Stress acidulated amphoteric molecules and mechanochromism via reversible intermolecular proton transfer

Article abstract of DOI:10.1039/c3cc42747a

Stress has been proved to acidulate amphoteric molecules and promote an intermolecular proton transfer, which results in a significant absorption and emission change. The stress acidulated amphoteric molecules open a new avenue for developing mechanochromic materials and anticipate many broad applications such as stress/pressure sensors and rewritable media.

Full text of DOI:10.1039/c3cc42747a

Chemical Communications
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Volume 49 | Number 59 | 28 July 2013 | Pages 6567–6690
ISSN 1359-7345
COMMUNICATION
Minjie Li, Sean Xiao-An Zhang et al.
Stress acidulated amphoteric molecules and mechanochromism via reversible
intermolecular proton transfer

ChemComm
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Stress acidulated amphoteric molecules and
mechanochromism via reversible intermolecular proton
transfer†‡
Yi Wang,a Minjie Li,*a Yumo Zhang,a Jin Yang,b Shaoyin Zhu,a Lan Sheng,a
Xudong Wang,a Bing Yanga and Sean Xiao-An Zhang*a
Cite this: Chem. Commun., 2013,
49, 6587
Received 15th April 2013,
Accepted 9th May 2013
DOI: 10.1039/c3cc42747a
www.rsc.org/chemcomm
Stress has been proved to acidulate amphoteric molecules and promote
an intermolecular proton transfer, which results in a significant absorp-
tion and emission change. The stress acidulated amphoteric molecules
open a new avenue for developing mechanochromic materials and
anticipate many broad applications such as stress/pressure sensors and
rewritable media.
Scheme 1 Chemical structures of twisted and conjugated amphoteric mole-
cules of AM, p-nitro-AM, o-nitro-AM and di-nitro-AM.
Stimuli-responsive intelligent materials with tunable colour or
fluorescence have attracted much attention due to their potential
To verify how significantly the acidity would be manipulated by
applications in sensors, fluorescent switches and optical recording the stress-induced molecular conformational change, four twisted
devices.1 However, compared with photo-, thermo- and electro- and conjugated amphoteric compounds, AM, p-nitro-AM, o-nitro-AM
chromic materials, the mechanochromic materials, which show and di-nitro-AM (Scheme 1), have been designed to investigate
colour changes upon grinding, compressing, shearing, and applica- whether the proton transfer could happen under external stress.
tion of other external mechanical forces, are still in their infancy.2 For AM, p-nitro-AM and o-nitro-AM, they all exist in neutral forms in
So far, most mechanochromic materials undergo conformational crystals with bright yellow colour (Fig. 1a, Fig. S4 and S5, ESI‡).
and (or) aggregation state changes upon application of external However, when the acidic group is 2,4-dinitrophenol, di-nitro-AM
mechanical forces.3 Moreover, the design strategies and in-depth takes a zwitterionic form with deep red colour (Fig. S6, ESI‡). X-ray
understanding of their mechanochromic mechanisms are still analysis of AM, p-nitro-AM and o-nitro-AM indicates that they all
inadequate.4 Herein, we propose to extend the effect of external have a twisted and bent conformation with dihedral angles between
forces on the molecular conformation to molecular reactivity to the phenolic plane and the indole plane approximately at 27.31, 18.11
explore reaction-dependent reversible mechanochromic materials. and 18.51, respectively (Fig. S3, ESI‡). This contorted conformation
Based on this concept, we envision that external stress can further will sharply disturb the molecular conjugation between the two
influence molecular acidity by changing the molecular conforma- moieties and make the electron withdrawing NQC motif thus
tion, especially coplanarization.5 And such a molecular acidity contribute less to the acidity of the phenolic moiety. The natural
change might finally induce a solid-state acid–base reaction that bond orbital (NBO) charge analysis further confirms that molecular
does not happen under ambient conditions, which makes the acidity can be enhanced significantly during a twisted-to-planar
traditional acid–base reaction mechanically responsive and opens conformational change for three molecules of AM, p-nitro-AM and
a new avenue for developing mechanochromic materials by means o-nitro-AM (Table S1, ESI‡). Thus, a proton transfer could be
of alternating molecular structures.
expected for these twisted and conjugated amphoteric molecules
upon external stress. And if such a proton transfer takes place, a
drastic bathochromic shift in UV-Vis spectra (around 100 nm)
resulting from the formation of zwitterionic salts will facilitate in
confirming the acidic enhancement (Fig. S1, ESI‡).
When subjected to external stress, these compounds exhibited
contrasting responses. For AM, o-nitro-AM and di-nitro-AM, the
absorption and emission remained unchanged after grinding their
solid powders in a mortar (Fig. S4 to S6, ESI‡), suggesting that the
expected proton transfer did not take place upon grinding. But for
p-nitro-AM, itsyellow colourturnedred immediately, withamaximum
