Pressure-Induced Blue-Shifted and Enhanced Emission: A Cooperative Effect between Aggregation-Induced Emission and Energy-Transfer Suppression

Article abstract of DOI:10.1021/jacs.9b11080

Most organic piezochromic materials exhibit red-shifted and quenched emission as pressure increases. However, an abnormal phenomenon of pressure-induced blue-shifted and enhanced emission is observed in a 9-(3-(1,2,2-triphenylvinyl)phenyl)anthracene cryst

Full text of DOI:10.1021/jacs.9b11080

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Communication
Pressure-Induced Blue-Shifted and Enhanced Emission: A Cooper-ative Effect
between Aggregation-Induced Emission and Energy-Transfer Suppression
Haichao Liu, Yarong Gu, Yuxiang Dai, Kai Wang, Shitong Zhang, Gang Chen, Bo Zou, and Bing Yang
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 10 Jan 2020
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Pressure-Induced Blue-Shifted and Enhanced Emission: A
Cooperative Effect between Aggregation-Induced Emission and
Energy-Transfer Suppression
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Haichao Liu,†,‡ Yarong Gu,║,ξ,‡ Yuxiang Dai,§,‡ Kai Wang,║,* Shitong Zhang,† Gang Chen,† Bo
Zou║,* and Bing Yang†,*
† State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University,
Changchun 130012 (P. R. China)
║
State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012 (P. R. China)
§ Department of Materials Physics and Chemistry, School of Materials Science and Engineering, Northeastern
University, Shenyang 110819 (P. R. China)
ξ Department of Electronics, Xinzhou Teachers University, Xinzhou, 034000 (P. R. China)
Supporting Information Placeholder
ABSTRACT: Most organic piezochromic materials exhibit
red-shifted and quenched emission as pressure increases.
However, an abnormal phenomenon of pressure-induced
blue-shifted and enhanced emission is observed in a 9-(3-
(1,2,2-triphenylvinyl)phenyl)anthracene crystal, which is
based on discrete π-π anthracene (AN) dimer stacking with
tetraphenyl ethylene (TPE) as a spacer. A blue-shifted
emission appears and strengthens when pressure is more
than 1.23 GPa, and it reaches the maximum when pressure is
4.28 GPa. This phenomenon is ascribed to the cooperative
effect between the aggregation-induced emission of TPE
units and energy-transfer suppression from TPE to an AN
excimer. This work reports
a new concept in the
piezochromic field and provides a novel strategy to achieve
luminescence from a high-lying excited state.
Piezochromic luminescent materials exhibit remarkable
emission color changes in response to external mechanical
stimuli (e.g., grinding, pressing, and stretching), and such
materials have been widely explored because of their
potential for applications in optical data storage, pressure
sensors, and optoelectronic devices.1-20 Their mechanism
mainly involves the changes in chemical structures (i.e.,
breaking and reforming chemical bonds),21-23 phase transition
(e.g., crystal-to-crystal24,25 and crystal-to-amorphous26-29),
conformational transformation,30-32 and excited-state
transformation (e.g., locally excited state to charge-transfer
state,33,34 triplet state to singlet state35,36, and monomer state
to excimer/exciplex state).37,38 However, more universal and
innovative strategies are required to design novel materials
for the specific applications of piezochromic luminescence
because current mechanisms usually cannot be integrated
from one system to another.
Figure 1. Molecular design for discrete AN dimer formation.
Molecular conformation and crystal packing were obtained
through single-crystal XRD experiment.
Among mechanical forces, isotropic hydrostatic pressure
from a diamond anvil cell (DAC) is a useful tool to
investigate
luminescent materials show
quenched emission as pressure increases as a result of the
formation of low-energy emission species and
structure–property
relationships.39
gradual red-shifted and
Most
a
a
a
nonradiative “dark” state (e.g., narrow-bandgap excimer).40-45
Although few cases have been reported with pressure-
induced blue-shifted emission,46-49 simultaneous pressure-
induced blue-shifted and enhanced emission has not been
reported in π-conjugated organic materials.6-20
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Tetraphenyl ethylene (TPE), a typical aggregation-induced
emission (AIE) luminogen,50,51 exhibits a pressure-induced
emission enhancement44,45 because of the inhibition of
intramolecular motions under pressure. By contrast, the
enhanced π-π interaction in an anthracene (AN) dimer
during compression usually leads to the gradual redshift and
quenching of excimer fluorescence.52-54 In our work, we
combined TPE and AN into one molecule to design a new
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compound,
triphenylvinyl)phenyl)anthracene
namely,
9-(3-(1,2,2-
which
(mTPE-AN),
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demonstrated a rare piezochromic luminescent behavior
upon the application of pressure in crystals: pressure-
induced emission blue-shift and enhancement by the
cooperation between AIE and suppressed energy transfer
(ET) from TPE to an AN excimer.
