Structure-dependent reversible mechanochromism of D-π-A triphenylamine derivatives

Article abstract of DOI:10.1016/j.tetlet.2018.12.024

Three simple non-planar D-π-A TPA-based isomers (TPA-o, TPA-m, and TPA-p) were designed and synthesized via Suzuki reaction with higher yields. It was found that all of the three compounds had intramolecular charge transfer (ICT) properties resulting from

Full text of DOI:10.1016/j.tetlet.2018.12.024

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Structure-dependent reversible mechanochromism of D-p-A
triphenylamine derivatives
b
Junhui Jia a,b, , Haixia Zhao
⇑
a Modern College of Humanities and Sciences of Shanxi Normal University, Linfen 041000, China
b Key Laboratory of Magnetic Molecules and Magnetic Information Material, Ministry of Education, College of Chemistry and Material, Shanxi Normal University, Linfen 041004, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three simple non-planar D-p-A TPA-based isomers (TPA-o, TPA-m, and TPA-p) were designed and syn-
Received 17 October 2018
Revised 7 December 2018
Accepted 11 December 2018
Available online xxxx
thesized via Suzuki reaction with higher yields. It was found that all of the three compounds had
intramolecular charge transfer (ICT) properties resulting from electron donation from triphenylamine
to the formyl moiety. Interestingly, TPA-based derivatives solids exhibited not only reversible mechano-
chromic behaviors, but structure-dependent emission properties. The solid emission studies illustrated
that their fluorescence color could change reversibly between bright blue (455 nm), blue (445 nm), cyan
(479 nm), and bright blue-green (497 nm), bright blue (460 nm), yellowish-green (511 nm) through
grinding and dichloromethane (DCM) fuming treatment, giving the large spectral red-shifts of 42, 15,
and 32 nm, respectively. The MFC activities of the three compounds are increased with the sequence
of TPA-o (42 nm) > TPA-p (32 nm) > TPA-m (15 nm). The single crystal structure, powder X-ray diffrac-
tion (PXRD) and DSC studies indicated that the reversible phase change from crystalline to amorphous
was responsible for MFC properties. This work demonstrated the feasibility of tuning the solid-state opti-
cal properties of fluorescent organic compounds by combining the simple alteration of chemical structure
and the physical change of aggregate morphology under external stimuli. And these results will be of
great help in understanding structure-property relationships of MFC mechanisms and designing new
more MFC materials.
Keywords:
Mechanochromic
Triphenylmine
Single crystal
Structure-dependent
Ó 2018 Published by Elsevier Ltd.
Introduction
phenylethene [3], 9, 10-divinylanthracene [4], phenothiazine [5],
triphenylamine [6] derivatives and so on. It was found that these
Over the past decades, continuous efforts have been made to
develop novel smart organic fluorescent materials. Among these
functional smart materials, mechanofluorochromic (MFC) com-
pounds have been rapidly developed because of their promising
applications in sensors, memory chips, security inks and optical
data storage devices [1], and so on. As is well known, MFC com-
pounds showed dynamically variable solid state emission in
response to mechanical force stimuli including grinding, shearing
or friction due to either phase transition between crystalline and
amorphous state or between two crystalline states. Partially, their
fluorescence can be recovered after solvent or thermal annealing
because the pristine crystal phase or molecular structure emerges
[2].
nonplanar molecules generally possess push-pull electron struc-
tures with large transition dipole moments, and the nonplanar
structures usually induce the loose molecular stacking in the crys-
tal states, and in this case their solid emission would be affected by
external forces [2].
However, MFC materials are still relatively limited in number
mainly because there are no clear strategies for the design of
MFC fluorophores. Therefore, it is still a great challenge to exploit
excellent MFC materials. Obviously, a deep understanding of struc-
ture-property relationship at the molecular level is a crucial factor
to the development of the MFC materials. Twisted conformation
[7], the alkyl chain length [8] and the D-A structure [9] have been
found to show a direct influence on the performance of the MFC
behaviors. And these influence factors provided useful strategies
to design more new MFC materials [7a].
