Piezochromic Luminescence of Donor–Acceptor Cocrystals: Distinct Responses to Anisotropic Grinding and Isotropic Compression

Article abstract of DOI:10.1002/anie.201810149

Piezochromic luminescent materials that exhibit distinct luminescence responses to different types of mechanical stresses have been emerging as new kinds of materials which are rarely investigated. Herein, we report a donor–acceptor (D–A) charge-transfer (CT) cocrystal, which shows distinct hypochromatic and bathochromatic shifts upon anisotropic grinding and isotropic compression, respectively. Detailed spectroscopic and structural analyses revealed that the hypochromatic shifted emission under grinding is attributable to a structural reorganization from a loosely segregated-stack to a mixed-stack, while the bathochromatic shifted emission originates from the formation of a tighter packing structure. We present rare evidence of a distinct luminescent response to anisotropic grinding and isotropic compression on the basis of structural rearrangement in a D–A cocrystal, and thus enriches our insight into piezochromic luminescence.

Full text of DOI:10.1002/anie.201810149

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Accepted Article
Title: Piezochromic Luminescence of Donor−Acceptor Co-crystal:
Distinct Responses to Anisotropic Grinding and Isotropic
Compression
Authors: Yingjie Liu, Qingxin Zeng, Bo Zou, Yu Liu, Bin Xu, and
Wenjing Tian
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Piezochromic Luminescence of Donor−Acceptor Co-crystal:
Distinct Responses to Anisotropic Grinding and Isotropic
Compression
Yingjie Liu, Qingxin Zeng, Bo Zou*, Yu Liu, Bin Xu*, and Wenjing Tian[a]
[
a]
[b]
[b]
[a]
[a]
Piezochromic luminescent materials that exhibit distinct
luminescence responses to different types of mechanical stresses
have been emerging as a kind of new important materials which are
rarely investigated. Here, we report a donor−acceptor (D-A) charge-
transfer (CT) co-crystal, which show a distinct hypochromatic and
bathochromatic shifts upon anisotropic grinding and isotropic
compression, respectively. Detailed spectroscopic and structural
analyses revealed that the hypochromatic shifted emission under
grinding is attributable to a structural reorganization from loosely
segregated-stack to mixed-stack, while the bathochromatic shifted
emission originates from the closer proximity to the formation of the
tight packing structure. The study presents a very rare evidence of
distinct luminescent response to anisotropic grinding and isotropic
compression on the basis of structural rearrangement in a D-A co-
crystal, and thus enriches the insight of the piezochromic
luminescence.
halogen bonds, charge-transfer (CT) interactions, electrostatic
interactions and host − guest Interactions, etc. The organic co-
crystal,[5] in which -conjugated components can self-assemble
in long term ordered structure based on the supramolecular
interactions is one of the most important supramolecular
assemblies. It is a promising way to forecast the presence of
multiple phases of co-crystal itself and its fluorescence changes
during the structural alteration.[6] Organic co-crystals with typical
mixed or segregated stacking of donor (D) and acceptor (A)
molecules show unique physicochemical properties since the
intermolecular D-A pairs give rise to the distinct band energies
originated from localized frontier molecular orbitals (FMOs).[7] In
these D-A co-crystals, the Coulombic interactions induced from
the large energetic offset between D and A lead to the intended
loosely packed stacking structure, which is benefit for a stimuli
responsive feature.[6a] For instance, Park et al. reported that the
luminescence of a crystalline donor–acceptor mixture film had
blue shift from 658nm to 472nm due to the structural
transformation from the mixed phase into demixed phase under
solvent vapor annealing, and then recover to the original color
under grinding or thermal stimuli.[6a] It is expected that a
piezochromic system can be achieved by using appropriate
Piezochromic luminescent materials with multi-color switching
are recently attracted enormous research interest due to their
appealing applications in advanced photonics, such as optical
recording, security ink, memory and sensors.[1] Various
piezochromic materials based on organic molecules, metal
complexes, and polymers have been devised showing actively
controllable luminescence characteristics response to external
mechanical stimuli, such as grinding, shearing, tension or
hydrostatic pressure.[2] However, there are only few reports on
materials that show the switchable luminescence responses to
different types of mechanical forces.[3] For example, Zhang and
co-workers demonstrated a boron diketonate crystal exhibiting a
blue-shifted emission under tension, but a red-shifted emission
under grinding and compressing.[3b] Yamaguchi et al. reported a
tetrathiazolylthiophene fluorophore which displays distinct
luminescent responses to grinding and compressing.[3a] But in
these cases, when treated by grinding, the original crystals
convert into amorphous powders, so that it is difficult to establish
the relationship between the fluorescent properties and
molecular packing structure of the grinded solids.
donor and acceptor pairs in which there is
a moderate
intermolecular interactions design for a flexible packed structure.