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A design strategy was proposed on the basis of molecular
structure, conformation, and packing. The meta-linkage of
mTPE-AN between an AN core and a one-sided TPE
substituent was designed for the possible formation of the
target dimer (Figure 1 and S7). The TPE substituent is
asymmetrically oriented along one side of the AN plane in
molecular conformation, which favors AN dimer formation
with antiparallel π-π stacking.55-58 For comparison,
a
counterpart of 9-(4-(1,2,2-triphenylvinyl)phenyl)anthracene
(pTPE-AN)59 was designed with a para-linkage between the
AN core and the one-sided TPE substituent. The TPE
substituent in the molecular conformation of pTPE-AN was
divided equally by the AN plane, resulting in a complete
deviation of π-π AN dimer stacking in crystals. mTPE-AN
was synthesized via a boride reaction followed by two-step
Suzuki coupling reactions, and pTPE-AN was produced via a
one-step Suzuki coupling reaction (Supporting Information).
Single crystals of mTPE-AN and pTPE-AN were cultivated
Figure 2. (a) Emission spectra, (b) fluorescent images, (c)
absorption spectra and (d) visible images of mTPE-AN
crystals under pressure from 1 atm to 10.26 GPa.
Here, shearing force was applied to a crystalline sample of
mTPE-AN by grinding for 60 min, but no change was
observed in the emission and XRD pattern (Figure S19). This
result indicated that the crystalline mTPE-AN sample was
stable enough and thus remained undisturbed by
grinding.8,57 As an alternative, the isotropic hydrostatic
pressure was directly exerted via a DAC on mTPE-AN crystal
(for experimental details, see Supporting Information), and
obvious three-step variations were observed (Figures 2, S20,
and S21). As pressure increased from 1 atm to 1.23 GPa,
mTPE-AN crystal showed the gradual red-shifted emission of
the AN excimer from green to yellow (herein called low-
energy emission band for clarity) and the decreased
intensity, together with the slightly red-shifted absorption
onsets. Once pressure increased above 1.23 GPa, the emission
band of approximately 438 nm (called high-energy emission
band) suddenly appeared and then gradually increased.
Moreover, the high-energy emission band reached its
maximum intensity at 4.28 GPa; the intensity of low-energy
emission band kept decreasing, giving rise to pure blue
emission. The absorption onset also kept the blue-shifted
trend until 4.28 GPa. With further compression of over 4.28
GPa, high- and low-energy emission bands demonstrated a
redshift and an intensity decline. The absorption onset was
unceasingly red-shifted. In compression, the blue-shift and
through
slow
solvent
evaporation
from
their
dichloromethane/methanol solutions. mTPE-AN crystals
showed a largely red-shifted, structureless, and broadened
emission spectrum (λmax = 518 nm) and prolonged lifetime (τ
= 191.70 ns; Figure S8-10) compared with those from single-
molecule emission. These findings agreed with the
characteristic of an AN excimer.52-54,60-62 mTPE-AN crystals
exhibited a high photoluminescence quantum yield (PLQY)
of 82.20% as a result of the excimer-induced emission
enhancement mechanism (Figure S11 in Supporting
Information).8,55-58 By comparison, pTPE-AN crystals
displayed a monomer-like blue emission with λmax = 432 nm,
short lifetime of τ = 8.05 ns, and PLQY of 68.24%.
