Flexible π-Conjugated 2,5-Diarylamino-Terephthalates: A New Class of Mechanochromic Luminophores with Tunable Aggregation States

Article abstract of DOI:10.1002/chem.202002712

The generation of different thermodynamically (meta)stable states is crucial for the development of smart solid-state luminescent materials. However, the design of luminophores with tunable aggregation states is remaining a grand challenge. Herein, we present a family of mechanochromic luminophores with tunable metastable states, based on the dynamically controllable π–π stacking of the flexible π-conjugated structure of 2,5-diarylamino-terephthalates in the solid state. The experimental data revealed that both the kinetically controlled metastable state and thermodynamic controlled stable state can be generated via tuning the intermolecular interactions such as π–π stacking and hydrogen bonds. As a result, the highly sensitive mechano-stimuli response of these luminophores was successfully achieved.

Full text of DOI:10.1002/chem.202002712

Accepted Article
Accepted Article
Title: Flexible π-Conjugated 2,5-Diarylamino-terephthalates: A
New Class of Mechanochromic Luminophores with Tunable
Aggregation States
Authors: Zhipeng Liu, Yi Liu, Fen Qi, Hui Yan, Zhiyong Jiang, and
Yuncong Chen
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202002712
Link to VoR: https://doi.org/10.1002/chem.202002712
01/2020

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Flexible -Conjugated 2,5-Diarylamino-terephthalates: A New
Class of Mechanochromic Luminophores with Tunable
Aggregation States
Zhipeng Liu,*[a] Yi Liu,[b] Fen Qi, [c] Hui Yan,[d] Zhiyong Jiang,[a,b] and Yuncong Chen*[c]
[a]
Prof. Z. Liu, Z. Jiang
College of Materials Science and Engineering,
Institution Nanjing Forestry University,
159 Longpan Road, Nanjing 210073, P.R. China.
E-mail: zpliu@njfu.edu.cn.
[b]
[c]
Y. Liu, Z. Jiang
Institute of Advanced Materials (IAM)
Nanjing Tech University (NanjingTech)
30 South Puzhu Road, Nanjing, 211816, P.R. China
F. Qi, Prof. Y. Chen
School of Chemistry and Chemical Engineering
Nanjing University
163 Xianlin Road, Nanjing 210023, P.R. China.
E-mail: chenyc@nju.edu.cn.
[b]
Prof. H. Yan
School of Pharmacy
Liaocheng University
1 Hunan Road, Liaocheng, 252000, P.R. China
Supporting information for this article is given via a link at the end of the document.
Abstract: The generation of different thermodynamically (meta)stable
states is crucial for the development of smart solid-state luminescent
materials. However, the design of luminophores with tunable
aggregation states is remaining a grand challenge. Herein, we present
a family of mechanochromic luminophores with tunable metastable
states, based on the dynamically controllable  stacking of the
flexible -conjugated structure of 2,5-diarylamino-terephthalates in
the solid-state. The experimental data revealed that both the
kinetically controlled metastable state and thermodynamic controlled
stable state can be generated via tuning the intermolecular
interactions such as  stacking and hydrogen bonds. As a result,
the highly sensitive mechano-stimuli response of these luminophores
has been successfully achieved.
chemical changes have been proposed.[1a] However, even
with the help of those proposed mechanisms, the
development of new mechanochromic luminescent materials
(MCLs) still heavily depends on random screening, which
makes the rational design of new materials, especially those
with high sensitivity to mechanical stimuli, remains a
challenging work.
Mechanical stimuli trigged phase transformation of a
luminophore from its metastable state to a more stable state
is a promising strategy to generate MCLs with high sensitivity
10d, 12]
and high contrast fluorogenic signal.[1a,
Hence,
luminophores, which can generate both of their metastable
and stable states in a controlled way, have gained a great
interest in the development of MCLs. Compared with
luminophores with large and rigid -configurations, flexible -
conjugated systems are preferred to generate kinetically
trapped metastable states, with luminescent properties that
are sensitive to the surrounding environment, which make
them promising platforms for the construction of stimulus-
sensitive materials.[10,12c] It can be predicted that if a
Introduction
Smart luminescent materials that exhibit tunable luminescent
properties in the solid-state under mechanical stimuli have
received considerable attention in the development of optical
data recording and storage devices, as well as fluorescent
sensors.[1] Several mechano-responsive luminescent
systems derivate from metal (Au,[2] Cu,[3] Pt[4] and Ir[5]) and
boron complexes,[6] anthracene-,[7] pyrene-,[8] and perylene
metastable state of flexible
-conjugated system is highly
sensitive to mechanical stimuli, it would be easily converted
to a more stable state in the presence of mechanical stimuli
such as shearing, grinding or rubbing, accompanied by the
distinct change of luminescence. Although flexible
-
bisimide-based
dyes,[9]
oligo(p-phenylenevinylene)
conjugated luminophores such as oligo(p-phenylenevinylene)
derivatives,[10] and aggregation-induced emission (AIE)
compounds[11] have been reported, and various mechanistic
explanations such as phase transformation, excimer
formation or dissolution and mechanical stress-induced
derivatives have been successfully employed for the
construction of MCLs,[10] the development of flexible
-
conjugated luminophores with tunable aggregation states
remains a grand challenge. In the present work, we report a
1
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new family of flexible
luminophores (MCL1-4) that show tunable metastable and
stable aggregation states.

