Luminescent switching and structural transition through multiple external stimuli based on organic molecular polymorphs

Article abstract of DOI:10.1039/c9tc00051h

Investigations on the relationship between multi-stimuli responsive luminescent properties and aggregation structures of polymorphs play a crucial role in developing organic multi-stimuli responsive luminescent (OMSRL) materials. Herein, we reported two p

Full text of DOI:10.1039/c9tc00051h

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Luminescent switching and structural transition
through multiple external stimuli based on
organic molecular polymorphs†
Cite this:DOI: 10.1039/c9tc00051h
a
a
Bo Shao,‡a Rihua Jin,‡b Aisen Li,a Yingjie Liu,a Bao Li,
Shuping Xu,
*
a
a
Weiqing Xu,
Bin Xua and Wenjing Tian
*
Investigations on the relationship between multi-stimuli responsive luminescent properties and aggregation
structures of polymorphs play a crucial role in developing organic multi-stimuli responsive luminescent
(OMSRL) materials. Herein, we reported two polymorphs (G-phase and O-phase) based on a novel molecule
9,10-bis-((E)-4-(pyridin-3-yl)styryl)anthracene (BP3SA) and their piezochromic properties, structural transitions,
and protonation–deprotonation effects under external stimuli. Photophysical characterization together with the
analysis of crystal structures indicated that the G-phase with J-type aggregation showed green emission, while
the O-phase with H-type aggregation presented orange emission. Red-shifted emissions could be observed
for the two polymorphs under mechanical grinding pressure due to the transition from the crystalline state to
the amorphous state. Then upon heating, the ground G-phase could partly recover the initial emission, while
the emission of the ground O-phase nearly turned to the initial emission of the G-phase after heating. Powder
X-ray diffraction patterns revealed that a structural transition from the O-phase to the G-phase could be
achieved by grinding and heating processes. High pressure experiments and theoretical calculations
demonstrated that the O-phase showed a more significant red-shift than the G-phase, and the molecular
geometry of the O-phase trended toward a more planar conformation than that of the G-phase under the
same hydrostatic pressure. It indicated that the O-phase had higher sensitivity to the hydrostatic pressure than
the G-phase because of the different intermolecular interactions inside the two crystalline phases. Additionally,
the protonation–deprotonation of the two polymorphs showed that acid stimuli could induce red-shifted
emission, while the emission partly recovered to the initial emission by alkali stimuli due to the change of the
intermolecular interactions.
Received 4th January 2019,
Accepted 13th February 2019
DOI: 10.1039/c9tc00051h
rsc.li/materials-c
aggregate structure and luminescent properties.5 Great effort has
been devoted to the research on organic molecular luminescent
Introduction
Organic luminescent materials have attracted more and more crystals and their multi-stimuli responsive luminescent properties.
attention due to their unique electronic structure and optical For example, piezochromic luminescent materials based on
properties.1 Organic multi-stimuli responsive luminescence twisted molecular structures, such as tetraphenylethylene,4,6
(OMSRL) materials are of great importance because of their divinylanthracene,5b,7 triphenylamine8 and boron-containing
structural tunability and widespread applications in the fields compounds9 and other compounds,10 have been investigated
of optoelectronics, such as sensors,2 switches,3 memories4 and through the manipulation of molecular packing modes/inter-
so on. As organic molecular luminescent crystals have definite molecular interactions by grinding or hydrostatic pressure
conformation and packing mode, this can provide a visualized treatments. Moreover, acid-dependent luminescent materials
model to precisely investigate the relationship between the containing an alkaline group, such as pyridine,6c,7a,11 diazine,12
piperidine,13 and other groups,14 have also been studied by
a State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, P. R. China. E-mail: wjtian@jlu.edu.cn, xusp@jlu.edu.cn
b Department of Neurosurgery, The First Hospital of Jilin University,
Changchun 130012, P. R. China
changing their conformations or frontier molecular orbitals
with acid.
Based on the research around the multi-stimuli responsive
properties of 9,10-bis((E)-2-(pyridin-3-yl)vinyl)anthracene (BP3VA)11b
in our group, we designed and synthesized a novel molecule
9,10-bis-((E)-4-(pyridin-3-yl)styryl)anthracene (BP3SA) by introducing
benzyl groups into BP3VA. Two polymorphs (G-phase and O-phase)
† Electronic supplementary information (ESI) available. CCDC 1581638–1581640.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c9tc00051h
‡ Two authors have equal contribution for this manuscript.
