Design and synthesis of piezochromic materials based on push-pull chromophores: A mechanistic perspective

Article abstract of DOI:10.1002/chem.201102929

Computational analysis predicts that intramolecular charge transfer (ICT) exists in anthraquinone imide (AQI) derivatives with electron-donating substituents at the 6-position, such as 4-methoxylphenyl, 4-N,N- dimethylaminophenyl, and thiophene. However, for those with electron-withdrawing ones, no clear ICT interaction could be observed. We predicted, on the basis of the simulation results, that AQI derivatives with electron-donating substituents would be piezochromic. To verify this hypothesis, the corresponding AQI derivatives with various substituents were synthesized. Absorption spectra recorded with a diffuse reflectance method on powders revealed that 4-methoxylphenyl-, 4-N,N-dimethylaminophenyl-, and thiophene-substituted AQI exhibited piezochromism, but not 4-nitrophenyl-substituted AQI, which is in good agreement with the simulation results. Interestingly, redshifts of both the lower and higher energy absorption bands were observed along with redshifts of the emission spectra. However, XRD patterns before and after being pressed presented no significant changes, which was different from known piezochromic molecules described in the literature. An unprecedented mechanism in which enhanced ICT from better conjugation between the donor and acceptor segments induced by the decrease of θ under pressure could be responsible for the piezochromism of aryl-substituted AQIs is proposed.

Full text of DOI:10.1002/chem.201102929

DOI: 10.1002/chem.201102929
Design and Synthesis of Piezochromic Materials Based on Push–Pull
Chromophores: A Mechanistic Perspective
Fengkun Chen, Jie Zhang, and Xinhua Wan*[a]
Abstract: Computational analysis pre-
dicts that intramolecular charge trans-
fer (ICT) exists in anthraquinone imide
(AQI) derivatives with electron-donat-
ing substituents at the 6-position, such
as 4-methoxylphenyl, 4-N,N-dimethyl-
recorded with
a
diffuse reflectance
shifts of the emission spectra. Howev-
er, XRD patterns before and after
being pressed presented no significant
changes, which was different from
known piezochromic molecules de-
scribed in the literature. An unprece-
dented mechanism in which enhanced
ICT from better conjugation between
the donor and acceptor segments in-
duced by the decrease of q under pres-
sure could be responsible for the piezo-
chromism of aryl-substituted AQIs is
proposed.
method on powders revealed that 4-
methoxylphenyl-, 4-N,N-dimethylami-
nophenyl-, and thiophene-substituted
AQI exhibited piezochromism, but not
4-nitrophenyl-substituted AQI, which is
in good agreement with the simulation
results. Interestingly, redshifts of both
the lower and higher energy absorption
bands were observed along with red-
ACHTUNGTRENNUNGaminophenyl, and thiophene. However,
for those with electron-withdrawing
ones, no clear ICT interaction could be
observed. We predicted, on the basis of
the simulation results, that AQI deriva-
tives with electron-donating substitu-
ents would be piezochromic. To verify
this hypothesis, the corresponding AQI
derivatives with various substituents
were synthesized. Absorption spectra
Keywords: charge transfer · chro-
mophores
· donor–acceptor sys-
tems · molecular modeling · piezo-
chromic
Introduction
talline state to amorphous phase induced by the external
stress.[19]
Push–pull chromophores with electron-donor (D) and -ac-
ceptor (A) architectures exhibiting intramolecular charge-
transfer (ICT) properties have attracted growing interest for
both the understanding of fundamental chemistry and phys-
ics and promising potential applications in diverse fields,
such as organic solar cells,[1–3] anion sensors,[4–6] nonlinear
optics,[7–10] and fluorescence imaging.[11] One of the most fas-
cinating advantages of such materials is that the p conjuga-
tion, band gap, and electro-optical properties could be effec-
tively tuned by judicious choice of the D–A couples,[12–15]
substituents,[16,17] and molecular architectures.[18] Such elec-
tro-optical properties could also be significantly tuned by
disrupting the D–A structures under external stimuli. For
example, the optical properties of a series of heteropolycy-
clic D–p–A fluorescent dyes could be tuned by mechanical
stress, which resulted from a phase transition from the crys-
The phenomenon of materials becoming colored under
pressure is referred to as piezochromism.[20] Polymers, such
as
poly(di-n-alkylsilanes)[21–23]
and
poly(3-alkylthio-
phene),[24,25] transition-metal complexes,[26–29] and also some
organic dyes[30–33] are reported to show piezochromic proper-
ties. Due to convenient tunability and easy detection, these
functional materials have promising practical applications,
such as mechano-sensors, security papers, data storage, and
also opto-electronic devices. For most reported piezochro-
mic materials, the phenomenon is mainly explained by per-
turbations of intermolecular interactions and this is support-
ed by spectroscopic and XRD analyses. For example, as de-
duced from XRD results, the face-to-face distance in head-
to-tail-attached poly(octylthiophene) was shortened by 9%
after pressing.[34] A decrease in the Ni Ni distance along the
ꢀ
c axis from 3.26 ꢀ at ambient pressure to 2.82 ꢀ at high
pressure was also believed to lead to the piezochromism of
bis(dimethylglyoximato)nickel(II).[35] Reversible intercon-
versions between different types of molecular assemblies, as
indicated by the disappearance of original reflection peaks
or the appearance of new peaks in XRD patterns, have also
been testified to induce piezochromism in organic dyes.