-conjugated mechanochromic
of MCL2-4 (Y-form, MCL2-4-Y) with bright orange luminescence
(560-584 nm) and increased f (917%) and τf (6.710.9 ns) can
be obtained. Moreover, MCL2-4-Y powders can also be obtained
by the method of rapid aggregation such as adding a large
amount of water into the tetrahydrofuran (THF) solution of MCL2-
4. These results strongly indicate that MCL1-3-R and MCL2-4-Y
are the thermodynamically stable and kinetically trapped
metastable states of MCL1-4, respectively.
Table 1. Photographs of MCL1-4 crystals under UV illumination
Y-form
R-form
MCL1
MCL2
MCL3
MCL4
—
—
Next, we examined the mechanical stimuli responses of
MCL1-4. As shown in Figure 2 and S1, MCL1-3-R MCL2-Y,
,
Figure1. Chemical structures of MCL1-4 and the proposed molecular packing
modes of MCL1-4 in the solid-state.
and MCL4-Y showed relatively poor mechanochromic
response. After shearing with the mortar, none emission
band shift was observed for MCL1-R and MCL3-R (Figure
S1a and S1d), while a 20 and 16 nm red-shifted em were
observed for MCL2-Y and MCL4-Y (Figure 2a and 2b),
accompanied by the emission color change from light to deep
Results and Discussion
The design of MCL1-4 relies on the dynamically controllable
orange for MCL2-Y and yellow to orange for MCL4-Y
,
 stacking of the flexible
-conjugated structure in the
respectively. Interestingly, the em of MCL2-R was blue-
shifted from 627 to 614 nm upon shearing force stimuli,
indicating the relatively unstable state of MCL2-R in
solid-state. Firstly, MCL1-4 are more likely to have a twisted
conformation and large Stokes shift because of their flexible
structure. As a result, molecular packing and energy transfer
caused emission quenching would be efficiently inhibited,
resulting in high emission intensity in the solid-state.[13]
Secondly, dimethyl 2,5-diphenylamino-terephthalates are
expected to generate different aggregation states
corresponding to their four different packing modes: 
comparison with MCL1-R and MCL3-R. MCL3-Y is highly
sensitive to mechanical stimuli and can be easily converted
to MCL3-R in the presence of shearing stimuli. After shearing
the MCL3-Y powders using a spatula or mortar, the yellow
fluorescence converted to red immediately, and the emission
spectrum of ground powders (MCL3-Y-G) is the same with
MCL3-R, suggesting the shearing stimuli triggered phase
transformation from MCL3-Y to MCL3-R (Figures 2b and
S1d). This phase transformation was further confirmed by
powder X-ray diffraction (PXRD) measurements. PXRD
results showed that MCL3-Y forms different molecular
stacking architectures compared with MCL3-R, while MCL3-
stacking between B and B’ (
I), A-B-C and A’-B’-C’ (II or III),
and and ’ (IV) (Figure 1). After incorporating long alkyl
A
A
chain or halogen atom to 2,5-diarylamino groups, different
molecular packing modes would be tuned by varying the
intermolecular interactions such as  stacking and
hydrogen bonds. Therefore, mechanical stimuli triggered
phase transformation from a metastable aggregation state to
a stable one is highly expected. MCL1-4 are constructed by
incorporating of arylamine groups to the 2,5-positions of
methyl terephthalates. The synthetic route for MCL1-4 is
shown in Scheme S1. The chemical structures of MCL1-4
are confirmed by NMR, mass data, and single-crystal X-ray
structure analysis.
We first investigated the photophysical properties of MCL1-4 in
the solid-state (Tables S12). Slowly evaporation of the
dichloromethane (DCM) solution of MCL1-3 resulted in the
precipitation of reddish-emissive crystals (R-form, MCL1-3-R). As
shown in Tables 1 and S2, MCL1-3-R shows weak red
fluorescence around 620 nm (em) with quantum yield (f) and
decay time (τf) ranging from 25% and 2.58.9 ns, respectively.
When the solvents are rapidly evaporated, yellow pristine powder
Y-G shows the same diffraction patterns with MCL3-R
,
suggesting the same molecular stacking structures of MCL3-
Y-G and MCL3-R (Figure 2c). In addition, compared with
MCL2-R and MCL2-Y, MCL2-R-G showed a different PXRD
pattern, indicating that a new molecular packing mode was
formed. In the case of MCL2-Y, and MCL4-Y, the initial
powder, and the ground powder showed similar PXRD
patterns (Figure S2a and S2c). This result indicates that no
phase transition has occurred.
The mechanical stimuli sensitivity of MCL2-Y, MCL2-R,
MCL3-Y, and MCL4-Y was also investigated using an
automatic roller pressing machine. As shown in Figure S3,
when the pressure is increased from 0 to 2 MPa, the MCL3-
Y
powders changed the colour from orange to dark orange,
and the emission colour from bright yellow to dark yellow.
2
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However, under the same pressure, MCL2-Y
,
MCL2-R, and
heating process, the DSC thermogram of MCL1-R
MCL2-R MCL3-Y, and MCL4-Y showed one endothermic
peak in the range of 124
160 oC. Such a low phase transition
temperature proves the flexible π-conjugated nature of
MCL1-4 During the cooling process, an exothermic
transition was detected around 72
115 oC, indicating the
, MCL2-Y,
MCL4-Y showed almost no colour change. Only under the
pressure of 10 MPa, the MCL2-Y MCL2-R, and MCL4-Y
,
,