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based on BP3SA were obtained by using a solvent evaporation and purified by column chromatography using dichloromethane
method. Piezochromic properties, structural transitions, together and methanol (100 : 1) to get a yellow powder with a yield of about
with the protonation–deprotonation effect were investigated for 88%. 1H NMR (500 MHz, CDCl3) d 8.96 (s, 2H), 8.65 (d, J = 4.0 Hz,
the two polymorphs. The luminescent switching and structural 2H), 8.43–8.41 (m, 4H), 8.09–8.00 (m, 4H), 7.83 (d, J = 8.5 Hz, 4H),
transition can be realized by grinding and further heating 7.71 (d, J = 8.0 Hz, 4H), 7.53–7.49 (m, 6H), 7.00 (s, 1H), 7.03(s, 1H).
processes. High pressure experiments together with the theoretical MS m/z: calcd for C40H28N2: 536.23; found 536.03.
calculation reveal that the O-phase is more sensitive to the hydro-
Single-crystal X-ray crystallography studies
static pressure than the G-phase because of the fewer interactions
among the adjacent molecules in the O-phase. In addition, the
protonation–deprotonation of the pyridine moieties has a significant
effect on the destruction of various C–HꢀꢀꢀN bonds in the G-phase,
resulting in more distinct luminescent switching of the G-phase
than that of the O-phase under acid and alkali stimuli.
Crystal diffraction data were obtained with graphite-
monochromated Mo/Cu Ka radiation using a Rigaku RAXISRAPID
diffractometer. The structure was solved by the direct method using
the SHELX program and refined with least-squares methods.
Anisotropic thermal parameters were refined for all of the non-
hydrogen atoms. Hydrogen atoms were added at the idealized
positions and refined using isotropic displacement.
Experimental section
Materials and methods
Diamond anvil cell technique
Hydrostatic pressure was obtained from a diamond anvil cell
(DAC) (BGI-type DAC) with 400 mm culet diamonds. A 0.25 mm
thick T301 stainless steel gasket (d = 5.00 mm) was drilled to
make a hole with a diameter of 0.2 mm as the sample chamber.
G-Phase crystal and O-phase crystal were placed in the sample
chamber, and a small ruby chip was introduced into the same
hole for in situ pressure calibration according to the fluores-
cence shift of the ruby R1 line. And then, silicone oil was added
as a pressure-transmitting medium (PTM), which can provide a
guarantee for obtaining the hydrostatic pressure according to
Pascal’s principle. The high-pressure fluorescence spectra
under hydrostatic conditions were measured using a fluores-
cence microscope (IX71, Olympus, 50ꢁ, NA = 0.5) equipped
with a spectrometer (Jobin Yvon iHR320). The light source was
a mercury lamp with an excitation wavelength of 365 nm.
All starting materials were purchased by commercial approaches
and used without further purification. THF was dried by distilla-
tion from sodium/benzophenone under nitrogen.
1H NMR spectra were obtained using a Bruker AVANCE 500 MHz
spectrometer. UV-vis absorption spectra were recorded by using a
Shimadzu UV-3100 spectrophotometer. Photoluminescence spectra
were collected with a Shimadzu RF-5301PC spectrophotometer
and a Maya 2000Pro. optical fibre spectrophotometer. Solid-state
PL efficiencies were measured by means of an integrating
sphere12 (C-701, Labsphere Inc.), with a 365 nm Ocean Optics
LLS-LED as the excitation source. The excitation light was
introduced into the sphere through an optical fibre.
Synthesis
BP3SA was synthesized by using the procedure shown in
Scheme S1 (ESI†) and characterized by 1H NMR, and mass
spectroscopies.