However, up to now there is nearly no reports on the organic D-
A co-crystals with distinct responses to different mechanical
stimuli.
Herein we report the distinct luminescent responses to
anisotropic grinding and isotropic compression from a D-A co-
crystal, called CT-R, in which 1, 4-bis-p-cyanostyrylbenzene
(
CNDSB) severs as donor (D) and TCNB as acceptor (A). It
shows a unique enhanced hypochromatic shifted emission
under grinding, but a remarkable bathochromic shifted emission
under hydrostatic pressure. An in-depth understanding of
luminescence switch has been elucidated through a precise
correlation of the chemical, structural, optical and thermal
properties
1
, 4-bis-p-cyanostyrylbenzene (CNDSB) was readily synthesized
In recent years, big progress has been made[4] in the study of
supramolecular assembly whose main driving forces are
noncovalent interactions, such as - interactions, hydrogen and
[8]
by the Wittig-Horner reaction (Scheme S1). Because of their
simple and rigid molecular structure, CNDSB and TCNB
together hold strong crystallization tendency. Red CT-R co-
crystals with block-like shape (Figure 1) were obtained by
solvent evaporation process, where CNDSB and TCNB were
mixed with a molar ratio of 4:3 in a dichloromethane solution.
Compared with the individual component CNDSB and TCNB,
CT-R showed a new, broad bathochromic-shifted absorption at
around 490 nm (Figure S1a). The red emission of co-crystal
peaked at 643 nm (Figure S1b), which is also greatly red-shift
compared with that of CNDSB and TCNB crystals. In addition,
the CT-R co-crystal showed high solid-state photoluminescence
[
[
a]
b]
Y. Liu, Prof. Dr. Y. Liu, Prof. Dr. B. Xu and Prof. Dr. W. Tian
State Key Laboratory of Supramolecular Structure and Materials,
Institute of Theoretical Chemistry, Jilin University
Qianjin Street No. 2699, Changchun 130012 (China)
Email: xubin@jlu.edu.cn
Q. Zeng, Prof. Dr. B. Zou
State Key Laboratory of Superhard Materials, Jilin University
Qianjin Street No. 2699, Changchun 130012 (China)
Email: zoubo@jlu.edu.cn
quantum yield (PLQY) (
redshifted absorption and emission features indicate existence
F
) up to 13.0% (Table S1). This
Supporting information for this article is given via a link at the end of
the document.
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ground sample were consistent with those of the pristine sample,
the relative intensity was greatly reduced. It is worthy to note
that the new diffraction peak position of the obtained ground
sample is completely different from that of the pristine crystalline
sample. It suggests that the initial molecular arrangements in the
co-crystal have been destructed and may be undergoing
reorganization to new crystallization state during the grinding
processes.
In addition, the switchable fluorescence feature can also occur
by the thermal stimulus. By heating the crystals at 145˚C for 12h
in an oven and cooling down to room temperature, the red-
emitting sample turned into orange-emitting solids (see Figure
S4 a, b). The heated samples exhibited a broad emission band
peaked at about 575 nm which is blue-shifted by about 68 nm
compared with that of the pristine crystals. Moreover, the heated
samples possessed a sharp PXRD signal (Figure S4c), which
matched well with that of ground sample. From the differential
scanning calorimetry (DSC) curves (Figure S5), there showed a
transition around 145 °C prior to melting at 293 °C, implying that
there should have a crystallization processes during thermal
treatments.[12]
On the other hand, CT-R co-crystal showed a significant
bathochromic shift in both emission and absorption under
hydrostatic pressure (Figure 2). The high-pressure experiments
were performed by using a diamond anvil cell (DAC) with
silicone oil as a pressure-transmitting medium. During the
compression process, the absorption spectra were gradually
red-shifted from around 490 nm to 640 nm and simultaneously
the red single crystal progressively became dark red. Besides,
the fluorescence maximum peak was gradually red-shifted and
its intensity gradually decreased, and eventually to a near
infrared emission at 744 nm when the pressure reached at 5.08
GPa. The large wavelength difference of 104 nm in the red shift
was observed. It should be noted that both of the absorption and
fluorescence spectra can recover to the original absorption and
emission band, respectively, when the hydrostatic pressure was
Figure 1. a) Molecular structure of the resulting CT-R co-crystal. b)
Fluorescence images, c) corresponding fluorescence spectra and PXRD
patterns of CT-R co-crystal before and after ground.
