Mechanochromic luminescent property of a polypeptide-based dendron

Article abstract of DOI:10.1039/c1cc10873e

We report herein the mechanochromic luminescent property of a dendritic polypeptide with a fluorescent aromatic moiety at the focal point. The different luminescent property of 1 under mechanical stimulus is attributed to the switch of self-assembled stru

Full text of DOI:10.1039/c1cc10873e

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Mechanochromic luminescent property of a polypeptide-based dendronw
Mingjun Teng,a Xinru Jia,*a Xiaofang Chen,a Zhiyong Maa and Yen Weib
Received 15th February 2011, Accepted 11th April 2011
DOI: 10.1039/c1cc10873e
We report herein the mechanochromic luminescent property of a
dendritic polypeptide with a fluorescent aromatic moiety at the
focal point. The different luminescent property of 1 under
mechanical stimulus is attributed to the switch of self-assembled
structures. Moreover, the photoluminescence property of 1 also
depends on the thermal history.
both a reversible mechanochromic and a thermal history
dependent luminescent property. The reversible mechanochromic
property of 1 arises from the switchable self-assemblies
between hexagonal packing and lamellar arrangement, thus
leading to the different color emission. The mechanism of the
conversion of fluorescent colors is based on the excimer-to-
excimer transition, which differs greatly from the previously
reported monomer-to-excimer mechanism.5d,6a,b,e Interestingly,
the emission property of 1 also relies on the thermal history.
To the best of our knowledge, this is the first report of
mechanochromic luminescent materials constructed from
polypeptide-based dendrons, and it refers to a conceptually
new mechanism of excimer-to-excimer transition.
Optical properties of solid materials are affected greatly by the
molecular packing patterns because the photophysical process
is associated with the distance, arrangements and interactions
of fluorophores.1,2 Mechanochromic luminescent materials
show photoluminescence and color change due to the altered
self-assembled structures under external forces such as grind-
ing and shearing, and are generally returned to the original
color after treatment with solvent or by heating.3 The easily
controlled luminescent property and the reversible switching
feature of such materials make them promising candidates for
applications in smart stimuli-responsive materials. Up to
now, limited examples with a mechanochromic luminescent
property have been reported.4,5 However, molecular assemblies
showing mechano-responsive luminescence are still rare, and
clarification of the mechanism to direct the molecular design
also remains a challenge.6
In our previous studies,7 we found the dendrons based on
polypeptide structure from amino acids, such as glycine and
aspartic acid (Gly-Asp), displayed versatile self-assembly
behaviors with different packing modes only on changing
the focal groups. The feasible modulation of self-assembled
structure from Gly-Asp dendritic branches inspired us to
design a molecule with simple structure but facile assembly
property to show a novel mechanochromic luminescence
property. Our effective molecular designing strategy here is
to combine a dendritic polypeptide and a luminophore (pyrene)
within a same molecule so as to fabricate a molecular assembly
showing a mechanochromic luminescence property.
Dendron 1 was synthesized by using glycine and aspartic
acid as the building blocks and was further modified with a
pyrenyl group at the focal point. 1 exhibited strong violet
luminescence in a solution of CH2Cl2 (2 Â 10À5 M). A
suspension was obtained by increasing the concentration of
1. When the suspension was drop-casted on a mortar or quartz
glass plate, the precipitates exhibited blue emission. It was
exciting that the blue colors changed to bright green after
grinding with a spatula or simply shearing the sample. The
bright green color could be reverted to the original blue
luminescence after treating the sample with CH2Cl2 and
drying (Fig. 1). The emission of a ground sample was also
changed by treating it with other solvents, such as methanol,
THF and chloroform.
To elucidate the mechanochromic property of 1, detailed
spectroscopy measurements were performed. The maximum
absorption band of solution, drop cast sample and ground
solid (Fig. S4) appeared at similar positions (around 345 nm),
suggesting that the electronic intermolecular interaction in the
ground state is negligible. Their luminescent behaviors varied
with each other as shown in Fig. 2a. The solution of 1 in
CH2Cl2 exhibited a violet emission band at 390 nm with a
The designed Gly-Asp-based dendron 1 (Scheme 1) with
pyrenyl group as the focal luminophore successfully displays
a Beijing National Laboratory for Molecular Sciences, and Key
Laboratory of Polymer Chemistry and Physics of the Ministry of
Education, College of Chemistry and Molecular Engineering,
Peking University, Beijing, China. E-mail: xrjia@pku.edu.cn;
Fax: +86-010-62751708
b Department of Chemistry, Tsinghua University, Beijing, China
w Electronic supplementary information (ESI) available: Synthesis of
1, fluorescence decay spectra, UV-vis spectra, optimized geometry of
1, thermal analysis profiles, SAXS patterns of 1 in different states. See
DOI: 10.1039/c1cc10873e
Scheme 1 Molecular structure of dendron 1.
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6078 Chem. Commun., 2011, 47, 6078–6080
This journal is The Royal Society of Chemistry 2011