A single-crystal X-ray diffraction (XRD) experiment under
ambient conditions was performed to confirm the feasibility
of the molecular design strategy as mentioned above (Figure
1). In mTPE-AN crystals, TPE units acted as spacers to isolate
the π-π AN dimer from one another. No interaction existed
between one dimer and adjacent dimers, that is, isolated and
discrete π-π dimer stacking in mTPE-AN crystals.55-58,61,62 The
interplanar π-π distance between two AN units in a dimer is
3.527 Å, and the overlapped area is approximately 32%,
indicating a strong π-π interaction (Figure S12).63 According
to density functional theory calculations, the natural
transition orbitals of the dimer indicate that the high PLQY
of mTPE-AN crystals can be ascribed to the hybridized local
and charge-transfer state (Figures S13–S16, see Supporting
Information for details).64-66 By contrast, pTPE-AN crystals
showed the nondimer stacking of AN (Figures S17 and S18).
enhancement of the emission spectra resulted in
a
discontinuous change in emission color from green and
yellow to blue, which was opposite of the majority of
observations in the piezochromic luminescent field.
However, after a high pressure of up to 12.48 GPa was exerted
on pTPE-AN crystal (Figure S22), continuous red-shifted and
quenched emission was observed from 432 nm to 532 nm.
This observation was different from that of mTPE-AN crystal
under pressure. Although blue-shifted emission has been
reported for carbon dots under hydrostatic pressure,47-49
carbon dots have no clear structural information at a
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molecular level. Consequently, studying the structure–
decreased nonradiation, resulting in the AIE behavior of TPE.
In Figure 3, almost the same emission and absorption bands
were observed between TPE crystal at 1 atm and mTPE-AN
crystal at 4.28 GPa, indicating that the blue-shifted emission
band in mTPE-AN crystal under pressure might originate
from the TPE units.
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property relationship is difficult. In this experiment, mTPE-
AN crystal had a definite molecular structure and packing
arrangement, which was highly desired to establish a
structure–property relationship.
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Two possibilities were responsible for the emergence of
the high-energy emission band of mTPE-AN crystal under
pressure: phase transition from dimer to monomer and
chemical reaction. On the one hand, the high-pressure
angle-dispersive XRD experiment showed that all
representative diffraction peaks during compression shifted
to high angles. No disappeared peaks as well as no sudden
new peaks were observed (Figure S23), excluding the
occurrence of phase transition. Moreover, all three axes of a
unit cell were shortened, and volume was reduced as
pressure increased. Thus, dissociating one AN dimer into two
AN monomers was impossible because mTPE-AN crystal
became much more closely packed throughout compression.
Actually, the low-energy emission band from the AN excimer
existed throughout compression and just accompanied with
a continuous redshift and a decreasing intensity. On the
other hand, after the crystalline mTPE-AN powder was
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The time-resolved spectrum and the excitation wavelength
dependence were obtained at room and low temperature to
understand the mechanism of pressure-induced emission
blue-shift and enhancement in mTPE-AN crystals. To some
extent, the effect of low temperature on the decrease in π-π
distance was the same as that of high pressure (Figure
S30).42 At room temperature (RT), mTPE-AN crystals showed
single-exponential fluorescence decay from the time-resolved
spectrum (Figure S31a), corresponding to a single pure
excited state of the AN excimer. Moreover, the excitation-
independent emission wavelength at RT suggested a rapid
internal conversion (IC) process, i.e., a complete ET from
TPE to an AN excimer (Figure S31b). However, mTPE-AN
crystals exhibited two-exponential fluorescence decay at 80 K
(Figure S31c). This result indicated a suppressed ET from
TPE (donor) to the AN excimer (acceptor).69 The excitation-
dependent emission spectra at 80 K revealed two emission
bands from TPE and the AN excimer and confirmed the
suppressed ET from TPE to the AN excimer (Figure S31d).
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irradiated with UV light for 32 h, its H NMR spectrum
remained the same as the one before irradiation (Figure
S24), excluding the photodimerization of the AN dimer.67,68
Moreover, the mass spectrum (MS) of mTPE-AN crystals
under 2.00 GPa was identical to the MS of a pristine sample
(Figure S25). This result indicated no chemical reaction in
mTPE-AN crystals.
Figure 4. Schematic diagram of pressure-induced emission
blue-shift and enhancement in mTPE-AN crystals.
Pressure-induced emission blue-shift and enhancement
during mTPE-AN crystal compression can be explained as
follows. From 1 atm to 1.23 GPa, the complete ET from TPE to
the AN excimer produces the only emission band of the AN
excimer because of the gradually increasing spectral overlap
between TPE emission and AN dimer absorption (Figure 3).