of CT excitations between CNDSB and TCNB, where CNDSB
molecules act as the donors (D) and TCNB as the acceptors
(
A).[6a, 9] This is consistent with the slightly longer fluorescence
lifetime of the red emission in CT-R co-crystal (17.5 ns, Figure
S3) against that of the yellow emission in CNDSB crystal
(
r
13.1ns). The radiation rate constant (k ) of CT-R are calculated
6
-1
to be 7.48×10 s , and it is very near to the typical value of a
CT complex.[
9b, 10]
This relatively slow radiation implies its CT
transition nature, further confirmed by electron spin resonance
ESR) measurements (Figure S3). A sharp signal was observed
(
centered at the magnetic field of 3506 G, indicating the presence
of unpaired electrons in the co-crystals.[11] The g-factor
calculated from the ESR theory is 2.0042, which is similar to the
value of free electron (2.0023). These behaviors disclose the CT
interactions between CNDSB and TCNB.
To our surprise, the emission of pristine co-crystals exhibited
obviously hypochromatic shift after grinding the samples (Figure
1). The ground powders showed bright orange fluorescence with
an increased PLQY up to 22% and a 33 nm spectral shift to 610
nm. The powder X-ray diffraction (PXRD) curve of the pristine
sample had sharp and high diffraction peaks (Figure 1c),
indicating a well-ordered crystalline structure of CT-R crystals.
The ground powders, however, showed a number of broad and
strong diffraction peaks. Although some resolvable peaks of the
Figure 2. a, b) Visible and fluorescence images of a single crystal of CT-R
under different hydrostatic pressures. c, d) Corresponding absorption and
fluorescence spectra. The value of the four curves in the magenta circle is
magnified tenfold.
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molecular
rearrangement
process to a packing structure
analogous to CT-O under
grinding, which also agreed
with thermal analysis.
The structural analysis of two
polymorphs of co-crystal could
give insights into the nature of
the emission in the crystalline
state and its piezochromic
luminescence behavior. CT-R
held the triclinic crystal system
and P -1 space group, while
CT-O was crystallizing in the
monoclinic crystal system and
space group P 21/n.[14] They
crystallized
in
different
stoichiometric packing ratios of
CNDSB (D) and TCNB (A),
which is 4:3 for CT-R and 1:1
for CT-O, respectively. CT-R
Figure 3. Single crystal under UV light (365 nm) and stacking modes of a) CT-R and c) CT-O. Hydrogen atoms are
omitted for clarity. d = 3.44 Å, d = 3.33 Å, d = 3.54 Å. C-H···N intermolecular interactions in b) CT-R and d) CT-O.
1 2 3
returned to ambient pressure (Figure S6). The unique
restorability in the luminescence behavior implies that the
molecular packing mode in CT-R crystal would not change
substantially during the high-pressure experiment processes.[13]
Therefore, these observations of CT-R co-crystals under the
different mechanical force stimuli suggest that this unique
piezochromic behavior may come from the different nature.
The luminescent materials that show such distinct luminescence
responses to anisotropic grinding and isotropic compression are
rarely presented, although some materials have been reported
to show the switchable luminescence under the external
mechanical force stimuli. First, the observed unique
piezochromism feature in the CT-R co-crystal is quite different
from the previous reports about the single component
exhibited the segregated stacking mode (Figure 3a), which was
consists of segregated D and A molecular columns with an
intermolecular D–D distance of 3.44 Å and D–A distance of 3.33
Å, thus giving rise to moderate - and CT interactions,
respectively. The two types of C-H···N hydrogen bonds (Figure
3b and Figure S11 and S12), as self-assembly driving forces
along distinct directions, make molecules aggregate and expand
to form linear molecular chains. In sharp contrast, CT-O
possessed the mixed stacking mode and consisted of a longer
[3]
luminescent materials, where the luminescence comes from
the two-component CT complex. In addition, the grinding
induced luminescence of CT-R shows not only enhanced solid
state efficiency, but also a hypochromatic shift to the short-
wavelength region, whereas the compression induced
luminescence tends to a bathochromic shifted and weakened
emission. These observations give us a greatest opportunity to
enrich the understanding of the mechanism of piezochromic
behavior based on organic luminescent materials.