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scattering profile of the as-prepared aggregates displayed
several major diffraction peaks at the low angle, whose
scattering vector ratio was 1 : O3 : 2 : O7 that could be ascribed
to (100), (110), (200), (210) reflection of a hexagonal columnar
packing. The calculated column diameter is 2.69 nm, which
corresponds to the optimized geometry of 1 with the molecular
length of 2.47 nm (Fig. S5). In such an assembled structure,
most of the pyrenyl moieties are supposed to partly overlap
with each other (Fig. S6), which is characterized by the blue
emission. However, the finely ground solid displayed a distinct
scattering profile with the scattering vector ratio of 1 : 2 : 3,
clearly verifying the transition from a hexagonal columnar
pattern to a lamellar packing with a d spacing of 2.68 nm,
approximately the calculated molecular size of 1. The focal
pyrenyl groups may mainly adopt a sandwich packing mode in
the layered nanostructure, leading to bright green emission
(Fig. S7). Noteworthily, after recrystallization from CH2Cl2,
the ground solid displayed a nearly identical diffraction
pattern with that of original drop cast aggregates emitting
blue luminescence (lem = 420 nm), demonstrating the restoration
of the molecular stacking (Fig. S8). Furthermore, elemental
analysis of the drop cast aggregates, ground solid and solvent-
treated sample showed the expected value with calculated data
on the basis of formula 1, excluding the solvent typically
involved in the solvatochromism system. Therefore, it is
rational that the different color emission of the sample under
external stimuli is induced by the change of self-assembled
structures, in which the focal pyrenyl moieties adopted a
different packing mode.
Fig. 1 (a) Drop cast aggregates of 1 under ambient light; (b) the same
sample under illumination of 365 nm; (c) ground sample; (d) reversion
to blue emission after simple CH2Cl2 treatment by adding solvent to
the sample; (e) further scratching the solvent-treated sample at the
center; (f) scratching the solvent-treated sample on the periphery.
Fig. 2 (a) Fluorescence spectra of 1 in various states: CH2Cl2
solution (black; 2 Â 10À5 M), drop cast aggregates (red), ground solid
(blue), (lex = 345 nm). (b) SAXS patterns of 1 in different states: drop
cast aggregates (red) and ground solid (black).
shoulder peak at 403 nm (F = 0.76, t = 11.2 ns). For the
drop-cast precipitates, the luminescence band red-shifted
to 420 nm with the disappearance of the shoulder peak
[F = 0.18, t1 = 3.0 ns (A1 = 0.50), t2 = 8.4 ns (A2 = 0.50)].
Surprisingly, on grinding the sample with a spatula, a
dramatically red-shifted luminescence band at 470 nm was
observed, leading to a sample with bright green luminescence
color and an enhanced emission quantum yield [F = 0.31,
t1 = 8.5 ns (A1 = 0.23), t2 = 33.4 ns (A2 = 0.77)]. The
dynamic recovery of emission colors between bright green and
blue was evident upon multiple cycles of grinding and treating
the sample with solvent. The emission band at 390 nm is
assigned to the monomeric emission of the pyrenyl moiety. In
contrast, the two broader and less structured emission bands
at longer wavelength (420 nm and 470 nm, respectively) are
suggestive of two different excimer-like components of pyrenyl
groups.8 As demonstrated by the previous papers,8c,d the
excimer at 420 nm (E1) is less stable with a partially over-
lapped packing and exhibits a short lifetime. On the other
hand, excimer at 470 nm (E2) in a sandwich packing is more
stable due to the enhancement of p–p interaction and displays
a long lifetime. Our results of the two excimers with different
lifetimes agreed with the above reported examples. It is
possible that the prohibition of the nonradiative decay process
in the stable E2 excimer (in ground solid) may lead to the
increase of lifetime and quantum yield when comparing with
the less stable E1 excimer (in drop cast sample). The distinct
fluorescent properties of ground solid and drop cast sample
may be related to the different molecular packing styles.
Small angle X-ray scattering (SAXS) measurement was
applied to obtain detailed information of self-assembled structures
in different forms of the solid. As illustrated in Fig. 2b, the
Fourier transform infrared spectroscopy (FTIR) measurement
was implemented to further confirm the different self-assembled
structures at molecular level (Fig. S9). For the drop cast
aggregates, the characteristic stretching absorption band of
the strong hydrogen bond of the amide N–H group was
observed at 3316 cmÀ1, remarkably lower than that of the
free N–H group (ca. 3400 cmÀ1). This band weakened, became
broad and shifted to higher wavenumber after treating the
sample by grinding, indicating the partial destruction of
hydrogen bonds and the change of assembled structures. The
E1 emission of drop cast aggregates is indicative of the
conformational restriction of pyrenyl moieties,8c which is
consistent with the less free translational and rotational
motion of molecules immobilized by strong hydrogen bonding.
The external force may alter the molecular interactions and
facilitate the rearrangement of molecules, resulting in a sand-
wich-type packing (E2) of the luminophores in ground solid.
Differential scanning calorimetry (DSC) measurement was
also performed to investigate the two solid states (Fig. S10).
The higher enthalpy at the melting point of drop cast aggregates
than that of ground solid indicated that the former was
thermodynamically more stable, although the free luminophore
packing in the excited state of drop cast aggregates is less
stable.
The photoluminescence property of 1 also depends on the
thermal history (Fig. S11). When the drop cast aggregates
were heated above melting temperature (B200 1C), the drastic
transition from blue to bright green emission occurred,
suggesting that a dynamic and disordered state might favor
the E2 packing mode, further confirming our observation
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Chem. Commun., 2011, 47, 6078–6080 6079