Up to date, some fluorophores with nonplanar twisted molecu-
lar conformation have often been reported for MFC, such as tetra-
Triphenylamine (TPA) is a very popular building block for the
construction of optoelectronic materials [10] resulting from its
multiple advantages and flexible syntheses of its derivatives. And
the nonplanar molecular structure and strong electron donating
⇑
Corresponding author at: Modern College of Humanities and Sciences of Shanxi
Normal University, Linfen 041000, China.
E-mail address: jiajunhui@sxnu.edu.cn (J. Jia).
https://doi.org/10.1016/j.tetlet.2018.12.024
0040-4039/Ó 2018 Published by Elsevier Ltd.
Please cite this article as: J. Jia and H. Zhao, Structure-dependent reversible mechanochromism of D-
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J. Jia, H. Zhao / Tetrahedron Letters xxx (xxxx) xxx
Scheme 2. Synthetic route to compounds TPA-o, TPA-m and TPA-p.
of intramolecular charge transfer (ICT) process. To further under-
stand the influence of different solvents on the fluorescence emis-
sion, Lippert-Mataga plots for the three compounds were shown in
Fig. S10. The slopes of the fitting line for TPA-o, TPA-m, and TPA-p
were 18,052, 21,851 and 17,930, respectively, which indicated an
obvious positive solvatochromic effect [11]. These phenomena
resulted in a donor–acceptor structure and an intramolecular
charge transfer (ICT) characteristic for the excited state, which
means that they are polar molecules [12]. And it can be confirmed
by quantum chemical calculations. The quantum chemical calcula-
tions were performed by density functional theory calculations
(DFT) at the B3LYP/6-31G (d, p) level. The calculated dipole
moments were 4.2687 D, 3.7027 D and 5.4438 D, respectively.
For deeper understanding of optical properties displayed by
TPA-o, TPA-m, and TPA-p, density functional theory (DFT)
calculations were performed on the three isomers at the
B3LYP/6-31G (d, p) level after optimizing their structures to the
lowest energy spatial conformation with the Gaussian 09 W
program. The electron distributions of the HOMOs and LUMOs
for the three isomers were shown in Fig. 2. It was found that the
HOMOs were mainly distributed to triphenylamine units. In sharp
contrast, the LUMOs were almost located at the formyl moieties.
Such spatially separated HOMOs and LUMOs were similar to that
Scheme 1. The structures of TPA derivatives.
ability provided the basis for developing solid state fluorescent
materials with MFC properties. To our best knowledge, systematic
studies of structural modification especially for substituent posi-
tion of acceptor groups in TPA derivatives on their fluorescence
properties including MFC properties have never been explored in
detail, and such studies are expected to provide structural insight
for designing new MFC molecules. Herein, we have attempted to
study the substituent position of acceptor group (formyl group)
in TPA derivatives on the molecular packing and solid state fluores-
cence especially MFC properties by synthesizing three TPA-based
D-p
-A molecules (Scheme 1).
Interestingly, the three D-p-A TPA-based derivatives gave struc-
ture-dependent reversible mechanochromism, which indicated the
acceptor substituent position exhibited a strong influence on the
molecular conformation and packing that significantly modulated
the MFC behaviors. The single crystal structure, powder X-ray
diffraction (PXRD) and DSC studies indicated that the reversible
phase change from crystalline to amorphous was responsible for
fluorescence switching. Thus, our present studies could provide
structural insight into designing new TPA-based MFC materials.
in many classical D-p-A cruciform [12]. Moreover, the stimulated
Results and discussion
absorption spectrum showed strong absorption bands with the
maximum of ca. 425 nm, 439 nm and 423 nm because of the tran-
sition from HOMO to LUMO (Figs. S11–13 and Tables S4–6). There-
fore, light excitation clearly induced electron transfer from the
donor units to the acceptors. These results further demonstrated
the three TPA-based derivatives possessing push-pull electron
structures. In addition, the dihedral angles between the A ring
and B ring in the optimized geometry are 128.67, 148.57 and
146.67 (Fig. S14). These large dihedral angles induce a non-planar
configuration. Therefore, these three TPA-based derivatives are
polar and nonplanar molecules and mechanochromic activities
are expected for them in the solid states.