a State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, 2699 Qianjin Street, Changchun, Jilin 130012,
P.R. China. E-mail: liminjie@jlu.edu.cn, seanzhang@jlu.edu.cn
b Key Lab of Polyoxometalate Science, Department of Chemistry, Northeast Normal
University, Changchun, Jilin 130024, P.R. China
† This article is part of the ChemComm ‘Mechanochemistry’ web themed issue.
‡ Electronic supplementary information (ESI) available: Detailed experimental
procedures and characterization data for the four amphoteric compounds. CCDC
895281–895283. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3cc42747a
c
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Fig. 1 (a) Optical microscopic images and molecular structures of p-nitro-AM in
neutral and zwitterionic isomers, (b) Kubelka–Munk diffuse reflectance absorption
spectra, (c) emission spectra and (d) a spray coating of p-nitro-AM on a paper, Chinese
characters of ‘‘chemistry’’ (up), letters ‘‘CHEMISTRY’’ (down) and two patterns (left and
right) were imprinted with stress only on the coated paper, and subsequently were
erased with solvents (all pictures were taken under ambient light).
Fig. 2 (a) N 1s XPS spectra, (b) IR spectra, (c) DSC curves and (d) XRD patterns of
p-nitro-AM with various treatments.
absorption shift from B450 nm to B550 nm (Fig. 1a and b), which is CQNH+ (2640 to 2250 cmÀ1 10
as well as the stronger stretching
)
consistent with the simulated UV-Vis spectral change for its neutral-to- absorption in 1200–1700 cmÀ1 for the ground p-nitro-AM were
zwitterionic transition (Fig. S1, ESI‡). And a new emission band consistent with the calculated IR spectral changes during the
centred at 652 nm was also observed (Fig. 1a and c). Moreover, the neutral-to-zwitterionic transition (Fig. S2, ESI‡), clearly indicating
newly appeared absorption and emission bands resembles well with the formation of the zwitterionic isomer. Based on the solid evidence
the absorption (lmax = 537 nm) and emission (lmax = 645 nm) spectra in hand, we concluded that proton transfer had occurred upon
of 6-N-BIPS,6 N-methylated derivative of p-nitro-AM, indicating that grinding. Pressing around 10 MPa induced an obvious proton
the zwitterionic isomer was generated (Fig. S7, ESI‡). Interestingly, the transfer for p-nitro-AM in solid tablets. And the higher the pressure,
red colour of the ground sample reverted to the original yellow the higher the transition ratio (Fig. S9a, ESI‡).
immediately upon wetting with ethanol, along with the concomitant
The red zwitterionic isomer was metastable, and it tended to
disappearance of red emission (Fig. 1b and c). Other solvents such as revert to a more stable neutral form at ambient pressure (Fig. S2,
methanol, ethyl acetate and dichloromethane, as well as heat treat- ESI‡). The reversion should at least undergo planar-to-twisted con-
ment, can also trigger this reversion. In order to demonstrate the formational conversion and zwitterionic-to-neutral chemical transi-
mechanochromic behaviour of p-nitro-AM more clearly, a solid thin tion. A complex DSC curve at 35 1C–140 1C for ground p-nitro-AM
film of p-nitro-AM was coated on a paper, which was yellow under further confirmed that a chain of conformational, chemical and
ambient light. Then the red Chinese characters of ‘‘chemistry’’, letters molecular packing transitions were involved in the recovery process.
‘‘CHEMISTRY’’ and two patterns were imprinted with stress only by DSC data correlated well with these transitions, which corresponded
different templates, and subsequently they could be erased with to the weak endothermic peak at around 50 1C, the first broad
solvents in several seconds (Fig. 1d). The excellent reversibility and exothermic peak at around 100 1C and the second exothermic peak
significant colour change under ambient light ensure various at around 124 1C, respectively. And the endothermic peak at 210 1C
potential applications of p-nitro-AM in optical recording, rewritable and the exothermic peak at 215 1C corresponded to the melting
media and mechanical sensing.
point and decomposition point, respectively (Fig. 2c).
The zwitterionic structure for ground p-nitro-AM was further
From XRD spectra, it seemed that proton transfer was
confirmed by X-ray photoelectron spectroscopy (XPS) and IR spectra related to
a crystal-to-amorphous phase transformation
analyses. For unground p-nitro-AM, only a single N 1s binding energy (Fig. 2d). In fact, the neutral and zwitterionic isomers were
(BE) peak at 398.7 eV for the neutral indole was observed.7 In con- not the inherent properties of crystal and amorphous states,
trast, the ground sample exhibited an additional peak at 401.3 eV, because the amorphous p-nitro-AM from the freeze-dryer was
which correlated well with the protonated imine group,8 suggesting still yellow in colour and in neutral form (Fig. S8, ESI‡). This is
the formation of indolium due to proton transfer from phenol to significantly different from most mechanochromic materials, whose
the nitrogen atom in indole.9 This peak disappeared upon wetting mechanochromism generally shows dependence on their phase
with ethanol (Fig. 2a). This result was further supported by IR analysis transformations.11 The independence of mechanochromism on