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mentioned above. Interestingly, different cooling rates would
endow the sample with either blue or bright green emission.
On slow cooling at a rate of less than 10 1C minÀ1, the sample
showed distinct blue emission (lem = 420 nm), a luminescent
feature of E1 excimer. In sharp contrast, fast cooling the
sample afforded it the optical character of bright green color
(lem = 470 nm). The morphology also varied if the sample was
subjected to different cooling rates. The triangular micro-
crystalline structure was observed in the slow cooling sample,
whereas amorphous aggregates were observed for fast quenching
solid (Fig. S12). X-ray diffraction measurement was performed
to further understand the microstructure of the sample treated
under different conditions (Fig. S13). Compared to the fast
quenching sample, the slow cooling one exhibited two
relatively sharper peaks at 2y = 3.91 and 6.31, which could
be indexed as (100) and (110) reflection of hexagonal columnar
packing with the d spacing identical to the as-prepared drop
cast aggregates mentioned above (Fig. 2b). The results
suggested that the slow cooling sample indeed shared the
same self-organized pattern with the as-prepared drop cast
aggregates. In addition, the sharp reflection peaks denoted the
existence of crystalline solid in the slow cooling sample, which
was consistent with the observation by AFM images. It has
been reported that the varied photoluminescence of mechano-
chromic materials originates from relatively well-defined
crystal state and less ordered solid, respectively.5d Thus, it is
reasonable to deduce that the distinct luminescence of 1 relates
to the varied morphologies and order degrees. Moreover,
when the slow cooling sample was subjected to shearing, the
blue to bright green transition was observed, and another
heating–slow cooling cycle would produce the blue emission
again. The compound with such reversible and multi-stimuli
responsive luminescent property may have potential applications
for reusable smart materials.
which showed a reversible mechanochromic luminescence
property. Our results firstly demonstrate that the polypeptide,
which can provide multiple noncovalent molecular interactions,
is an effective framework for preparing mechanochromic
materials. Unlike the previously reported monomer-to-
excimer transition mechanism, dendron 1 is a rarely reported
mechanochromic example based on the mechanism of
excimer-to-excimer transition that offers a promising strategy
for tuning the luminescent property. Specially, 1 also exhibited
a thermal history dependent luminescent property. This newly
emerged amino acids-based smart material may enrich the
limited family of mechanochromic materials and further
establish a deep understanding between molecular self-
assembled structures and luminescent property. Our design
strategy of combining the polypeptides structure and aromatic
groups in the same molecule can be applied to other systems
due to the wide applicability of polypeptides.
This work is financially supported by the National Natural
Science Foundation of China (20974004) to X.-R. Jia and the
National Basic Research Program of China (973 program,
15120055114). We thank Mr Ting Lei for a useful discussion.
Notes and references
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Based on the above data, the unusual stimuli-responsive
behavior of 1 can be understood. The molecular interactions
and arrangement of dendritic branches and luminophores,
which finally determine the apparent phase structures, can
be tuned by stimulus of force or thermal history. The dendritic
block tends to self-assemble when crystallizing from solution
or cooling at a comparable rate from the melt. In this way, the
pyrenyl groups are packed in a confined environment, which
may force them to be partially overlapped with blue emission.
The alteration of hydrogen bonding under the stimulus of
external force contributes to the variation of self-assembled
structures in which the pyrenyl groups changed to a sandwich
like arrangement. Different packing modes of pyrenyl groups
could be detected by the different emission colors from the
sample. On the other hand, fast quenching of the drop-cast
sample from the isotropic state may fix the dynamic behavior
in the isotropic state and endow it with similar bright green
emission.
In summary, we successfully designed and prepared a novel
dendritic molecule based on polypeptide from amino acids,
c
6080 Chem. Commun., 2011, 47, 6078–6080
This journal is The Royal Society of Chemistry 2011