Kato and Sagara recently reported multiluminescent color
switching from a single-luminophore liquid crystal by me-
chanical stimuli.[36] Reversible change between different mo-
lecularly assembled structures is considered to contribute to
the coloration. Although a variety of piezochromic materials
[a] F. Chen, Dr. J. Zhang, Prof. X. Wan
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Polymer Chemistry
and Physics of Ministry of Education
College of Chemistry and Molecular Engineering
Peking University, Beijing 100871 (P.R. China)
Fax : (+86)10-62751708
E-mail: xhwan@pku.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102929.
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FULL PAPER
have been reported recently, the design principles of piezo-
chromic materials have rarely been reported.[37] Recently,
we reported a series of anthraquinone imide (AQI)-based
molecules with a D–A structure, which changed color from
yellow to red when pressed or ground.[38] It was found that
the lower-energy absorption band exhibiting a redshift
under pressure had an intramolecular charge-transfer (ICT)
nature and the higher-energy one was assigned to the p–p*
transition and also revealed a simultaneous redshift. Howev-
er, no significant change was observed in the XRD patterns
after being pressed. No available mechanism, such as
changes of intermolecular interactions or changes in molecu-
lar assemblies or chemical reactions,[30] is applicable in our
case. Therefore, we were motivated to perform in-depth ex-
perimental and theoretical investigations on the underlying
mechanism of the piezochromism of AQI-containing mole-
cules.
Results and Discussion
Molecular design by computational simulation: Based on
our hypothesis, a series of molecules with diverse substitu-
ents (Figure 1), that is, electron-donating ones, such as 4-meth-
AHCTUNGTREGoNNNU xylphenyl, 4-N,N-dimethylaminophenyl, and thiophene
groups, and electron-withdrawing 4-nitrophenyl, were de-
signed to demonstrate the existence of ICT interactions and
also to tune the electronic structures. The optimized geome-
try of each compound was calculated at the B3LYP/6-31G
(d, p) level. As shown in Figure 1, there is a dihedral angle
between the substituent aryl ring (or thiophene ring) and
the rigid planar AQI core in each compound ranges from 20
to 368 due to the subtle balance between two competitive in-
teractions: the o-hydrogen repulsion, which prefers a twisted
structure, and p-electron conjugation, leading to a planar
structure.[39] This moderate angle affords the possibility of
Herein, we provide a new insight into the piezochromic
mechanism that is different from those described above. We
propose that the origin of piezochromism in aryl-substituted
AQIs is enhanced ICT resulting from decreased torsion
angles between the electron-donating and -withdrawing seg-
ments in the D–A molecules caused by external pressure. To
verify this postulation, a series of AQI-based push–pull mol-
ecules with electron-donating and -withdrawing substituents
were designed with the assistance of prophase computation-
al simulations. For molecules with significant ICT interac-
tions and torsion angles, piezochromic properties were ob-
served, whereas they were absent in molecules without ef-
fective intramolecular interactions. Photoluminescence (PL)
experiments of the solids illustrated that the emission spec-
tra were also redshifted to a similar extent as that of absorp-
tion spectra under pressure. The possibility of the effects of
overlapping enhanced intermolecular electronic were also
investigated and eventually it was excluded by experimental
results from XRD analyses of these compound powders and
optical spectroscopic characterization of molecularly dis-
persed polymer blends of these compounds.
pressure-induced torsion around the CACTHUNGTERNNUCG bond between the
substituent and the rigid planar core and then regulation of
the efficiency of D–A conjugation by external pressure; this
is further confirmed by computation results. The energy bar-
riers for transition from twisted to planar structures were es-
timated to be 5.91 kJmolꢀ1 for CH3O-Ph-AQI, 4.65 kJmolꢀ1
for DMA-Ph-AQI, 6.56 kJmolꢀ1 for Nitro-Ph-AQI, and
0.65 kJmolꢀ1 for Th-AQI. Such small energy barriers indi-
cate that the D–A molecules are very flexible for the tor-
sional motion from twisted geometries to coplanar ones.[40]
The electronic structures and the frontier molecular orbi-
tal (MO) energies of each compound were calculated at the
same level and are shown in Figure 2. From the electron
density distribution of the HOMO and LUMO of CH3O-
Ph-AQI, it is evident that the largest electron coefficients in
the HOMO are located along the whole donating moiety,
whereas the coefficients in the LUMO are concentrated on
the anthraquinone imide core. This result indicated that
upon photoexcitation there might be considerable ICT,
which was further verified by time-dependent (TD) DFT re-
sults. According to TD-DFT calculations in the gas phase
(Table 1), the lowest energy absorption of CH3O-Ph-AQI
appeared at 523 nm, corresponding to the HOMO–LUMO
transition with an oscillator strength (f) of 0.09. The higher
energy one was positioned at 362 nm, corresponding to the
Figure 1. Designed AQI derivatives and their optimized geometries. (F: dihedral angles between the benzene/thiophene ring and anthraquinone core.)