powders can observe an obvious colour change. These
results further confirm that MCL3-Y is sensitive to
.
mechanical stimuli than MCL2-Y
,
MCL2-R, and MCL4-Y
.

The high-contrast and highly sensitive mechanochromic
fluorescence of MCL3-Y was further realized in a recording
device. An as-spin cast film of MCL3 was prepared by spin-
coating of MCL3 on a glass substrate from a THF solution.
The as-spin cast film exhibited strong yellow fluorescence,
which is coincidence with the emission of MCL3-Y. The
yellow emission changed to red high sensitivity in response
to very small shear forces, allowing for sensitive piezo-writing
with a high contrast ratio (Figure 2d, Supplementary Video 1-
2).
formation of different crystalline states (Figure S5). These
results are consistent with the aforementioned PXRD results,
which rationalize the irreversibility of the MCL behaviour of
MCL2-4
.
To understand the generation, photophysical and
mechanochromic properties of metastable and stable states
of MCL1-4, the photophysical properties in solution state and
the molecular packing mode in the crystalline state of MCL1-
4
were investigated. In THF solution, the absorption
maximum and shoulder bands of MCL1-4 are around 465
On the other hand, the reversibility of MCL behaviour was
and 315 nm, respectively, which can be assigned to the
explored by subjecting the ground samples to heat treatment. intramolecular charge transfer (ICT) band and * band,
After heating the ground powders at 160 oC, the original
respectively (Figure S6a). MCL1-4 show weak fluorescence
fluorescence can be restored except for MCL2-R-G, which
with an emission maximum of around 565 nm (Figure S6b).
shows that MCL2-Y-G
higher thermochromic sensitivity than MCL2-R-G (Figure
S1b 1e). Unexpectedly, the recovered powders showed
,
MCL3-Y-G, and MCL4-Y-G have
The f and τf are determined to be 8% and 3.8 ns for MCL1,
3% and 2.2 ns for MCL2, 2% and 0.69 ns for MCL3, and 2%
and 0.8 ns for MCL4, respectively. The large Stokes shift of
3584 a significant conformation
4035 cm-1 suggested
change in the excited state, which should be attributed to the
flexible structure of MCL1-4 (Table S2). The solvent effects
on the absorption and fluorescence of MCL1-4 are examined.
The absorption bands of MCL1-4 are hardly affected by
solvent polarity, while the emissions of these compounds are
quenched and red-shifted with the increment of solvent
polarity, supporting the ICT nature of MCL1-4 (Figures S7
and S8).

poor sensitivity compared to the original form, indicating that
these compounds are less reversible. As shown in Figure S2,
the heated ground powders showed different PXRD
diffraction patterns compared with the initial powders and the
ground powders, indicating a phase transition caused by the
heating-cooling cycle. Thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) further proved
this speculation. The TGA trace showed that MCL1-4 are
stable at temperatures up to 220 oC (Figure S4). During the
Figure 2. Fluorescence spectra and photographs of (a) MCL2-Y, MCL2-Y-G, MCL2-R-G, and MCL2-R and (b) MCL3-Y, MCL3-Y-G, and MCL4-Y and MCL4-Y-G
