A dipeptide-based multicolored-switching luminescent solid material: When molecular assemblies meet mechanochemical reaction

Article abstract of DOI:10.1002/anie.201200320

Color schemes: A mechanochromic material composed of two types of peptides bearing a pyrene group and rhodamine B moieties, respectively, displays multiluminescent colors, such as blue, green, and reddish in one sample (see picture). The mechanochromic behavior is based on a combined switching of molecular assemblies and chemical structure. Copyright

Full text of DOI:10.1002/anie.201200320

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DOI: 10.1002/anie.201200320
Mechanochromic Luminescence
A Dipeptide-Based Multicolored-Switching Luminescent Solid
Material: When Molecular Assemblies Meet Mechanochemical
Reaction**
Ming-Jun Teng, Xin-Ru Jia,* Xiao-Fang Chen, and Yen Wei
Tuning the fluorescence properties of solid-state materials
reversibly and dynamically has attracted much attention for
their potential applications in displays, sensors, fluorescent
probes, and photoelectronic devices.[1] The newly emerged
mechanochromic luminescent (ML) materials showing rever-
sible luminescent color switching upon mechanical stimulus,
termed piezochromism, are particularly suited to this pur-
pose. As has been reported, one promising approach to afford
the materials with reversible luminescent color transition in
situ is to control the microstructures of the molecular
assemblies by force.[2] Another is changing the chemical
structures by mechanical stimuli, which has the advantages of
good color stability and significant color transformation.[3] Up
to now, ML materials displaying two-colored switching have
been achieved due to their specific mechanistic models, but
very few examples show mechanically induced multicolored
transition. Recently, an excellent example of a tricolored
mechanochromic liquid crystal was reported by Kato et al., in
which reversible color switching was accompanied by differ-
ent molecular packing architectures.[4] However, the develop-
ment of a simple and effective strategy to prepare multi-
colored-emission ML materials, especially in the solid state,
remains a huge challenge but facilitates advances because it
not only possesses significance in fundamental research but
also can create new applications of luminescent materials.
We initiated research to address the versatile stimuli-
responsive properties of polypeptide molecules and discov-
ered their polymorphic self-assembly properties.[5] Motivated
by our previous findings that polychromic emission colors
could be achieved by introducing functional moieties to
polypeptide, we further co-assembled dipeptide 1 with
a pyrene group and 2 with lactam rhodamine B (Rh-B)
moieties to obtain their co-assemblies (hereinafter referred to
as MC-1/2). We expected that MC-1/2 would be endowed with
the characteristics of the moieties and would show multi-
luminescent colors switching from blue to green and a reddish
color by applying a mechanical stimulus. Compared to one-
component materials with only two-color transitions,[5b] the
co-assemblies displayed unique multicolored switching from
blue to green and reddish.
Dipeptide 1, bearing a pyrene moiety, displayed reversible
mechanochromic behavior between blue (lem = 410 nm) and
green (lem = 480 nm) due to the formation of different pyrene
excimers in the crystalline and amorphous states.[5b] Surpris-
ingly, we unexpectedly discovered that the dipeptide 2 can act
as a mechanical light switch in the solid state. The as-prepared
powder 2 showed low emission intensity. After shearing
heavily with a spatula, a bright red luminescence at around
580 nm developed (Figure S1a in the Supporting Informa-
tion), which was assigned to the emission of Rh-B opening
isomer with a more conjugated structure.[6] The fluorescence
intensity contrast between sheared and as-prepared samples
was larger than 10. A combination of optical measurements
was conducted to confirm that the generation of reddish
luminescence originated from the ring-opening reaction
rather than the change of Rh-B stacking. Firstly, comparison
between the ground samples in diluted solution and in the
solid state reveals a similar red luminescence (Figure S2a in
the Supporting Information). Secondly, the UV/Vis spectrum
exhibits the characteristic absorption peak of Rh-B ring-
opening isomer at around 550 nm (Figure S2b in the Support-
ing Information). It is worth mentioning that the sample can
[*] M. J. Teng, Prof. X. R. Jia, Dr. X. F. Chen
Beijing National Laboratory for Molecular Sciences, Key Laboratory
of Polymer Chemistry and Physics of the Ministry of Education,
College of Chemistry and Molecular Engineering, Peking University
Beijing 100871 (China)
E-mail: xrjia@pku.edu.cn
Prof. Y. Wei
Department of Chemistry, Tsinghua University
Beijing 100084 (China)
[**] This work was financially supported by the National Natural Science
Foundation of China (21174005) to X.-R.J. and partially supported
by the National Basic Research Program of China (973 program,
No. 2011CB933300). We thank Prof. Wei Wang for useful discus-
sions, Dr. Guichao Kuang for the kind help in manuscript revision,
and Hai He, Ting Lei, and Yun Liu for suggestions and help in the
experiments.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200320. It includes the
synthesis procedure, preparation of MC-1/2, SEM images, mecha-
nochromic behavior of 2, and DSC profiles.