DMA-Ph-AQI= N-butyl-6-(4-dimethylanilino)anthraquinone-2,3-dicarboxylic imide, CH3O-Ph-AQI=N-butyl-6-(4-methoxyphenyl)anthraquinone-2,3-
dicarboxylic imide, Nitro-Ph-AQI=N-butyl-6-(4-nitrophenyl)anthraquinone-2,3-dicarboxylic imide, Th-AQI=N-butyl-6-(2-thiophenyl)anthraquinone-
2,3-dicarboxylic imide.
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X. Wan et al.
tion of CH3O-Ph-AQI between the twisted and coplanar ge-
ometries (Figure 3), which was in agreement with a decrease
in the band gap by 0.14 eV due to better conjugation be-
tween the D and A segments. A redshift of the higher
energy absorption was observed as well. As shown in
Figure 3, the oscillator strength was also enhanced, for ex-
ample, the oscillator strength of the lower-energy absorption
increased from 0.090 to 0.104.
As demonstrated by the calculation results mentioned
above, the electronic structures, and ICT properties, could
be tuned by varying the substituents on the AQI core.
Meanwhile, a decrease in the dihedral angles in all four of
the compounds designed could result in redshift of both the
higher and lower energy absorptions in molecules with effi-
cient D–A conjugation. How-
Figure 2. The frontier MOs and energies of the AQI derivatives obtained
by DFT calculations. HOMO=highest occupied molecular orbital,
LUMO=lowest occupied molecular orbital, SUMO=second unoccupied
molecular orbital.
Table 1. Absorption spectra obtained by TD-DFT methods at the B3LYP/6-31GACHTNUGTRNEUNG(d,p) optimized geometries.
ever, due to the disappearance
of ICT in Nitro-Ph-AQI, no
such redshift could be ob-
served.
abs
max
Compounds
l
[nm]
f
Main configurations
523
362
680
425
409
495
0.090
0.039
0.101
0.106
0.244
0.093
HOMO-0!LUMO+0 (+99%)
CH3O-Ph-AQI
HOMO-0!LUMO+1 (+45%) HOMO-3!LUMO+0 (38%)
HOMO-0!LUMO+0 (+99%)
DMA-Ph-AQI
Th-AQI
HOMO-0!LUMO+1 (+72%) HOMO-0!LUMO+2 (+26%)
HOMO-0!LUMO+2 (+73%) HOMO-0!LUMO+1 (26%)
HOMO-0!LUMO+0 (+98%)
Synthesis: Schemes 1 and 2 il-
lustrate the synthetic ap-
proaches of AQIs with differ-
400
395
351
0.006
0.103
0.042
HOMO-6!L+0 (+81%) HOMO-0!LUMO+0 (+5%)
HOMO-0!LUMO+0 (+86%)
ent
electron-donating
and
Nitro-Ph-AQI
-withdrawing substituents. 4-
Methoxyphenyl and 4-dimethy-
HOMO-2!LUMO+0 (+83%) HOMO-5!LUMO+0 (+5%)
laminophenyl are electron-do-
p–p* transition (HOMO-0–LUMO+1), and the oscillator
strength was 0.039. Similar stimulated optical transitions
were obtained for DMA-Ph-AQI and Th-AQI. However, in
the case of Nitro-Ph-AQI, the electron density of the HOMO
is distributed across the full molecular skeleton and that of
the LUMO mainly located at the anthraquinone core, indi-
cating weak ICT. Consequently, the p–p* transition was pre-
dicted to appear at a high energy level at around 395 nm.
Interestingly, simulation results also indicated that the ab-
sorption of those compounds exhibiting clear ICT showed
a dependence on the dihedral angle. For instance, there was
a redshift of about 10 nm of the longest-wavelength absorp-
nating groups; the latter has a stronger donating capacity. 4-
Nitrophenyl is an electron-withdrawing substituent. The pre-
cursor Br-AQI was synthesized by multistep reactions with
a procedure reported by our group.[38] Palladium-catalyzed
Suzuki coupling reactions were carried out between the Br-
substituted AQI and corresponding phenylboronic acids
under standard conditions to afford the products in good
yields.
Scheme 1. Synthetic route for AQIs with different phenyl substituents.
Br-AQI=6-bromoanthraquinone-2,3-dicarboxylic imide.
