
7.51‒7.46 (m, 2H), 7.40 ppm (dd,
J = 8.5, 7.1 Hz, 4H). 13C NMR (125 MHz):
165.54, 161.48, 135.26, 132.13, 129.05, 126.92, 126.73, 123.27, 122.98 ppm.
Anal. Calcd for C22H12Br2N4O2: C, 50.41; H, 2.31; Br, 30.49; N, 10.69; O, 6.10.
Found: C, 50.45; H, 2.33; N, 10.65%.
N4,N4,N4'',N4''-Tetraphenyl-4',5'-bis(5-phenyl-1,3,4-oxadiazol-2-yl)-[1,1':2',1
''-terphenyl]-4,4''-diamine (DOT)
.
5,5'-(4,5-Dibromo-1,2-phenylene)-bis(2-phenyl-1,3,4-oxadiazole (0.41 g,
0.78 mmol), (4-(diphenylamino)phenyl)boronic acid (0.54 g, 1.88 mmol),
Pd(PPh3)4 (63 mg, 54 umol), K2CO3 (0.16 g, 1.16 mmol), THF(20 mL), and
H2O (4 mL) were added into an 100 mL of flask and purged with nitrogen. The
o
mixture was stirred for 48 h at 80 C. The extracted organic phase with
dichloromethane was dried over anhydrous magnesium sulphate and separated
by a column chromatography on silica gel using ethyl acetate/dichloromethane
(3/50) as the eluent. The separated product was further purified by
re-crystallization from chloroform and vacuum sublimation. The yield is 85%
(0.57 g). 1H NMR (500 MHz): δ 8.20 (s, 2H), 7.94 (d, J = 7.7 Hz, 4H), 7.48 (t, J =
7.5 Hz, 2H), 7.40 (t, J = 7.7 Hz, 4H), 7.28 (d, J = 7.8 Hz, 8H), 7.13 (dd, J = 10.9, 8.6
13
Hz, 12H), 7.09–6.98 ppm (m, 8H). C NMR (125 MHz): δ 165.15, 163.34, 147.45,
147.35, 143.75, 132.67, 132.57, 131.81, 130.45, 129.32, 128.98, 126.85, 124.68,

123.38, 123.24, 122.51, 121.68 ppm. Anal. Calcd for C58H40N6O2: C, 81.67; H,
4.73; N, 9.85; O, 3.75. Found: C, 81.75; H, 4.70; N, 9.89%.
3. Results and discussion
3.1 Synthesis and solution photo-physical properties
The synthetic route and structure of DOT are depicted in the Scheme 1.
Commercially available 1,2-dibromo-4,5-dimethylbenzene was oxidized to
4,5-dibromophthalic acid in an acceptable yield of 24%. This dried product was
treated with dichlorosulfoxide, followed by reacting with 5-phenyltetrazole to
afford 5,5'-(4,5-dibromo-1,2-phenylene)-bis(2-phenyl-1,3,4-oxadiazole). This is
an useful intermediate because it can undergo the C-N coupling, the Heck
coupling, and the Suzuki coupling to produce diverse conjugated cruciforms.
When (4-(diphenylamino)phenyl)boronic acid was used to perform the Suzuki
coupling, the target compound DOT was obtained in a high yield of 85%. The
chemical composition and structure were confirmed by NMR spectra and
elemental analysis. DOT is soluble in common organic solvents, and the
solutions with the concentration of 10 uM in hexane (Hex), triethylamine
(TEA), butyl ether (BE), isopropyl ether (IPE), ethyl acetate (EA),
tetrahydrofuran (THF), dichloromethane (DCM), acetonitrile (MeCN), and
dimethyl formamide (DMF) are prepared. DOT solutions emit blue to orange
fluorescence with peak emission wavelengths ranging from 453 to 575 nm
upon increasing the solvent polarity (Figure 1), displaying a strong
solvatochromic effect with a large spectral shift of 123 nm from hexane to DMF.

Figure 2 depicts the plot of the Stokes shift (va ‒ vf) versus the solvent polarity
function . The slope fitted according to the Lippert–Mataga equation affords a
f
two-section curve. Although the two slopes are different, the continuing upward
trends with the increase of solvent polarity are observed. Moreover, the
quantum chemical calculations demonstrate that the HOMO electrons wholly
localize on triphenylamine moieties and the LUMO electrons all concentrate on
phenyl-oxadiazole branches (right side in Figure 2), a typical spatial separation
of the frontier molecular orbital with few HOMO/LUMO overlap. These results
indicate that DOT obviously shows the polarity-sensitive charge transfer (CT)
state nature.
Figure 1. The fluorescence photos and emission spectra of DOT in different
solvents under the excitation of 365 nm UV light.