(c) PXRD spectra of MCL3-Y, MCL3-Y-G, and MCL3-R. (d) Fluorescence and colour change upon drawing of an “I” on a thin film of MCL3-Y on the glass. The
drawing was made by sequential sketching with a spatula. The excitation wavelength for the fluorescence spectra measuring and photograph taking are 450 and
365 nm, respectively.
3
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When water as poor solvent was added into THF solution
of MCL1 at varied volumetric fractions (fw), the emission
intensity was distinctly decreased with the increment of fw
from 0 to 70%, accompanied by red-shifted em from 565 to
585 nm (Figure 3a and 3e), demonstrating the aggregation
caused emission quenching (ACQ).[14] When fw reaches 80%,
the emission wavelength was blue-shifted from 585 to 570
nm, and the emission intensity was slightly increased in
comparison with that of fw = 70%. Then the emission
wavelength was red-shifted and the intensity was decreased
again with fw from 90 to 99%. This progress suggests that
unstable aggregates were formed with fw = 80%, and these
aggregates may be easily converted to MCL1-R with
increased aggregation induced by high fw. MCL2 and MCL4
showed decreased emission with fw increasing from 0 to 40%
for MCL2 (Figure 3b and 3f) and 0 to 70% for MCL4 (Figure
3d and 3h), respectively. However, when fw reaches to 50%
and 80%, a new fluorescence peak at 565 nm appeared and
better mechanical response of MCL2-R and MCL2-Y than
MCL1-R
.
enhanced with fw increasing. Similar to MCL2 and MCL4
,
MCL3 showed ACQ caused by fw increment from 0 to 50%.
The emission intensity was then slightly increased with fw
ranging from 60 to 80%, and decreased with fw = 90%,
demonstrating the unstable state of the AIE aggregates.
Finally, these unstable aggregates were transformed into
stable MCL3-Y aggregates to give an intense fluorescence
peak at 562 nm with fw = 95% (Figure 3c and 3g). These
results demonstrate that MCL2-4 are preferred to form the Y-
form aggregates during the fast molecular aggregation
process. The aggregation-induced intramolecular motions
restriction and twisted ICT inhibition may responsible for the
AIE and blue-shifted emission.
To further disclose the relationship between molecular
packing mode and mechanofluorogenic properties of MCL1-
4
, single-crystal X-ray analysis was performed. Single
crystals of MCL1-R MCL2-R MCL2-Y MCL3-R, and
,
,
,
MCL4-Y were obtained by slow evaporation of DCM solution
of these compounds. Unfortunately, single crystals of MCL3-
Y
suitable for X-ray analysis cannot be obtained due to the
poor quality of the crystals. The ORTEP drawings and
molecular packing structures of MCL1-R MCL2-R MCL2-Y
MCL3-R, and MCL4-Y are shown in Figures 4 and S9 18.
All the molecules in their crystals showed twisted
conformations due to the flexible -conjugation structure.
MCL1-R and MCL2-R crystallized in a P21/c lattice, while
MCL2-Y MCL3-R, and MCL4-Y crystallized in a P-1 lattice
(Table S3). Each MCL1-4 molecules are connected via
interactions such as  stacking and H-bonds. In MCL1-R
 stacking between phenyl rings and B’ (3.47 for
MCL1-R) dominated the type- molecular packing mode
(Figure 1), while weak C-H···O, H-bond and C-H···
,
,
,