The thiophene unit is also an electron-donating substitu-
ent with a donating capacity similar to that of the 4-methox-
yphenyl group. 5-Methylthiophene-2-boronic acid was ini-
tially used to synthesize the thiophene-substituted AQI. Un-
fortunately, only a byproduct was obtained under the same
coupling reaction conditions; this byproduct was further
1
confirmed to be the unsubstituted AQI by H NMR spec-
troscopy with the loss of the halogen atom (Figure S1 in the
Supporting Information). Alternatively, Stille coupling was
employed to prepare the thiophene-substituted AQI, as
demonstrated in Scheme 2, through a modified synthetic
Figure 3. Calculated absorption spectra of CH3O-Ph-AQI in twisted and
coplanar geometries.
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Piezochromic Materials Based on Push–Pull Chromophores
FULL PAPER
The clear splits of the proton
resonances of the thiophene
ring also confirm the chemical
structure of Th-AQI.
Piezochromic properties: As
predicted by simulation results,
color changes under external
pressure are expected in CH3O-
Scheme 2. Synthetic route for an AQI with a thiophenyl substituent.
Ph-AQI, DMA-Ph-AQI, and
route. The coupling reaction was employed before imidiza-
tion to enhance the solubility of intermediate products. The
pure product was obtained as a green–yellow powder in
high yield.
Th-AQI, whereas no clear change is expected for Nitro-Ph-
AQI.
To verify our speculation, solid powders of these com-
pounds were pressed or ground with a spoon to investigate
the piezochromic properties. As a result, all compounds
except Nitro-Ph-AQI exhibited color changes after being
pressed, that is, from orange to deep red for CH3O-Ph-AQI,
dark purple to black for DMA-Ph-AQI, and green–yellow
to red for Th-AQI. The color was retained after removal of
pressure, but changed reversibly back to the original by re-
dissolving the pressed powder in organic solvents (such as
CH2Cl2) and drying. However, light yellow Nitro-Ph-AQI
did not show any significant color change after pressure.
The absorption spectra of these compounds before and after
the employment of 0.9 GPa of pressure also gives quantita-
tive evidence of the color change (Figure 4). Two bands can
The chemical structures of the synthesized AQI deriva-
tives were fully characterized by 1H NMR spectroscopy,
high-resolution mass spectrometry, and elemental analysis.
1
In the H NMR spectra, the chemical shifts of the protons at
the benzene ring in CH3O-Ph-AQI were located at d=7.73
and 7.07 ppm. When the methoxyl was replaced by N,N-di-
methylanilino, the chemical shifts changed to d=7.71 and
6.87 ppm, indicating a stronger donating capacity of the
N,N-dimethylanilino unit. A low-field shift of the resonances
of these protons was observed in the 4-nitrophenyl-substitut-
ed AQI Nitro-Ph-AQI, which was consistent with the elec-
tron-withdrawing property of the 4-nitrophenyl substituent.
Figure 4. Absorption spectra recorded with a diffuse reflectance method before and after the employment of pressure on a) CH3O-Ph-AQI, b) DMA-Ph-
AQI, c) Nitro-Ph-AQI, and d) Th-AQI.
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X. Wan et al.
be seen in the spectra shown in Figure 4a, b and d: one in
the visible region, corresponding to the observed color, and
the other in the ultraviolet region. After pressure (0.9 GPa)
was applied, the whole absorption spectrum, including both
the peak maximum and the band edge, were redshifted. For
compounds with donating substituents, a redshift of about
10 nm in the peak maximum was observed, which was in
good agreement with simulation results. The onset of ab-
sorption bands displayed even larger redshifts (Table S1 in
the Supporting Information), for instance, 69 nm for DMA-
Ph-AQI, which may be responsible for the observed signifi-
cant color changes rather than the redshift of the peak maxi-
mum.
Interestingly, piezochromic luminescence was also ob-
served in those piezochromic compounds. As shown in
Figure 5, a redshift of 14 nm in the emission peak of CH3O-
Ph-AQI excited at 310 nm was observed, which was consis-
tent with that of the lowest-energy absorption. Piezochromic
luminescent materials, which
Figure 5. Emission spectra of CH3O-Ph-AQI excited at 310 nm before
and after the employment of pressure. (Inset: photograph of the partially
pressed film under UV radiation.)