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recover to the original state by heating. The ring opening–
closing transition upon grinding and heating could be
repeated for multiple cycles (Figure S1b in the Supporting
Information). Although the Rh-B moiety is well known for its
sensitivity to pH and cations,[6,7] it is rarely reported to be
mechanoresponsive.
this way, we can readily develop three different colors in one
paper.
In the fluorescence spectra (Figure 1e), the original blue
emission band centered at 410 nm indicates a typical feature
of the partially overlapped excimer of pyrene groups. Upon
shearing, the emission band red-shifts to 480 nm for the green
aggregates, which is ascribed to another pyrene excimer with
sandwich packing.[5b] Notably, besides the emission band at
480 nm, a new peak at around 580 nm was detected for the
reddish powder. This new band rationally results from the
ring-opening form of Rh-B moieties. As shown in Figure 1 f,
the intensity of the emission band at around 580 nm (I580) and
the ratio I580/I480 increased with further force perturbation,
which accounted for the continuous luminescence transition
from green to the reddish color. The tunable emission
property of MC-1/2 in the visible-light range is expected to
expand the utility of the materials in various applications (see
below).
The microstructure of co-aggregates was investigated in
detail by scanning electron microscopy (SEM), X-ray dif-
fraction (XRD), and differential scanning calorimetry (DSC).
Compared to the smooth surface of microwires from free
1 prepared by similar procedures from cyclohexane (Fig-
ure 2b), the compound deposited on the rough surface of
MC-1/2 microwires (ca. several hundred nanometers in width,
tens of micrometers in length) might be from 2, as shown by
Taking advantage of the complementary optical proper-
ties and different mechanochromic behaviors of 1 and 2, we
developed a new ML material by incorporating dipeptide 2
into the microstructure of 1 through a self-assembly process.
In a representative procedure, 1 was dissolved in chloroform
followed by addition of 2 powder. The MC-1/2 co-assemblies,
which displayed multicolored ML properties, were obtained
by dilution of the stock solution in excess cyclohexane. The
aggregates initially displayed a blue color. After force
perturbation, such as slight shearing with a spatula, the
color transition from blue to distinct green occurred. Inter-
estingly, the green emission switched to a new reddish color
with further mechanical action (Figure 1a). To show this
switching more clearly, we distributed the sample smoothly on
a piece of filter paper, and it showed color transitions from
blue to green then to reddish upon shearing (Figure 1b–d). In
Figure 1. a) MC-1/2 aggregates showing different emission colors
under force perturbation (irradiated by 365 nm UV light) and their
color restoration by heating. Blue: as-prepared aggregates without
force perturbation; green: slight shearing of the aggregates with
a spatula; reddish: further shearing of the green aggregates.
b–d) Fluorescence images of b) MC-1/2 on a filter paper with blue
luminescence; c) force-induced green luminescence pattern; d) force-
induced reddish luminescence pattern and three different emission
colors in one paper. e,f) Fluorescence (FL) spectra of e) the samples
with blue, green, and reddish emission colors and the sheared sample
after annealing, and f) the tunable fluorescence property under force
perturbation.
Figure 2. SEM images of a) MC-1/2 blue aggregates obtained from
cyclohexane; b) free 1 aggregates; c) MC-1/2 green aggregates, and
d) MC-1/2 reddish aggregates. e) XRD patterns of the free 1 and MC-
1/2 aggregates with different colors. f) IR spectra of the MC-1/2
aggregates with different colors (the absorption is normalized to the
C–H stretching band at 2927 cmÀ1).
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the SEM images (see Figure 2a). This assumption is firstly
evidenced by the smooth surface being restored after washing
the MC-1/2 microwires with methanol (Figure S3b in the
Supporting Information). Further evidence is provided by the
following three facts. 1) No reddish luminescence is observed
under an external force for the microwires after washing.