Figure 3. Fluorescence spectra of MCL1 (a), MCL2 (b), MCL3 (c), and MCL4
(d) in THF-water binary solvents (1.0 × 10-5 mol L-1) with varied fw (ex = 450 nm),
and emission intensity (em) and wavelength changes of MCL1 (e), MCL2 (f),
MCL3 (g), and MCL4 (h) with different varied fw.

,
,
MCL3-R was found to packed in type-IV packing mode. Strong -
 interactions between the phenyl rings A and C’ (3.47 Å) and the
weak C11-H11···O3 (3.48 Å, 148o) H-bond interactions were
observed within the packing structure. This head-to-tail type 
stacking should attribute its red-shifted emission wavelength.
Moreover, C19-H19···Br1 (3.82 Å, 157o) H-bond was also found
within the crystal, which facilities the organization of long alkyl
chains. The head-to-tail type  stacking and H-bond
synergistically stabilized MCL3-R, resulting in its poor mechanical
response. Molecules within the MCL4-Y crystals were packed in
type-III packing mode. The flexible conjugated -skeletons were
arranged shoulder to shoulder, with the help of strong 
interactions between phenyl rings B and B’, and weak C-H···
interactions between A and A’ (C11-H11··· 3.76 Å, from H atom
to phenyl center). Most importantly, strong N1-H···Cl1 (3.47 Å,
B
Å
I