When pressure exceeds 1.23 GPa, the absorption of the AN
dimer weakens because of the forbidden transition (Figure
S32) and the disappearance of AN monomer absorption,
while the emission spectra of TPE remain almost unchanged.
Thus, the continuously reducing overlap between TPE
emission and AN dimer absorption leads to the suppressed
ET,[70] which is responsible for the emerging high-energy
emission band of TPE. TPE units aggregate into a tighter
packing as pressure increases, thereby triggering the AIE
mechanism of the enhancement of the high-energy emission
band (Figure 4). Once pressure exceeds 4.28 GPa, the high-
energy emission band is gradually quenched probably
because a stronger π-π interaction between TPE units leads
to the formation of the energy-trapping “dark” state. For the
Figure 3. Normalized absorption and emission spectra of
TPE crystals at 1 atm and mTPE-AN crystals at 1 atm ~ 4.28
GPa. Dotted curve depicts the quenched emission spectrum
of TPE to estimate the spectral overlap for ET process from
TPE to AN dimer.
Pressure-dependent structural simulation in mTPE-AN
crystal showed the decreasing π-π distance of the AN dimer
and the TPE dimer, corresponding to the enhancement of
intermolecular interactions with increasing pressure through
Hirshfeld surface analysis (Figures S26-S29, Table S2, and
Table S3). As pressure further increased, more interactions
were combined, and intramolecular vibrations and rotations
of the TPE units could be effectively restricted for the
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redshift and quenching of the low-energy emission band
Bing Yang: 0000-0003-4827-0926
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with increasing pressure, the energy gap dramatically
narrows between the potential energy curves of the excimer
at an excited state and the dimer at a ground state because of
the decreased π-π distance. Consequently, the nonradiative
IC rate is aggravated according to the energy gap law. When
the released pressure is 1 atm, the emission band of the AN
excimer becomes reversible, but a small spectral residue of
the high-energy emission band is observed (Figures S33 and
S34). This spectral residue might result from the hysteresis
effect that TPE units experience difficulty in promptly
restoring their geometries to the original state because of the
large structural deformation at pressures above 1.23 GPa.
Author Contributions
The manuscript was written through contributions of all
authors. / All authors have given approval to the final version
of the manuscript. / ‡These authors contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
9
This work is supported by the National Natural Science
Foundation of China (Nos. 91833304, 51673083, 51873077,
51803071 and 21725304), the National Basic Research Program
of China (No. 2016YFB0401001), the Fundamental Research
Funds for the Central Universities (No. N170203007), the
Postdoctoral Innovation Talent Support Project (No.
BX20180121), the China Postdoctoral Science Foundation
(Nos. 2018M631801 and 2018M641767), the Postdoctoral
Foundation of Northeastern University (No. 20180301) and
the China Scholarships Council (No. 201906085035). ADXRD
experiments were performed at the BL15U1 beamline,
Shanghai Synchrotron Radiation Facility (SSRF).
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In conclusion, mTPE-AN, a new piezochromic compound,
was designed and synthesized, and it consisted of two
functional moieties with a meta-linkage: an AIE-featured
TPE and an excimer-forming AN. The discrete π-π dimer
stacking of the AN was obtained in crystal with TPE unit as a
spacer. An abnormal phenomenon of pressure-induced
emission blue-shift and enhancement of mTPE-AN crystal
was discovered. This phenomenon was an extremely rare
case in the field of piezochromic luminescence. Theoretical
and experimental investigations demonstrated that the blue-
shifted emission band originates from the TPE units in the
mTPE-AN crystal. The suppressed ET from TPE to AN dimer
induces the appearance of a blue-shifted band from 1.23 GPa
to 4.28 GPa, and the AIE mechanism of TPE units
contributes to the enhancement of the blue-shifted emission
band, which is cooperatively responsible for pressure-
induced emission blue-shift and enhancement. This work
reports a novel principle for a new class of blue-shifted and
enhanced piezochromic luminescent materials by using the
combination between ET suppression and AIE activation.
This study also presents an ideal model to improve the
understanding of high-lying excited-state emission in
fundamental photophysics.
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