Figure 2. The plot of the Stokes shifts (νa−νf) versus the solvent polarity function (f)
for DOT in various solvents and the calculated frontier molecular orbital nature.
3.2 Solid-state fluorescence and thermal properties
Organic luminophores can aggregate together in different stacking modes,
depending on external stimuli (or preparation) conditions. Investigations on
MFC materials and phenomenon have attracted much attention in the past
decade.[35-37] However, the molecules with two-dimensional cruciform
configuration are rarely exploited as MFC candidates. We investigate the MFC
behavior of DOT solid under simple mechanical grinding on a glass plate using
a metal spade. Powder X-ray diffraction patterns clearly indicate the grinding
has induced a phase transformation from crystalline to amorphous states
(Figure 3a). The diffraction pattern of the pristine DOT solid shows sharp and
intense diffraction peaks, indicating a well-ordered microcrystalline structure.
In contrast, the diffractogram of the ground DOT solid displays broad and

depressed reflections. As shown in Figure 4, the pristine solid emits blue
fluorescence (462 nm) and can be readily changed into a green emission (518
nm) upon grinding, affording a remarkable color change and a large spectral
shift of 56 nm. The grinding-induced amorphization is further evidenced by the
differential scanning calorimetry (DSC) analysis (Figure 3b). DSC curves
reveal that the pristine solid has no thermal transition before isotropic melting
o
transition (257 C), but the ground sample shows a glass transition temperature
o
o
at 114 C and a cold crystallization temperature at 150 C. Notably, both
pristine and ground solids exhibit the high fluorescence efficiency (47% and
56%, respectively). To examine MFC recovery, the ground sample is fumed on
various volatile organic solvents and annealed between the glass transition and
o
the cold crystallization temperatures (114‒150 C). It is found that the
fluorescence color is only blue-shifted to blue-green and can not be recovered
to the pristine blue color, signifying the not easy to crystallization of the
amorphous state. We further anneal the ground sample above the cold
o
crystallization temperature, such as at 170 C, and find that the fluorescence
color can be recovered to the pristine blue color. Regrinding the fumed and
annealed samples can produce the same MFC effectiveness as the first grinding
(Figure 4). Thus, the large and reversible MFC behavior happens only upon
grinding and high-temperature annealing and is ascribed to the phase transition
between crystalline and amorphous states.

Figure 3. (a) X-ray diffraction curves and (b) DSC curves of DOT solids under
pristine crystalline state and ground state.
Figure 4. The fluorescence photos and emission spectra of DOT solids under
different external stimuli.
Fortunately, we have obtained the analyzable single crystal of DOT
(CCDC number: 1965820), which enable us to gain an insight into the

molecular packing mode to understand the high solid-state fluorescence
efficiency and the destructible crystal structure. Single crystal analysis indicates
the dihedral angles between the peripheral phenyl and oxadiazyl units are not
large (8.58o and 18.4o, Figure 5) due to the weak intramolecular repulsion.
However, the dihedral angles between the central benzene and oxadiazyl rings
are large (33.9o and 37.7o) owing to the strong mutual repulsion of two adjacent
oxadiazole rings. As expected, when two phenyls are ortho-linked to the same
benzene ring, the very stronger mutual repulsion has resulted in the larger
dihedral angles between the central benzene and the two connected phenyls
(51.0o and 66.8o). Triphenylamine moieties are in the inherent propeller
conformation. The two relatively strong interacted molecules are also shown in
Figure 5, and 12 CH···π and 4 CH···N interactions are observed, which is not
much and strong relatively for such a large-size molecule. Thus, DOT adopts a
highly twisted conformation to loosely pack together with weak intermolecular
interactions and without π-π interactions (Figure 5b), which should be favorable
for the fluorescence emission and the disordering destruction of crystal
structure under mechanical stimuli.

Figure 5. The cell structure, molecular dihedral angle, and the intermolecular
interactions between two closely adjacent molecules in the single crystal.
3.3 Electroluminescence properties
Except for the admirable MFC and unique recovery behavior, DOT
exhibits high fluorescence efficiency not only in solution and crystalline states
but also in amorphous ground and film states. The fluorescence efficiency for
the vacuum-evaporated film is up to 64.7%, which is even higher than the
ground state (56%). In view of the high fluorescence efficiency and and the
donor- and acceptor-containing structure as well as the relatively high glass
transition temperature (Figure 3b), DOT is expected to be a promising
candidate as an efficient emitter for organic light-emitting diodes (OLED).