(Fig. 2b). Compared with the unground sample, both the decrease of phase transformation for p-nitro-AM will make these twisted and
O–H stretching absorption in phenol (3265 cmÀ1, 1365 cmÀ1 and conjugated amphoteric molecules more practical for developing
1252 cmÀ1) and the appearance of N–H stretching absorption in mechanochromic materials.
c
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phenol and the neighboring nitro as well as a relatively long distance
of the intermolecular O–HÁÁÁN make the intermolecular proton
transfer impossible under such limited acidic enhancement by
grinding. In contrast, p-nitro-AM exhibits a desired stress triggered
intermolecular proton transfer property, which can be summarized
as a result of the relatively shorter intermolecular hydrogen bond
length, the absence of intramolecular neighboring O–HÁÁÁN restric-
tion, and an appropriate acidity–basicity matching.
In summary, stress triggered molecular acidulation and
mechanochromism via proton transfer were observed for the
first time in twisted and conjugated amphoteric molecules. Both
experimental data and theoretical calculations support strongly
that external mechanical forces do manipulate molecular acidity
in the solid state. In addition to the acidity–basicity matching,
the hydrogen bonding distance and neighboring restriction also
play an important role in the intermolecular proton transfer.
This newly demonstrated important concept of stress acidula-
tion and promoting proton transfer not only opens a new avenue
for mechanochromic materials, but also inspires and accelerates
further development of stress/pressure sensors, rewritable media
and stress sensitive artificial nervous system, etc.
Fig. 3 Partial view of the crystal packing of AM, p-nitro-AM and o-nitro-AM.
The hydrogen bonds are indicated by the dashed lines (red: oxygen atom, blue:
nitrogen atom, white: hydrogen atom).
The proton transfer is believed to take place in the following
manner. First of all, the electron withdrawing indole moiety enhanced
the acidity of the intramolecular phenolic moiety due to a more planar
conformation under external stress, which provides the possibility for
phenol to give the proton to the nitrogen atom in the indole moiety.
Secondly, the strong multiple intermolecular hydrogen bonds
O–HÁÁÁN [HÁÁÁN = 1.869 Å] (Fig. 3) between the phenol moiety of
one molecule and the indole moiety of another provided the channels
for proton transfer. Finally, the hydrogen bond length in O–HÁÁÁNwas
shortened under external stress and it eventually transformed to
OÀÁÁÁH–N+ simultaneously, resulting in the zwitterionic form.
This work was supported by the National Science Foundation
of China (Grant No. 21072025). The authors also acknowledge Dr
Shubin Zhao for helpful discussion relating to this project.
The neutral-to-zwitterionic transition ratio was dependent on the
pressure (Fig. S9a, ESI‡) and grinding time (Fig. S9b, ESI‡). Take
hand grinding for example, one hour of grinding was enough to
induce maximum transition for 0.04 g of p-nitro-AM. The transition
ratio for this sample was estimated based on N 1s XPS spectra and
in situ fluorescence spectra. The results show that the neutral-to-
zwitterionic transition ratio was approximately 9.3% with hand
grinding (Fig. S10, ESI‡). Although the conversion was less than
10%, it showed a clear colour change from yellow to red (Fig. 1a).
Overall, we clearly proved that stress can enhance molecular
acidity by forcing molecules to be more planar, but the enhancement
is relatively weak with current concept proving molecules. Among
the four amphoteric compounds, only p-nitro-AM exhibits an inter-
molecular proton transfer property upon stress. The primary factor
for the proton transfer is acidity–basicity matching between the
amphoteric functional groups. For di-nitro-AM, two strong electron-
withdrawing nitro groups significantly increase the acidity (pKa =
3.94) of phenolic groups,12 thus resulting in a stable zwitterionic
isomer. Owing to the absence of the electron-withdrawing nitro
group in its acidic moiety, AM has a relatively weak acidity (pKa =
10.59, Fig. S11, ESI‡). This is not strong enough to enable the proton
transfer even under external stress, regardless of numerous inter-
molecular strong hydrogen bonds O–HÁÁÁN [HÁÁÁN = 1.983 Å] as
proton transfer channels (Fig. 3). Comparing o-nitro-AM with p-nitro-
AM, the acidity difference is not remarkable, because the pKa and
pKb of o-nitro-AM and p-nitro-AM are very close (Fig. S12 and S13,
ESI‡). The dihedral angles between the phenolic plane and the
indole plane are also close at 18.51 and 18.11 (Fig. S3, ESI‡),
respectively, which means that the effect of external stress on the
molecular conformation should be similar. The significant differ-
ence lies in hydrogen bonds. In o-nitro-AM, there are two different
hydrogen bonds, one is the intramolecular hydrogen bond O–HÁÁÁN
[HÁÁÁN = 1.946 Å] associated to the hydroxyl and neighboring nitro
group, the other is the intermolecular hydrogen bond O–HÁÁÁN
[HÁÁÁN = 2.430 Å]. The intramolecular O–HÁÁÁN restriction between
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