change their fluorescence emis-
sion upon external pressure
stimuli, have been attracting
significant research interest due
to the possibility to control
fluorescence in solids without
chemical modification.[41,42]
Crystal structures: The phe-
nomenon that emission spectra
change with pressure, that is,
piezochromic
luminescence,
was mainly explained in the lit-
erature by modulation of the
molecular packing of structures
through pressure.[38,43–45] Intui-
tively, pressure should shorten
the distance between adjacent
molecules, and consequently,
enhance the possibilities of
electronic orbital overlap. How-
ever, the endogenous require-
Figure 6. a) Single-crystal X-ray structure of compound CH3O-Ph-AQI and stacking patterns along the b) b, c)
a and d) c axes.[67]
ment for such an interaction to dominate is a rather planar
molecular architecture and cofacial arrangement.[35,46]
Single-crystal XRD was employed to get insights into the
molecular arrangements in the solid state. The crystal struc-
ture and molecular stacking are shown in Figure 6 (see the
Supporting Information for more details). There is indeed
a torsion angle between the benzene ring and the AQI core
(about 88). Even though it is smaller than the angle (348)
calculated by the molecular simulation in the gas state, a red-
shift and intensification of the absorptions are also predicta-
ble if the molecule twists from the structure with a relatively
small angle as 88 to a planar one. In a single-crystal cell,
there are four molecules arranged in a herringbone pattern
with an intermolecular distance of 3.78 ꢀ (Figure 6b) and
a slip angle between two molecules of about 65.58(Figure 6c,
d). For molecules with herringbone stacking structures, the
p–p overlap between adjacent molecules is believed to be
minimal.[47–49]
Because only a few pieces of very thin platelet crystals of
compound CH3O-Ph-AQI were available, it was not practi-
cal to investigate the change of molecular arrangement after
pressing by using single-crystal XRD. Instead X-ray powder
scattering was used for the powder samples of all com-
pounds. As demonstrated in Figure 7, no significant differ-
ence was observed between the X-ray scattering patterns of
the Th-AQI powders before and after pressure, except for
a negligible shift in the scattering patterns that could be at-
tributed to experimental error. Such a result explicitly ex-
cludes the possibility that changes in intermolecular interac-
tions are responsible for piezochromism.
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Piezochromic Materials Based on Push–Pull Chromophores
FULL PAPER
Table 2. Calculated MO energies and corresponding energy gaps.
[a]
[b]
HOMO LUMO SUMO DEL–H [eV] DES–H [eV]
[eV]
[eV]
[eV]
(l [nm])
(l [nm])
CH3O-Ph-AQI ꢀ6.08
DMA-Ph-AQI ꢀ5.37
ꢀ3.26
ꢀ3.14
ꢀ3.34
ꢀ3.60
ꢀ2.08
ꢀ1.96
ꢀ2.23
–
2.82 (440)
2.23 (556)
2.99 (415)
3.70 (335)
4.0 (310)
3.41 (364)
4.10 (305)
–
Th-AQI
ꢀ6.33
Nitro-Ph-AQI ꢀ7.30
[a] L–H=LUMO–HOMO. [b] S–H=SUMO–HOMO.
stronger donating substituent also illustrates the significant
ICT character. The energy gaps of these lowest-energy ab-
sorptions are comparable to the estimated energy gaps be-
tween the HOMO and LUMO (Table 2).
The ICT absorption character was further confirmed by
protonation/neutralization experiments in solutions of
DMA-Ph-AQI. The pink solution became almost colorless
upon addition of TFA accompanied with the disappearance
of the longest-wavelength absorption band, implying that
protonation prohibited ICT.[52] Upon neutralization with
TEA, the original ICT band was regenerated along with the
deep color (inset in Figure 8). Similar phenomena have been
Figure 7. XRD patterns of Th-AQI powders before and after the employ-
ment of pressure.
Characteristic ICT of D–A compounds: Based on our specu-
lation that ICT of D–A compounds may be responsible for
piezochromism, the significant ICT character of these com-
pounds was investigated by UV/Vis absorption spectroscopy
in CH2Cl2 at room temperature (Figure 8). For compounds
reported in other push–pull molecules with a N,N-dimeth
AHCTUNGTRENNUNG
ACHTUNGTRENNUNGyl-
anilino unit.[6,53,54] Deep-colored solids (orange for CH3O-
Ph-AQI, green–yellow for Th-AQI, and dark purple for
DMA-Ph-AQI) could be attributed to the clear ICT-based
absorption. No notable ICT absorption of Nitro-Ph-AQI
was observed in the visible region due to the lack of effec-
tive D–A conjugation and, as a result, the solid is light col-
ored.
Furthermore, for molecules with ICT absorptions, the ab-
sorbance of the lowest-energy bands in all solvents used
(i.e., CH2Cl2, CHCl3, DMF, toluene, cyclohexane) followed
Beer–Lambert law over a 100-fold change in concentration
(i.e., 10ꢀ3–10ꢀ5 m; see Figure S2 in the Supporting Informa-
tion). Considering the fact that the solutions were saturated
at 10ꢀ3 m and no band shift was observed, no intermolecular
interactions existed in solution.[55]
Solvent polarity is known to have a great effect on ICT
character. To further confirm the ICT nature of the lowest-
energy absorption, UV/Vis and PL spectra in solvents with
different polarity were recorded. Normalized UV/Vis ab-
sorption and fluorescence spectra of Th-AQI in different
solvents (CH2Cl2, CHCl3, and toluene) are shown in
Figure 9. Although the absorption bands show little depen-
Figure 8. UV/Vis spectra of the AQI derivatives in CH2Cl2 (2.5ꢂ10ꢀ5 m).