Surprisingly, further mixing CNDSB and TCNB with a molar ratio
of 1:1 in a dichloromethane solution, bright orange emitting
crystals with high PLQY (26.2%), named CT-O, were obtained
by solvent evaporation process. The emission of CT-O was
located at around 600 nm (Figure S7), which is close to that of
ground sample of CT-R crystals. More importantly, the PXRD
pattern of CT-O is fully consistent with ground samples of CT-R,
demonstrating that the obtained ground samples adopt a same
molecular arrangement as those of the CT-O polymorph (Figure
S9). Moreover, upon grinding the CT-R crystals, the Raman
peaks (Figure S10) located at the wavenumber range of 1150-
1
650 cm−1 slightly shifted towards high frequency vibration, and
Figure 4. Transition modeling schematic of molecular stacking mode in CT-R
co-crystal under different mechanical force stimuli.
finally coincided with the Raman spectrum of CT-O. The
comparisons in both PXRD and Raman spectra before and after
grinding the co-crystals suggest that CT-R undergoes
a
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D-A intermolecular distance of 3.54 Å along b axis (Figure 3c). Acknowledgements
There are almost no - interactions between adjacent CNDSB
molecules, whereas the two types of intermolecular C-H···N
This work was supported by the Natural Science Foundation of
China (51773080, 21674041, 51573068, 21725304), Program
for Changbaishan Scholars of Jilin Province, Jilin Province
project (20160101305JC), and the “Talents Cultivation
Program“ of Jilin University.
interactions (Figure 3d, Figure S13 and S14) result in unique
mixed alterative molecular columns. These relatively weak
intermolecular interactions and the intended loosely packed
molecular arrangement are necessary for a stimuli responsive
feature.
Putting all the experimental data together enable us to draw a
clear overall picture of the unique piezochromic luminescent
behavior of CT-R co-crystal (Figure 4). Upon the grinding, the
loosely stacking molecular columns in CT-R may happen to slide
along the molecular stretching orientation since the strong and
directional C-H···N interactions within the molecular chains,
where the slide direction between adjacent molecular columns
could be opposite due to the anisotropic shear force. A structural
reorganization thus can be realized from the segregated-stack to
the mixed-stack, resulting in the weaken - and CT interactions
among CNDSB and TCNB. Accordingly, the observed enhanced
and blue shifted emission of the ground samples possibly
originated from integrating of the weaken exciton coupling
between the pairs of neighboring CNDSB and CT transition
strength. On the other hand, the hydrostatic pressure induces
molecules to form tighter packing structure,[3a, 13] which was
demonstrated through the optimized lattice parameters of CT-R
under the external stress calculated by the CASTEP module
Keywords: anisotropic grinding • donor−acceptor co-crystal •
isotropic compression • molecular rearrangement • piezochromic
materials
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In summary, we have successfully demonstrated the distinct
luminescent responsive feature to anisotropic grinding and
isotropic compression based on a donor-acceptor co-crystal CT-
R. The intermolecular CT, -, C-H···N interactions are the
driving forces for self-assembly and result in the loosely
segregated packing mode. Grinding of the crystals leads to a
structural rearrangement from segregated-stack to mixed-stack
in which - and CT interactions are weakened and hence
results in a blue-shifted and enhanced emission. In contrast, the
hydrostatic pressure leads to a remarkable red-shifted emission,
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Structure
solution
and
2467−2477.
refinement were conducted using SHELX-97 and Olex2 1.2.
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Entry for the Table of Contents
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Herein we report the distinct
Yingjie Liu, Qingxin Zeng, Bo Zou*,
Yu Liu, Bin Xu* and Wenjing Tian
luminescent
responses
to
anisotropic grinding and isotropic
compression from a D-A co-crystal,
called CT-R. It shows a unique
Page No. – Page No.
Piezochromic Luminescence of
Donor−Acceptor Co-crystal:
Distinct Responses to Anisotropic
Grinding and Isotropic
enhanced
hypochromatic
shift
emission under grinding, but
a
remarkable
emission
pressure.
bathochromic-shift
under
An
hydrostatic
in-depth
Compression
understanding of luminescence
switch has been elucidated.
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