Synthesis
The synthetic routes to TPA-o, TPA-m, and TPA-p are depicted
in Scheme 2. They were synthesized by the Suzuki coupling reac-
tion of formylphenylboronic acid derivatives with 4-bromo-N, N-
diphenylaniline. The target molecules were purified by silica gel
chromatography, with their structures confirmed by the NMR
spectroscopy, high resolution mass spectroscopy and elemental
analysis (Figs. S1–9).
Optical properties
Mechanofluorochromic properties
The measured UV-vis and fluorescence spectra of TPA deriva-
tives TPA-o, TPA-m, and TPA-p in solvents with different polarity
were shown in Fig. 1 and the photophysical data were listed in
Tables S1–S3. It was found that the absorption spectra of these
three compounds displayed obvious polarity independence like
the common ICT molecules. In contrast, the fluorescence bands of
TPA-o, TPA-m, and TPA-p showed significant red-shift with a
noticeable decrease in fluorescence intensity and an increasing
Stokes shift when increasing the solvent polarity. For example,
when TPA-o was dissolved in cyclohexane, the maximum emission
As discussed above, TPA-o, TPA-m, and TPA-p have nonplanar
molecular conformations, and a D-p-A conjugated system with
ICT character, so we predicted that they would have MFC proper-
ties. The fluorescence responses of the three isomers toward grind-
ing and fuming processes were illustrated in Figs. 3 and 4. It was
found that TPA-o exhibited a significant red-shifted emission from
the as-prepared powder (455 nm) to the ground state (497 nm)
with broadened fluorescence emission spectrum, indicating the
highly contrast mechanochromism effects with about 42 nm, along
band was at about 404 nm and its Stokes shift was 2348 cmÀ1
,
with the decreasing fluorescence intensity. Additionally, the UF of
when the solvent polarity was increased, the emission band was
red-shifted to 540 nm and the Stokes shift reached up to
8752 cmÀ1 in DMF. An increased Stokes shift in polar solvents
and the obvious red-shift of emission band indicated the existence
the ground powder was measured as 0.18, which is a little lower
than that of the pristine crystal (0.21). Notably, when the ground
powder was fumed with CH2Cl2 vapor for about 15 s, the powder
returned to its original color. The emission color of TPA-o could
Please cite this article as: J. Jia and H. Zhao, Structure-dependent reversible mechanochromism of D-
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J. Jia, H. Zhao / Tetrahedron Letters xxx (xxxx) xxx
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Fig. 1. (a) Absorption and (b) emission spectra of TPA derivatives in different solvents.
be switched between blue and bluish green reversibly by grinding
and CH2Cl2 fuming. This change of emitting color was reversible
under grinding and solvent fuming (Fig. S15). Similarly, the fluo-
rescence color of TPA-p after grinding was converted from cyan
to yellowish green with about 32 nm red-shift. Interestingly, the
solid fluorescence quantum yield increased from 0.24 to 0.28
before and after grinding, and the process could be reversed by
fuming with CH2Cl2. And this reversible change of emissive color
can be repeated for many times without fatigue under alternating
grinding and solvent fuming procedures, indicating reversibility
and durability under external stimuli (Fig. S16). In contrast, TPA-
m only presented a 15 nm red-shift from 445 nm to 460 nm with
the solid fluorescence quantum yield increased from 0.23 to 0.25,
which indicated the smallest fluorescence spectrum shifts in the
three isomers under mechanical force stimuli. And the grinding
sample of TPA-m returned to the original color after fuming with
CH2Cl2 for about 20 s. Moreover, this reversible change of emissive
color can be repeated for many times (Fig. S17). These comparative
results demonstrate that the substituent position of acceptor group
(formyl group) in TPA derivatives strongly influenced the solid
state and mechanochromic fluorescence, which meant they
showed structure-dependent reversible mechanochromism.