Devices with the structure ITO/HATCN (5 nm)/TAPC or NPB (40 nm)/DOT
(20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) are fabricated, where HATCN,
TAPC (device 1) or NPB (device 2), and TPBI are used as the hole-injection,
hole-transport, and electron-transport/ hole blocking layers, respectively. Both
device 1 and 2 exhibit same green-emitting electroluminescence (EL), and their
EL spectra are also similar to the fluorescence spectrum of the vacuum-
evaporated film (Figure 6).
Figure 6. The absorption and photo-luminescence spectra of vacuum
evaporated film and the electroluminescence spectra for the devices of
ITO/HATCN/HTM/DOT/TPBi/LiF/Al, where the hole transport materials
(HTM) are TAPC (device 1) and NPB (device 2), respectively.
The turn-on voltages of both devices are low at 2.69 V and 2.65 V,
respectively. However, in spite of the same EL spectra and turn-on voltages,
both devices exhibit very different EL performance (Figure 7). The maximum
brightness is 14350 and 24500 cd/m2 for devices 1 and device 2, respectively,

and the maximum current and power efficiency are 6.28 cd/A and 5.92 lm/W
for device 1 and 9.27 cd/A and 7.05 lm/W for device 2. The low efficiency
roll-off is also observed. For example, the current efficiency is 9.15, 9.21, 8.12
cd/A for the device 2 at the luminance of 100, 1000, and 10000 cd/m2,
respectively. These results indicate that DOT is an efficient emitter for
fluorescence-based EL devices. It is noted that organic luminophors with strong
twisted conjugation skeleton and loose intermolecular stacking are commonly
unfavorable for carrier transporting. For that matter, the thinner emitter layer
and the doped devices could boost the DOT potential to significantly improve
EL performance. The device can exhibit better EL performance by employing
NPB as the hole transport layer. Therefore, we predict that the device
performance can be further improved by optimizing device compositions and
structures, which is underway in our laboratory.

Figure 7. The current efficiency and power efficiency versus EL luminance
curves for the device 1 (ITO/HATCN/TAPC/DOT/TPBi/LiF/Al) (a) and the
device 2 (ITO/HATCN/NPB/DOT/TPBi/LiF/Al) (b). Inserts: The current
density–voltage–brightness characteristics.
4. Conclusions
We have designed and synthesized a fully aromatic benzene-centered
cruciform emitter (DOT) with two adjacent donors and two adjacent acceptors
as the branches. The crowded substitution of four aryl branches on the same
benzene ring has resulted in a strongly twisted conformation and loosely
stacking structure, which render luminorphors the destructible crystal structure
and the high solid-state fluorescence efficiency. This kind of luminescent
cruciforms could serve as promising mechanofluorochromism and
light-emitting diode materials. We have demonstrated that DOT crystal can
readily produce amorphization with a large spectral shift under simple
mechanical grinding, and the non-doped devices exhibit the high luminance,

current efficiency, power efficiency, and the low turn-on voltage and efficiency
roll-off. This work has also provided an effective synthetic strategy for a new
class of fully aromatic benzene-centered cruciforms, and many isomers and
analogues can be prepared by employing different xylenes and donors, which
and the device optimization are underway in our laboratory.
5. Acknowledgements
Financial support for this research was provided by the National Natural Scienc
e Foundation of China (No. 51673105, 51873095 and 51573081), Natural Scien
ce Foundation of Shandong Province (No. ZR2018MB024), the Key Project of
Higher Educational Science and Technology Program of Shandong Province of
China (No. J18KZ001) and the Natural Science Foundation of Qingdao City of
China (No. 16-5-1-89-jch). We thank the open project of The State Key Laborat
ory of Supramolecular Structure and Materials of Jilin University (SKLSSM-20
19033).
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Figure 5. The cell structure, molecular dihedral angle, and the intermolecular
interactions between two closely adjacent molecules in the single crystal.

Highlight:
1. A new two-dimensional conjugated molecule is designed and synthesized
2. Crossed donor-acceptor molecules show high ligh-emitting efficiency
3. Cruciforms with strong twisted structure are easy to mechanofluorochromism\\
4. A non-doped EL device with low efficiency off is achieved

Xin Xu and Lei Xu conducted the synthesis and the photophysical
characterizations of the materials. Lei Xu and Lingtai Yue fabricated the
OLED for the materials and estimated the OLED performances. Qikun
Sun analyzed the single crystal structure. Yaguang Wang did the
theoretical calculations. Guangshui Yu is responsible for the basic testing
and analysis of thermal and electrochemical properties. Haichang Zhang
tested and analyzed the mechanofluorochromism properties. Xin Xu and
Lei Xu wrote the paper, and all the authors revised it. Shanfeng Xue and
Wenjun Yang supervised the whole work. Xin Xu and Lei Xu contributed
equally to this work.