interactions such as C46-H46···O6 (3.07 Å, 106o) give fewer
contributions. The closely-packed MCL1 molecules in
MCL1-R should contribute to its poor mechanical response.
Interactions including  stacking between phenyl rings
and B’ (3.44 and 3.42 Å in MCL2-R and MCL2-Y
respectively) and weak C-H···C between the phenyl rings
and A’ were observed in MCL2-R and MCL2-Y to give
almost the same type-II molecular packing mode. However,
their long alkyl chains arrayed slightly differently. In
comparison with MCL1-R, the introduction of long alkane
chains made the looser packing structure, leading to the
B
,
A
4
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138 o) H-bond was observed within the crystal, which should be
helpful for the stabilization of the molecular arrangement of each
molecule, and may give a major contribution for the formation of
MCL4-Y as stable aggregation states rather than the type-IV
packing mode like MCL3-R.
observed within the single crystal of MCL4-Y. In addition, no other
strong intermolecular interactions, such as  stacking, were
observed. The weak intermolecular interaction makes the MCL3-
Y more unstable than that of MCL2-Y and MCL4-Y. Accordingly,
the excellent mechanical fluorescence properties of MCL3 can be
rationalized.
Table 2. Dihedral angles between rings B and A, and rings B and C of MCL1-4
Compounds
MCL1
Y-form
-
R-form
46.7/43.4°
55.8°
MCL2
64.0°
-
MCL3
40.1°
MCL4
76.1°
-
Conclusion
In summary,
a
family of dimethyl 2,5-diphenylamino-
terephthalate-based mechanochromic luminophores (MCL1-4)
that display tunable molecular packing mode in the solid-state,
has been designed and synthesized. By varying the
intermolecular interactions such as  stacking and hydrogen
bonds between molecular assemblies, different kinetically and/or
thermodynamically controlled molecular packing modes with
highly contrast emission colors have been generated. Moreover,
highly sensitive mechanical stimuli fluorescence response of
these luminophores has been successfully achieved by the
transformation from the metastable state to the stable state. The
understandings obtained from the experimental investigations will
inspire the development of a new class of mechanochromic
luminescent materials with tunable responsibility and provides
important insights into the solid-state luminescent properties of
molecules with flexible -conjugated structure.
Figure 4. Molecular packing structures of MCL1-R, MCL2-R, MCL3-R, MCL3-
Y, and MCL4-Y.
As mentioned above, the formation of yellowish-emissive
crystals is attributed to the shoulder to shoulder type arrangement
of flexible conjugated -skeleton and intermolecular H-bond.
While reddish-emissive crystals are preferred to formed via 
stacking interactions between phenyl rings B and B’ or A and C’.
As shown in Table 2, the emission color of these two aggregates
is closely related to the dihedral angles between rings B and A
and rings B and C. The calculated dihedral angles between rings
B and A and rings B and C are same to each other, demonstrating
the high symmetry of the molecules in the packing structure. The
dihedral angles of 6476o in Y-form are larger than those of 40-
56o in R-form, demonstrating the more twisted molecular structure
in Y-form than R-form. On the other hand, the substituted groups
such as long alkyl chains and halogen atoms also play important
roles in tuning the generation of Y-form and R-form aggregates.
Because MCL2-Y, MCL3-Y, and MCL4-Y showed intense
PXRD peaks around 2 = 4o, but the R-form did not. In addition,
MCL2-Y and MCL4-Y showed type-III molecular packing mode.
Therefore, although we did not obtain the crystal structure of
MCL3-Y, based on the information obtained from the XRD and
single-crystal X-ray analysis, it can be expected that MCL3-Y may
have the similar packing mode as MCL4-Y. However, because
the electronegativity of the bromine atom is lower than that of
chlorine atom, the intermolecular N-H···Br H-bonds in MCL3-Y
should be weakened than N-H···Cl H-bonds in MCL4-Y, which
makes MCL3-Y has higher energy than MCL4-Y.[15] The
theoretical calculation results primarily proved this speculation. In
the optimized MCL3-Y molecular packing structure (Figure S19),
the length of the intermolecular N-H···Br H-bonds is about 4.03 Å
(140o), which is longer than the N-H···Cl (3.47 Å, 138o) H-bond
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21971115) and the Key University
Science Research Project of Jiangsu Province (17KJA150004).
We thank Dr. Dan Tian and Mr. Houting Liu for single-crystal
structure solvation.
Keywords: Mechanochromic materials • Aggregation-induced
emission • Dyes and Pigments • Fluorescence • Tuneable
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10.1002/chem.202002712
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A new class of mechanochromic luminophores with tuneable aggregation states has been developed.
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