Crystal structures of TPA-o, TPA-m, and TPA-p
The single-crystal structures of solids (Table S7, Figs. 5 and 6
and S18–19) were investigated to further understand the MFC
mechanism. Good single crystals suited for single-crystal analysis
could be easily obtained through slow evaporation of the solvent.
For TPA-p (CCDC 1562914) [13], the single crystal of TPA-p is
orthorhombic, and each crystal cell has two molecules. And there
were two different types of molecular configurations in the crystal,
two types of strong CAHÁ Á ÁO bonds (2.50 Å and 2.68 Å) and 25
types of CAHÁ Á Á
p bonds with distances ranging from 2.98 to
3.44 Å were observed [13], the effective intermolecular interac-
tions could surely contribute much to the rigidity of crystal and
would suppress the intramolecular rotation in the crystalline state,
leading to strong luminescence. Similarly, multiple intermolecular
interactions existed in the crystal of TPA-m (Fig. 5). One TPA-m
molecule has multiple intermolecular weak interactions with three
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Fig. 2. The optimized geometry, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TPA-o, TPA-m, and TPA-p.
Fig. 3. Normalized PL spectra of TPA-o (kex = 420 nm), TPA-m (kex = 400 nm), and TPA-p (kex = 415 nm) in different solid-states: pristine, grinding and fuming.
Please cite this article as: J. Jia and H. Zhao, Structure-dependent reversible mechanochromism of D-
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J. Jia, H. Zhao / Tetrahedron Letters xxx (xxxx) xxx
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Fig. 6. Molecular packing of TPA-o in single crystals with CAHÁ Á Á
p
(2.89, 2.73 Å),
CAHÁ Á ÁO (2.61, 2.46 Å), and Á Á Á (3.35 Å).
p
p
ground solid of TPA-m was recorded. As shown in Fig. S21, the
absorption band in the long wavelength region consists of two
absorption bands with maximum peaks at 292 and 398 nm. Con-
sidering the absorption peak located at 336 nm in cyclohexane,
the two absorption peaks at 292 and 398 nm correspond to H-
and J-aggregations, respectively, which is consistent with that
obtained by single-crystal structure analysis [15]. And it was found
Fig. 4. Photos of TPA-o, TPA-m and TPA-p colour changes under grinding and
fuming stimuli.
that the distances corresponding to the CAHÁ Á Á
single crystal of TPA-m were shorter than those in the single crys-
interactions were stronger
p stacking in the
tal of TPA-p, implying that the CAHÁ Á Á
p
between molecules TPA-m than molecules TPA-p. As for the TPA-o
crystal, the single crystal of TPA-o is monoclinic and each crystal
cell has only one molecule. But one TPA-o molecule has multiple
intermolecular weak interactions with six other molecules, includ-
other molecules, including CAHÁ Á ÁO, CAHÁ Á Á
p and weak hydrogen
bond. However, the phenyl ring connecting to formyl moiety and
triphenylamine group are not coplanar, and the included angle of
the plane between two rings is 120.20° (Fig. S20). 1D stacking
was observed along the b-axis (Fig. 5a). Moreover, the 1D stacking
has two kinds of aggregations. Two TPA-m molecules in one pack-
ing type adopt an antiparallel arrangement with interplanar dis-
tance of 2.82 Å, which indicate H-aggregation [14]. Two kinds of
J-aggregate could be observed. The distances between two adja-
cent benzene rings of triphenylamine were 2.82 Å, 2.84 Å, 2.85 Å
and 2.82 Å, respectively. The absorption spectrum of pristine and
ing two types of CAHÁ Á ÁO (2.61, 2.46 Å), two types of CAHÁ Á Á
p
(2.89, 2.73 Å) and a kind of (3.35 Å) (Fig. 6), which indicated
pÁ Á Áp
the least reactions were present in the TPA-o crystal among the
three crystals. Moreover, the monoclinic crystal system would also
weaken the rigidity of crystal and lead to the molecular readily
being destroyed by external mechanical stimuli to form amor-
phous aggregates, which results in a larger fluorescence spectrum
Fig. 5. Molecular packing of TPA-m in single crystals with hydrogen bond (2.37 Å), CAHÁ Á ÁO (2.69, 2.66, 2.61 Å), and CAHÁ Á Á
p
(2.82, 2.84, 2.85, 2.88 Å).