DFT calculations at GGA-PBE level under different external
stress
Compound 1 (9,10-bis(chloromethyl)anthracene) and com-
pound 2 (tetraethyl anthracene-9,10-diylbis(methylene)diphos-
phonate) were synthesized according to the previously reported
methods. Compound 2 (1.435 g, 3.00 mmol) was stirred with
4-bromobenzaldehyde (2.755 g, 14.89 mmol) in THF (90 mL)
under nitrogen. t-BuOK (1.346 g, 11.20 mmol) in THF (60 mL)
was added dropwise to the solution, and then the mixture was
stirred for 12 h at 50 1C. After removing the solvent (THF), the
resulting precipitate was filtered, washed by methanol and
purified by column chromatography using dichloromethane
and petroleum ether. Compound 3 (0.871 g) was obtained in a
The single crystal DFT calculations under different applied
pressures were performed by means of the CASTEP module in
Material Studio. The exchange–correlation (XC) effects were
described by utilizing the generalized gradient approximation
(GGA) with Perdew–Burke–Ernzerhof (PBE). A TS scheme was
used for dispersion corrections. This calculation was done with
a plane wave set using norm-conserving pseudopotentials with
750 eV energy cut off. For geometry optimization, the energy
tolerance was 2.0 ꢁ 10ꢂ5 eV per atom with a force tolerance of
0.05 eV Åꢂ1, a maximum displacement of 2.0 ꢁ 10ꢂ3 Å, and a
maximum stress tolerance of 0.10 GPa. For the self-consistent
field (SCF) calculation, the energy tolerance was set to 1.0 ꢁ
10ꢂ6 eV per atom.
1
yield of 54%. H NMR (500 MHz, CDCl3) d 8.37–8.35 (m, 4H),
7.93 (d, J =20.0 Hz, 2H), 7.57 (q, J = 8.5 Hz, 8H), 7.50–7.19
(m, 4H), 6.88 (d, J = 15.0 Hz, 2H).
Compound 3 (270.10 mg, 0.50 mmol), 3-pyridineboronic
acid pinacol ester (256.33 mg, 1.15 mmol), Pd(Pph3)4 (34.67 mg,
0.03 mmol) and 2 M Na2CO3 (8.3 mL) were stirred with back-
flow in THF (12.3 mL) under nitrogen at 65 1C for 24 h. The
Results and discussion
Crystal structures
mixture was cooled to ambient temperature. After removing the Single crystals have a highly ordered molecular packing structure
solvent, the resulting precipitate was dissolved with CH2Cl2, that makes them the ideal model to investigate the relationship
and extracted with H2O. The organic phase was concentrated between the aggregation structure and luminescence properties.
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measured to be 3.7 Å, as shown in Fig. 1j. Thus, owing to the
large overlap and short distance between the adjacent anthra-
cene planes, there was relatively strong p–p stacking.7a
Photophysical properties
The photophysical properties of BP3SA were investigated in
THF solution and the solid state. As shown in Fig. 2a, the
maximum absorption of BP3SA in THF solution appeared at
417 nm, and the corresponding emission spectrum exhibited a
peak at 638 nm. A large Stokes shift (4.43 ꢁ 104 cmꢂ1) and low
photoluminescence quantum yield (PLQY) (0.19 of BP3SA) were
observed in solution. When the molecules were in the solid
state form, the luminescence intensity shows a sharp increase.
The maximum absorption of the pristine BP3SA powder
appeared at 433 nm, and the emission spectrum exhibited a
peak at 509 nm and PLQY up to 0.55. In a crystalline state, both
crystals possessed high PLQY which were 0.98 for G-phase and
0.48 for O-phase, respectively. Additionally, the two crystalline
phases showed a red-shifted emission from 511 nm to 575 nm
and a decrease in PLQY from the G-phase to the O-phase
(Fig. 2b), which could be understood through their packing
structures from J-type to H-type aggregation. Meanwhile, the
theoretical study shown in Fig. S9 (ESI†) also indicated that the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of the O-phase were
delocalized a little more, stabilizing the molecules in the
excited states and giving rise to the decrease in the band gap
(Fig. S9, ESI†). Thus, the O-phase had a smaller band gap
3.15 eV, which could be in accordance with the shift of the
PL spectrum to a longer wavelength.
Fig. 1 Images and molecular conformation of G-phase (a and b), O-phase
(c and d). (e) The perspective view of G-phase cells along the c-axis. (f) The
intermolecular interactions between adjacent molecules in G-phase.
(g) The vertical distance, the centre distance and the pitch angle in G-phase.
(h) The perspective view of O-phase cells along the b-axis. (i) The
intermolecular interactions between adjacent molecules in O-phase. (j) The
vertical distance, the centre distance and the pitch angle in O-phase.