2) Similar XRD patterns of free 1 and MC-1/2 were detected.
Free 2 powder shows broad diffraction peaks relative to those
of free 1 aggregates, thus indicating that 2 is in an amorphous
state (Figure S5 in the Supporting Information). The com-
parable XRD patterns of the co-assembled sample MC-1/2
and free 1 aggregates (Figure 2e) reveal that the internal
packing style of 1 is not significantly affected after introducing
2. 3) Similar phase transition temperatures between 1 and
MC-1/2 imply microphase separation between 1 and 2 in the
co-assemblies of MC-1/2 (Figure S4 in the Supporting Infor-
mation). The procedure for the preparation of MC-1/2
guarantees that the ML behavior of each component can be
retained and works without interference. The superiority of
this preparation method is clearly revealed by the fact that the
emission switching is facile and effective, whereas simply
mixing 1 and 2 powders shows poor emission manipulation
(Figure S6 in the Supporting Information). Noncovalent
interactions, such as hydrogen bonding between the dipeptide
backbones, may be responsible for the co-aggregation.[8]
Based on the above results and analysis, the multicolored
transition may derive from the change of self-assembled
microstructures of 1 and the spiro-ring-opening reaction of 2
upon application of an external force. The initial force
perturbation may lead to disruption of the 1 crystalline
microstructure, which accounts for the color switching from
blue to green. The triggering of the ring-opening reaction of 2,
which induces the reddish color, does not occur until deeper
damage of the co-assemblies is observed.
The multicolored luminescence switching is reversible.
After annealing the sheared sample at 1208C for 15 min, the
sample restores its original blue luminescence from different
grinding states (Figure 1a). Recovery of the blue color was
reconfirmed by fluorescence measurement. After thermal
treatment, the emission band at 410 nm became dominant,
accompanied by the disappearance of the emission bands at
480 and 580 nm (Figure 1e). Two factors may be responsible
for the color recovery: 1) the crystallinity of 1 is improved by
annealing, which facilitates the recovery of the blue emission
at 410 nm;[5b] and 2) spirocyclic Rh-B will be re-formed from
the opening isomer after heating,[9] thus leading to the
disappearance of the reddish luminescence.
As a result of the wide coverage of the emission spectra
and facile tuning of each emission component, such as the
emission intensity at 580 nm, it is possible to control the
fluorescence of MC-1/2 to offer different intermediate colors
between green and reddish through carefully tuning the ratio
I580/I480 by shearing. The related study is ongoing. In addition,
the direction to quantitatively elucidate the mechanochromic
behavior is important yet challenging, and is in progress in our
laboratory.
In summary, we have prepared a multicolored mechano-
chromic material by taking advantage of two distinct modu-
lation approaches to bridge the supramolecular structures and
mechanochemical reaction. Correspondingly, the multicol-
ored transitions involve two different kinds of luminescence
change: 1) the emission wavelength shift from blue to green,
and 2) the unusual switching of emission intensity at long
wavelength (nonluminescence to reddish luminescence). To
the best of our knowledge, this is the first report to correlate
the luminescence property with molecular assembly and
mechanochemical reaction to afford novel multicolored ML
switching. Such a binary system not only displays unique
advantages in the multiple outputs, but also in the facile
preparation method by exploiting already available dyes,
which avoids tedious molecular synthesis and is of great
importance in practical applications.
SEM, XRD, and IR measurements were applied to gain
insight into the mechanical effect on the materials. SEM
images provide directly visualized evidence for the morphol-
ogy transformation from the wirelike ordered microstructure
(blue emission) to the blurred solid (green emission), and
further amorphous powder with random distribution (reddish
emission; Figure 2a,c,d). The stepwise morphology trans-
formation of MC-1/2 was echoed by XRD measurements
(Figure 2e). For the blue aggregates, the well-resolved
diffraction peaks indicated a good crystalline lattice. After
slight grinding, the disappearance of several diffraction peaks
at the low-angle region represented damage of the self-
assembled architecture. Further grinding resulted in the
decreasing of the corresponding diffraction peaks, thus
demonstrating more significant damage of the materials.
More detailed information is provided by IR spectroscopy
Received: January 12, 2012
Published online: May 15, 2012
Keywords: color transition · luminescence ·
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mechanochromic fluorescence · peptides · self-assembly
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