Please cite this article as: J. Jia and H. Zhao, Structure-dependent reversible mechanochromism of D-
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p-A triphenylamine derivatives, Tetrahedron Letters,

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J. Jia, H. Zhao / Tetrahedron Letters xxx (xxxx) xxx
Fig. 7. Powder wide-angle X-ray diffraction patterns of TPA-o, TPA-m, and TPA-p in different states at room temperature.
shift. Above all, the molecular conformational and packing differ-
ences induced optical properties upon external mechanical stimuli.
To deeper investigate the MFC mechanism of TPA-o, TPA-m,
and TPA-p, powder X-ray diffraction (PXRD) and differential scan-
ning calorimetry (DSC) measurements were conducted for both
pristine and ground samples. As shown in Fig. 7, the XRD curves
of TPA-based derivatives exhibited different structures of molecu-
lar aggregation stations before and after grinding. And it was
clearly found that the as-synthesized samples displayed many
intense and sharp diffraction peaks, indicating the ordered
arrangement forms of crystals. In contrast, after grinding, many
reflection peaks weakened or disappeared, and the diffraction
curves became diffuse, which indicated an amorphous structure.
After exposing the ground samples to CH2Cl2 vapour, the sharp
and intense diffraction peaks reappeared. Their reappearance
demonstrated that the amorphous powder restored its pristine
crystallinity upon solvent fumigation. Thus, the powder XRD
results revealed that the MFC mechanism of TPA-based derivatives
could be ascribed to a morphological transition between crystalline
and amorphous states upon the ground/fumed process. Ground-
induced structural transition character of TPA-o, TPA-m, and
TPA-p probably leaded to changes in packing, which allowed a
more planar structure and perhaps better dipole-dipole inter-
molecular interactions, leading to a red shift of fluorescence [16].
Additionally, the DSC curves of ground powder of T TPA-o, TPA-
m, and TPA-p (Fig. 8) show clear exothermic transition peaks at
87.6 °C, 53.5 °C, and 81.7 °C, respectively, which can be ascribed
to their coldcrystallization process [16,17]. This exothermic transi-
tion peak also illustrated that the ground solid was in a metastable
phase. Therefore, the mechanofluorochromism of those molecules
probably originated from the transformation between the highly
ordered form and the amorphous state.
Conclusions
In summary, three simple non-planar D-p-A TPA-based deriva-
tives (TPA-o, TPA-m, and TPA-p) were designed and synthesized
via Suzuki reaction with higher yields to explore the influence of
acceptor substituent position on photophysical properties. It was
found that all of the three compounds had ICT properties resulting
from electron donation from triphenylamine to the formyl moiety.
Interestingly, the three isomers presented structure-dependent
reversible mechanochromism through grinding and dichloro-
methane (DCM) fuming treatment. And the general trend of
DkMFC
(kgrinding–kpristine) was TPA-o (42 nm) > TPA-p (32 nm) > TPA-
m (15 nm). And single crystal analysis indicated that the molecular
conformational and packing differences induced optical properties
upon external mechanical stimuli. Moreover, powder X-ray diffrac-
tion (PXRD) and DSC studies indicated that the reversible phase
change from crystalline to amorphous was responsible for MFC
properties. Moreover, several other organic MFC luminogens were
under research to further obtain deep understanding of the struc-
ture-packing-property relationship.
Please cite this article as: J. Jia and H. Zhao, Structure-dependent reversible mechanochromism of D-
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p-A triphenylamine derivatives, Tetrahedron Letters,

J. Jia, H. Zhao / Tetrahedron Letters xxx (xxxx) xxx
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Fig. 8. DSC curves of TPA-o, TPA-m, and TPA-p under pristine and ground states.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (21805175), the Natural Science Foun-
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Please cite this article as: J. Jia and H. Zhao, Structure-dependent reversible mechanochromism of D-
https://doi.org/10.1016/j.tetlet.2018.12.024
p-A triphenylamine derivatives, Tetrahedron Letters,