By solvent evaporation in a dichloromethane solution, the G-phase
and O-phase suited for X-ray structural analysis were obtained.
Through the analysis of the two crystals, we found that the
molecular conformations of the G-phase maintained larger
torsion angles y (Fig. 1b), and the packing structures of the
G-phase and O-phase were totally different from each other.
Piezochromic behavior
%
G-Phase crystals were triclinic, and the space group was P1.
As shown in Fig. 3a, a dramatic luminescence change Dl =
68 nm in the G-phase was observed after grinding along with
the colour change from green (511 nm) to orange (579 nm), and
it recovered to yellow green (544 nm) by heating at 150 1C for
5 min. In Fig. 3b, the luminescence change Dl = 3 nm from
575 to 578 nm was shown after grinding the O-phase. Different
from the result of heating of the ground G-phase powder, the
emission of the O-phase shifted to 538 nm, after being heated
under the same condition. These results suggested that the
aggregation structure was changed during the grinding and
heating process, contributing to the structural transition.
As shown in Fig. 1f and Fig. S4 (ESI†), the main interactions
between the adjacent molecules in G-phase crystals were C–Hꢀꢀꢀp
and C–HꢀꢀꢀN interactions (for detailed data see Table S2, ESI†).
Along the b axis, the phenyl moiety along the long axis of one
molecule acted as the H donor and the corresponding anthracene
core of an adjacent molecule acted as the acceptor. In C–HꢀꢀꢀN
interactions, the phenyl and vinyl moieties acted as the H donor,
and the pyridyl moiety of the adjacent molecule acted as the H
acceptor. For the G-phase, the molecules adopted a stacking
mode with J-type aggregation along the b axis (Fig. 1g, w = 39.51)
and there was no overlap between the central anthracene planes,
indicating that no pꢀ ꢀ ꢀp interactions were formed in the G-phase
(dcentre = 9.84 Å, and dvertical = 3.14 Å between the adjacent
molecules).
O-Phase crystals belonged to the monoclinic system, P21/c
space group with the molecules adopting a stacking mode similar
to the H-type aggregation along the c axis (Fig. 1j, w = 73.31).15 In
Fig. 1i, each molecule interacted with the adjacent molecule via
pꢀ ꢀ ꢀp and C–Hꢀ ꢀ ꢀp interactions (for detailed data see Table S3,
ESI†). The adjacent anthracene planes overlapped with other
molecules by around 30%. The centre distance dcentre between
two molecules in the O-phase was measured to be 4.8 Å, and the
vertical distance dvertical between two anthracene planes was
Fig. 2 UV-vis spectra (a) and PL spectra (b) of BP3SA in THF, powder, G-phase
and O-phase. The excitation wavelength is 365 nm. (Inset: The molecular
structure of BP3SA.)
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Fig. 4 (a) Images of G-phase and O-phase under different applied
pressures (‘r0’ represents ‘release pressure to 0’). (b and c) PL spectra of
G-phase and O-phase under different applied pressures. The excitation
wavelength is 365 nm.
Fig. 3 (a and b) PL spectra of G-phase and O-phase upon different
treatments with excitation at 365 nm (inset: images of G-phase and O-phase
upon different treatments). (c) PXRD patterns of G-phase and O-phase upon
different treatments. (d) DSC patterns of G-phase and ground O-phase.
and l1 and l2 refer to the maximum emission-peak wavelengths at
pressures of p1 and p2, respectively).6e The k value of the G-phase
was calculated to be 29.1 nm GPaꢂ1 at around 2.0 GPa, while the k
value of the O-phase was 33.2 nm GPaꢂ1, which indicated a better
sensitivity of the O-phase to the pressure.17 On the other hand, the
origin of different sensitivities could be understood from the
intermolecular interactions in the crystal structure. The various
intense C–HꢀꢀꢀN interactions could help to rigidify the molecular
conformation and lock the molecular rotations of the G-phase,
resisting the effect from the external pressure, thereby showing
a lower sensitivity to the pressure. In contrast, owing to the lack of
C–HꢀꢀꢀN interactions, the O-phase could sensitively response to the
external hydrostatic pressure with a higher sensitivity.