TFA=trifluoroacetic acid, TEA=triethylamine.
AHCTUNGTREGdNNNU ence on the solvent polarity, significant redshifts in the
emission spectra with increasing solvent polarity were ob-
served. This relatively large Stokes shift in polar solvents is
a typical emission originating from relaxed ICT states. The
Lippert–Mataga equation [Eq. (1)] is usually employed to
explain the relationship between the Stokes shift and the
solvent polarity [Eq. (2)] as follow:[56–58]
with electron-donating substituents, multiple absorption
bands appear. CH3O-Ph-AQI and Th-AQI have almost the
same absorption spectra with two bands: one centered at
about 310 nm (4.0 eV) and the other at around 420 nm
(2.95 eV). The former band can be attributed to a p–p*
transition corresponding to the transition between the
HOMO and SUMO, as indicated in Table 2, whereas the
lowest-energy bands originate from the ICT transitions.[50,51]
For DMA-Ph-AQI, these two peaks were redshifted to 340
(3.65) and 530 nm (2.34 eV), respectively. This large redshift
of the lowest-energy structureless absorption band with the
2Df
2
ð1Þ
ð2Þ
Dv ¼ vabs ꢀ vf ¼
ðe ꢀ 1Þ
ðDmÞ þ constant
hca
ðn2 ꢀ 1Þ
Df ¼
ꢀ
ð2e þ 1Þ ð2n2 þ 1Þ
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X. Wan et al.
dipole moments of the excited and ground states of CH3O-
Ph-AQI.
The fluorescence quantum yields of CH3O-Ph-AQI and
Th-AQI were measured in chloroform and toluene. With
the increasing polarities of the solvents from toluene to di-
chloromethane, the fluorescence quantum yield decreased
from 9.1 to 5.4% for Th-AQI and from 12.6% in toluene
for CH3O-Ph-AQI to completely quenched in dichlorome-
thane. These results are consistent with the expected effects
of ICT on fluorescence quantum yield, that is, the fluores-
cence efficiency should decrease with increasing strength of
ICT.[12] The completely quenched fluorescence in the case of
DMA-Ph-AQI may be attributed to nonradiative energy dis-
sipation and the highly polar excited state.[60]
By comparing the emission spectra in Figures 5 and 9, we
found that the powder emission spectrum before pressure
was similar to that of a dilute solution in nonpolar solvent
corresponding to the ICT emission. In crystals of intramolec-
ular D–A compounds, fluorescence emission could result
from intramolecular exciplexes, intermolecular exciplexes,
and intermolecular excimers due to different interaction
pathways between the D and the A.[61] However, no other
new emission band appeared in the solid state compared
with the emission in solution; this would rule out the possi-
bility that emission in the solid state originated from inter-
molecular exciplexes and excimers.
Figure 9. Normalized UV/Vis absorption and fluorescence spectra of Th-
AQI in different solvents.
in which Dv is the Stokes shift; vabs and vf are the wavenum-
bers of absorption and emission peaks, respectively; h is
Planckꢃs constant; c is the speed of light; a is the Onsager
cavity radius; and Dm ¼ me ꢀ mg is the dipole moment differ-
ence between the excited and ground states. The orienta-
tional polarizability of the solvent (Df) could be deduced
from the dielectric constant (e) and the refractive index (n)
of the solvent, according to Equation (2).
The Lippert relationship in Th-AQI between the Stokes
shift and solvent polarities is illustrated in Figure 10. As de-
duced from Equation (1), the difference in the dipole
moment between the ground and excited states could be ob-
tained from the slope of fitted Lippert plot. According to
Lippertꢃs suggestion,[59] the cavity radius for nonspherical
molecules can be approximated to 40% of their longest axis,
considering that the longest axis is estimated to be about
17.4 and 14.7 ꢀ for CH3O-Ph-AQI and Th-AQI, respective-
ly, we can conclude a slightly larger change between the
Mechanism of piezochromism: As predicted by the simula-
tion results, the series of well-designed AQI derivatives with
D–A architecture were piezochromic. From results for the
crystal structures before and after pressure, the possibility of
changes in intermolecular interactions due to shorter molec-
ular distances or crystal structures, which were considered to
be the main contributions to piezochromism, would be ruled
out. Photo-optical measurements in solution confirmed the
ICT nature of the lowest-energy absorption and redshifts of
the whole absorption spectra under pressure were illustrated
in the solid state. To further verify the fact that the solid
emission resulted from intramolecular interactions, but not
intermolecular interactions, the absorption and emission
spectra of blend films of D–A compounds dispersed in
poly(methyl methACTHNUGRTENUNGacrylate) (PMMA) were measured
(Figure 11). The concentration (0.05 wt%) was low enough
to ensure molecular dispersion.
As shown in Figure 11, the absorption spectrum of Th-
AQI in PMMA was the same as that of Th-AQI molecularly
dispersed in the nonpolar solvent toluene. As indicated by
concentration-dependence results, there were no intermolec-
ular interactions. A slight redshift of the emission spectra in
PMMA was observed similar to those in polar solvents,
which may result from the larger dielectric constant of
PMMA.