Density functional theory (DFT) calculations by using the
CASTEP module in Material Studio were performed on the basis
of the structural data of a single crystal, for further investigating
the change in the molecular geometry and packing structure
under external pressure.6h The optimized lattice parameters and
geometrical parameters under the external stresses of 0, 1, 2 and
3 GPa were obtained and listed in Tables S7–S10 (ESI†). As the
parameters show, the molecular geometries trended toward the
planarization conformation and the molecular packing became
tighter with the increase in the external stress. It was noted
that the change in the molecular geometry and the molecular
packing was bigger in the O-phase, according to the lattice
parameters and roll angles, which was in accordance with the fewer
varieties of intermolecular interactions in the O-phase, contributing
to a higher sensitivity of the O-phase to the pressure.
In order to understand the structural transition between the
G-phase and O-phase, PXRD of the two crystals under different
treatments was performed. The diffraction patterns of the
ground G-phase and O-phase exhibited a wide swell, indicating
that the initial aggregation state was changed by grinding, from
different crystalline states to amorphous states. After heating,
the diffraction patterns of the two samples showed the same
three sharp and intense reflections which were consistent with
the PXRD pattern of the G-phase. Moreover, the differential
scanning calorimetry (DSC) measurements of the G-phase and
ground O-phase were performed, as shown in Fig. 3d. Both the
G-phase and ground O-phase exhibited two identical peaks at
190 1C and around 345 1C, but an extra endothermal peak at
161 1C could be clearly observed in the ground O-phase,
indicating that the ground O-phase exhibited a metastable
amorphous phase and converted to a stable G-phase via an
endothermal process.16 It suggested that the crystal structures
of BP3SA could be readily influenced and transformed by the
external stimuli.
In order to further understand the piezochromic behaviour
of the two crystals, the effect of hydrostatic pressure on the
luminescence of BP3SA was investigated. As shown in Fig. 4, as
the applied pressure increased, the luminescence emissions
of the G-phase and O-phase had gradual red-shifts. For the
G-phase, the emission changed from green (511 nm) to deep
red (660 nm) when the pressure increased from 0 to 5.27 GPa.
As for the O-phase, the emission colour changed from orange
(575 nm) to red (641 nm) with the increase in the pressure from
0 to 2.02 GPa. When the pressure turned back to 0 GPa, the
Protonation–deprotonation effect
emission of the G-phase partially recovered to the original state. The protonation–deprotonation effect of the two crystalline
But that of the O-phase recovered to the original state completely, phases was investigated. As shown in Fig. 5a, the fumigation
indicating its good reversibility under the pressure stimulus. In with HCl vapor had a significant effect on the luminescence of
order to understand the different sensitivities of the G-phase and the G-phase, turning it from green (511 nm) to red (610 nm),
O-phase, we calculated the ratio of the wavelength and pressure by while the O-phase showed a weak luminescence switching from
l1 ꢂ l2
575 to 595 nm. The acidified G-phase turned to green emission
(blue-shifting from 610 to 530 nm) by fuming with diethylamine
the following equation: k ¼
(k represents the sensitivity,
p1 ꢂ p2
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crystals (Fig. S8, ESI†). Thus, the original crystals could not
transfer to C-HCl by protonation. To gain further understanding
of the relationship between the fluorescence change in the
protonation process, density functional theory calculations were
performed for C-HCl (Fig. S9, ESI†). Since the protonation of
nitrogen atoms could greatly enhance the electron-withdrawing
ability of pyridyl,11a,18 the electron cloud of the LUMO spreads
on the pyridine group compared to those of the G-phase and
O-phase. Thus, the C-HCl molecule had a smaller band gap
1.18 eV, which was compatible with the observed red-shifted
emission of the G-phase and O-phase after the protonation.
To explore the application of materials, we carried out
Fig. 5 (a and b) PL spectra of the initial two crystalline phases under different
stimuli. (c) PXRD patterns of the initial two crystalline phases under different
stimuli. (d) The butterfly patterns smeared partly and fumed by HCl vapor.