When considering the intensification and redshifting of
both the absorption and emission bands under pressure, con-
sistent with simulation results, we proposed that the piezo-
chromism of AQI derivatives resulted from the decrease of
the torsion angle between the electron-donating and -ac-
Figure 10. Plot of Stokes shift (Dn) versus the solvent polarity function
(Df) for Th-AQI. Solid lines are the fitted results.
4564
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4558 – 4567

Piezochromic Materials Based on Push–Pull Chromophores
FULL PAPER
paves the way for exploring diverse piezochromic materials.
More significant piezochromism and piezochromic lumines-
cence also could be expected with judiciously designed mo-
lecular architectures, such as branched molecules with differ-
ent D–A geometries, which are under investigation in our
lab.
Experimental Section
Materials: 4-Methoxyphenylboronic acid (98%, J&KCHEMICA), 4-ni-
trophenylboronic acid (97%, J&KCHEMICA), 4-[N,N-dimethyl-
AHCTUNGTERG(NNUN amino)]phenylboronic acid (98%, Acros), 5-methylthiophene-2-boronic
acid (98%, J&KCHEMICA), and 2-(tributylstanyl)thiophene (97%, Al-
drich) were used as purchased. Br-AQI was synthesized by following the
procedure reported previously.[65] [Pd
ACHTNUTRGNEUG(N PPh3)4] was synthesized in our lab-
Figure 11. Absorption and emission spectra of Th-AQI dispersed in
PMMA (0.05 wt%) and in toluene (2.5ꢂ10ꢀ5 m).
oratory. DMF was purified by distillation under vacuum and stored over
4 ꢀ molecular sieves prior to use. Other solvents and reagents were com-
mercially available and used without further purification unless otherwise
specified.
cepting segments, and thus, more coplanar structures, as il-
lustrated in Figure 12. The pressure-induced coplanarization
might lead to better conjugation between the D and A moi-
eties, thus intensifying and redshifting the absorption and
emission bands.[62,63] In fact, a slight quantum yield increase
of 0.2% was also found in the solid state of Th-AQI from
0.3 to 0.5%. Such intramolecular planarization, together
with synergetic J-type aggregation, was also employed to ex-
plain the enhanced emission of organic fluorescent nanopar-
ticles.[64] Therefore, the redshifted absorption and emission,
and the slightly enhanced fluorescence intensity, might be
attributed to the pressure-induced decrease of the dihedral
angle between the D and A moieties, thus leading to en-
hanced intramolecular coupling or conjugation.
Synthesis of DMA-Ph-AQI: Br-AQI (0.4745 g, 1.15 mmol), 4-[N,N-
dimethyl
ACHTUNGTRENNUNG
(0.4 g, 4.76 mmol), and [PdAHCTNUGTRENNNUG
a 50 mL three-necked round-bottomed flask fitted with a magnetic stirrer
bar and a condenser. After being degassed and refilled with nitrogen
three times, benzene (20 mL), ethanol (5 mL), and distilled water
(10 mL) were added under nitrogen. Then, the mixture was heated to
reflux for 4 h. After cooling, the reaction mixture separated into two
phases. The organic layer was isolated and the aqueous layer was extract-
ed with CH2Cl2 (3ꢂ30 mL). The combined organic layer was washed suc-
cessively with water and dried over anhydrous Na2SO4. The solvent was
removed under vacuum. The residue was purified by column chromatog-
raphy on silica gel (with CH2Cl2 as eluent) to give DMA-Ph-AQI
(0.4678 g, 90%). 1H NMR (400 MHz, CDCl3): d=8.79 (s, 2H), 8.54 (d,
J=3.5 Hz, 1H), 8.37 (d, J=8.2 Hz, 1H), 8.06 (q, J1 =8.2, J2 =1.9 Hz,
1H), 7.72 (d, J=8.8 Hz, 2H), 6.88 (br, 2H), 3.79 (t, J=7.3 Hz, 2H), 3.07
(s, 6H), 1.73–1.69 (m, 2H), 1.43–1.37 (m, 2H), 0.99 ppm (t, J=7.3 Hz,
3H); 13C NMR (100 MHz, CDCl3):
d=182.0, 181.1, 166.8, 166.7, 147.5,
138.4, 138.2, 135.9, 135.6, 133.5, 131.4,
130.3, 128.6, 128.2, 124.3, 122.4, 112.8,
40.5, 38.4, 30.5, 20.1, 13.6 ppm;
HRMS: m/z calcd for C28H25N2O4
[M+H]+: 453.1814; found: 453.1821;
Figure 12. An illustration of the proposed piezochromic mechanism.
elemental analysis calcd (%) for
C28H24N2O4: C 74.32, H 5.35, N 6.19;
found: C 74.52, H 5.45, N 6.21.
Conclusion
Synthesis of CH3O-Ph-AQI: CH3O-Ph-AQI was synthesized in 87%
yield by
a
similar method to DMA-Ph-AQI. 1H NMR (400 MHz,
Based on our previous work on piezochromic D–A mole-
cules, a series of D–A molecules containing AQI moieties
with different electron-donating or -withdrawing substitu-
ents were designed with the assistance of molecular simula-
tions and prepared to investigate the mechanism of piezo-
chromism in this type of molecule. Compounds with effec-
tive ICT interactions exhibit significant color changes under
pressure, whereas Nitro-Ph-AQI, which does not have an
ICT interaction, does not. Changes in the intermolecular in-
teractions have been proven not to contribute significantly
to piezochromism by spectroscopic and X-ray measure-
ments. Intramolecular interactions might be responsible for
piezochromism because pressure would induce a decrease in
the dihedral angle between the D and A moieties, and con-
sequently, result in better conjugation. Such a mechanism
CDCl3): d=8.80 (d, J=1.7 Hz, 2H), 8.55 (d, J=1.9 Hz, 1H), 8.41 (d, J=
8.1 Hz, 1H), 8.07 (q, J1 =8.1, J2 =1.9 Hz, 1H), 7.72 (d, J=8.8 Hz, 2H),
7.06 (d, J=8.8 Hz, 2H), 3.90 (s, 3H), 3.80 (t, J=7.3 Hz, 2H), 1.75–1.67
(m, 2H), 1.43–1.37 (m, 2H), 0.99 ppm (t, J=7.3 Hz, 3H); 13C NMR
(100 MHz, CDCl3): d=181.8, 181.2, 166.6, 160.8, 147.3, 138.3, 138.2,
136.0, 135.9, 133.6, 132.3, 131.1, 130.9, 128.6, 125.2, 122.5, 114.8, 55.5,
38.5, 30.5, 20.1, 13.6 ppm; HRMS: m/z calcd for C27H22NO5 [M+H]+:
440.1498; found: 440.1497; elemental analysis calcd (%) for C27H21NO5:
C 73.79, H 4.82, N 3.19; found: C 73.75, H 5.00, N 3.17.
Synthesis of Nitro-Ph-AQI: Nitro-Ph-AQI was synthesized in 33% yield
by a similar method to DMA-Ph-AQI. 1H NMR (400 MHz, CDCl3): d=
8.83 (d, J=2.2 Hz, 2H), 8.63 (d, J=1.7 Hz, 1H), 8.52 (d, J=8.1 Hz, 1H),
8.42 (d, J=8.7 Hz, 2H), 8.15 (q, J1 =8.1, J2 =1.7 Hz, 1H), 7.92 (d, J=
8.7 Hz, 2H), 3.80 (t, J=7.3 Hz, 2H), 1.75–1.68 (m, 2H), 1.43–1.37 (m,
2H), 0.99 ppm (t, J=7.3 Hz, 3H); HRMS: m/z calcd for C26H19N2O6
[M+H]+: 455.1243; found: 455.1238; elemental analysis calcd (%) for
C26H18N2O6: C 68.72, H 3.99, N 6.16; found: C 68.79, H 4.26, N 6.05.
Chem. Eur. J. 2012, 18, 4558 – 4567
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4565

X. Wan et al.
Synthesis of Th-AQI: Diethyl 6-bromo-9,10-dioxo-9,10-dihydroanthra-
cene-2,3-dicarboxylate (0.6468 g, 1.5 mmol) and [Pd(PPh3)4] (170 mg,
Acknowledgements
AHCTUNGTRENNUNG
0.153 mmol) were added to a 100 mL three-necked round-bottomed flask
fitted with a magnetic stirrer bar and a condenser. After being degassed
and refilled with nitrogen three times, anhydrous DMF (15 mL) and 2-
(tributylstanyl)thiophene (0.6 g, 1.6 mmol) were added under nitrogen.
The mixture was stirred at 808C for 24 h. After the removal of DMF
under vacuum, the resulting solid was purified by column chromatogra-
phy on silica gel (with CH2Cl2 as eluent) in a yield of about 92%.
We thank the financial support of the National Natural Science Founda-
tion of China (no. 20834001) and the Research Fund for Doctoral Pro-
gram of Higher Education of MOE of China (no. 20060001029). Helpful
discussions with Prof. H. Jiang are greatly appreciated.
This solid (0.6 g), together with KOH (4 g) and a mixture of ethanol and
water (60 mL, v/v: 1/2), were added to a 250 mL flask and the mixture
was heated to reflux overnight. The mixture was acidified with dilute hy-
drochloric acid solution to pHꢁ 2. The solid that precipitated was fil-
tered, washed with water, and dried to afford the hydrolysate (0.5 g,
95%).
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1
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[67] CCDC-867438 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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Received: September 18, 2011
Published online: March 5, 2012
Chem. Eur. J. 2012, 18, 4558 – 4567
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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