(DEA). The acidified O-phase also showed green emission (shifting multiple external treatments on a butterfly-shape filter paper
from 595 to 559 nm) and a shoulder peak at 517 nm by treating which was processed in a fluorescent dye solution. As shown in
with DEA vapor. PXRD analysis (Fig. 5c) showed that after being Fig. 5d, the original empty pattern was transferred into the vivid
fumed by HCl vapor for 2 min, all of the characteristic peaks of the one by partly smearing, and after fuming with HCl vapor, the
G-phase vanished, and a new wide swell arose implying that the colour of the butterfly obviously changed from green to orange
crystalline state of the initial phase was destroyed and turned to an red. The results indicated that this kind of material had
amorphous state.6c With the exposure to HCl vapor, the protons practical applications in high security inks.
tended to attach to the exposed nitrogen in order to protonate
the molecules. Meanwhile, incoming chloride ions were apt to
form strong hydrogen bonds with the protonated molecules.
Conclusions
Thus, the fluorescence emission of the protonated sample
In summary, we have demonstrated remarkable luminescence
showed a red-shift. When being fumed by DEA vapor, protons
switching upon grinding force, hydrostatic pressure, and acid
were captured from the protonated pyridine by DEA vapor, and
and alkali treatments based on two polymorphs of the novel
molecule BP3SA. The G-phase with J-type aggregation shows
green emission, while the O-phase with H-type aggregation
the two crystalline phases showed blue-shifted luminescent
emission. But the amorphous state for the protonated G-phase
could hardly be restored by the DEA vapor. As for the O-phase,
presents orange emission. Upon grinding, the change in the
the crystalline state was not destroyed under the protonation
aggregation states results in the red-shifts in luminescence of
and deprotonation processes due to the different intermolecular
the two polymorphs. Upon further heating, the ground samples
interactions inside the two phases. In the crystal structure of the
can partly recover their initial emissive colours. In addition, a
G-phase, the nitrogen atom of a pyridine group could form
structural transition from the O-phase to the G-phase appears
during grinding and further heating processes. According to high-
strong C–Hꢀ ꢀ ꢀN interactions with the adjacent molecules which
would be broken by the incoming HCl due to the protonation of
pressure experiments and theoretical calculations, fewer various
the nitrogen atom of the pyridine group, resulting in the
intermolecular interactions are beneficial for higher sensitivity to
destruction of the crystalline phase. Conversely, there were no
the hydrostatic pressure. The protonation–deprotonation of the two
polymorphs indicates that fumigation with HCl/DEA vapor leads to
C–Hꢀ ꢀ ꢀN interactions in the O-phase, thus, the initial inter-
molecular interactions were not destroyed by protonating and
the destruction and reconstruction of the noncovalent C–HꢀꢀꢀN
deprotonating, as proved by the intense diffraction patterns. And
the more hypsochromic luminescent emission with a weak
shoulder peak of the alkalified O-phase might be attributed to
bonds, resulting in more distinct luminescence switching in
the G-phase than the O-phase. This study on BP3SA provides
comprehensive insight into the relationship between crystal
a faint change from the molecular conformation on the surface
structures and stimuli responsive properties, and further opens
of crystals under the successive stimuli of acid and alkali.
up an avenue for obtaining OMSRL materials.
In order to understand the acidichromism in depth, single
crystals of protonated BP3SA (C-HCl Fig. S6a, ESI†) were obtained
by using the solvent evaporation method. When a droplet of HCl Conflicts of interest
was added in a mixed BP3SA solution of dichloromethane and
There are no conflicts to declare.
methanol (volume ratio = 2 : 1), protonated crystals C-HCl can
be obtained and show orange luminescent emission at 580 nm
(Fig. S7, ESI†). X-ray structural analysis revealed that the
molecular crystals were in the symmetrical protonation states,
Acknowledgements
with abundant strong multiple X–Hꢀ ꢀ ꢀClꢂ (X = O, N) and This work was supported by the Natural Science Foundation of
C–Hꢀ ꢀ ꢀp interactions in C-HCl crystals (for detailed data see China (21835001, 51773080, 21674041, 51573068, and 21221063),
Fig. S6 and Table S4, ESI†), which was consistent with the Program for Changbaishan Scholars of Jilin Province, Jilin Province
analysis of the protonated crystals. Furthermore, the PXRD project (20160101305JC), and the ‘‘Talents Cultivation Program’’ of
pattern of C-HCl was not consistent with those of the protonated Jilin University.
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N. Gao, K. Wang, B. Zou, B. Yang and Y. G. Ma, Adv. Funct.
Mater., 2017, 27, 7.
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