A novel aggregation-induced emission enhancement triggered by the assembly of a chiral gelator: from non-emissive nanofibers to emissive micro-loops

Article abstract of DOI:10.1039/c6cc08808b

In this study, a novel aggregation-induced emission (AIE) enhancement triggered by the self-assembly of chiral gelator is described. Tuning of molecular chirality in situ triggers different assemblies of superstructures exhibiting fluorescence. This novel AIE material can constitute an emerging library of chiral supramolecules for turn-on fluorescent sensors. It will also help in better understanding the effects of chiral factors on the photophysical process.

Full text of DOI:10.1039/c6cc08808b

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
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A novel aggregation-induced emission enhancement triggered by
the assembly of chiral gelator: From non-emissive nanofibers to
emissive micro-loops
Received xx xx ,
Accepted xx xx
Wenrui Chen, Guangyan Qing and Taolei Sun*ab
‡a
‡a
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this study, a novel aggregation-induced emission (AIE)
enhancement triggered by the self-assembly of chiral gelator is
described. Tuning of molecular chirality in situ triggers different
assemblies of superstructures exhibiting fluorescence. This novel
AIE material can constitute an emerging library of chiral
supramolecules for turn-on fluorescent sensors. It will also help in
better understanding the effects of chiral factors on the
photophysical process.
Supramolecular self-assembly1 is ubiquitous in nature; in
2
addition, it plays crucial roles in drug delivery, chiral
3
4
separation, and biosensing. The understanding associated
with the integration of self-assembled approaches to modified
AIE-active fluorogenic molecules is a promising direction for the
research on supramolecular materials and their optical
Fig. 1 Chemical structure (a) and circular dichroism spectra (b) of a pair of
-5
C
3
-symmetric molecules (10 M in ethyl acetate, 25
°
C).
5
applications, which can be significantly extended to biological
because of their multiple hydrogen bonding sites and chiral
centers, which were crucial for the regulation of assembly
applications for which high concentrations are crucial. The
integration of AIE with chiral self-assembled superstructures is
attracting increasing attention because of their advantages.6
Herein, a novel AIE phenomenon via the integration of chiral
9
modes. The 1, 3, 5-trisubstituted phenyl group was introduced
3
as the core for constructing the C -symmetric trefoil-like
structure (Fig. 1). First, L-Asp-L-Phe and D-Asp-D-Phe were
prepared by solid-phase peptide synthesis (SPPS). L,L-G1 and
D,D-G1 were then synthesized in 78% yield (Fig. S1, ESI†). Both
L,L-G1 and D,D-G1 formed macroscopic organogels in methanol,
with their microstructures containing three-dimensional cross-
linked fibers (Fig. S2, ESI†).
Interestingly, L,L-G1 and D,D-G1 exhibited contrasting
micropatterns in ethyl acetate (Fig. S3, ESI†). As shown in the
atomic force microscope (AFM) images in Fig. 2, long, twinning
fibers and isolated micro-loops were clearly observed on the
mica surface, indicating that chiral signals has been transferred
from a single-molecule level to a supramolecular scale.10 An
average height of 5nm was observed for L,L-G1 aggregates (Fig.
2a), typical of coiled superhelix structures. These intertwined
“tape reels” appeared to be rolled from a single fiber into
several spiral ribbons by hierarchical self-assembly. However,
aggregated superstructures is described.
3
C -symmetric
molecules are used as the building blocks because their
excellent framework can easily afford supramolecular gels via
7
non-covalent interactions. Upon chiral aggregation, nanofibers
and micro-loops are formed on the mica surface. Moreover, the
loop-like aggregates exhibit typical AIE characteristics
attributed to multiple intermolecular interactions.8
In this study, a pair of dipeptide sequences (L-Asp-L-Phe
and D-Asp-D-Phe) were chosen as the fundamental unit
a
State Key laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan, 430070, China.
School of Chemistry, Chemical Engineering and Life Science, Wuhan University of
b
Technology, 122 Luoshi Road, Wuhan, 430070, China.
*
E-mail: suntl@whut.edu.cn
†Electronic Supplementary Information (ESI) available: experimental details,
characterization, macroscopic organogel images, AFM images, SEM images, ATR- the D,D-G1 aggregates exhibited an unusual type of a loop-like
FTIR, nano-FTIR, and s-SNOM tests.
See DOI: 10.1039/x0xx00000x
These authors contributed equally to this work
structure with an average height of 140 nm (Fig. 2b). Micro-
loops with a diameter of 9
μ
m and a thickness of 400 nm were
‡
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Fig. 2 AFM images of the aggregates on mica substrates in the tapping mode:
a) L,L-G1; (b) D,D-G1; Young’s modulus of aggregates measured by AFM in
Fig. 3 Fluorescence microscopy images of L,L-G1 aggregates (a) and D,D-G1
aggregates (b), Insets: SEM images of the superstructures obtained on gold-
coated mica surface; PL spectra of L,L(D,D)-G1 (c) and L,L(D,D)-G2 to L(D)-
G5 (d) in ethyl acetate; and fluorescence integral area ratio for the selected
ten molecules with different peptide sequences (e). Concentration: 10-4 M,
(
the PeakForce QNM mode (c); and the dynamic assembly of L,L-G1 and D,D-
G1 observed by dynamic light scattering (DLS) measurements (d).
also observed, which was consistent with scanning electron
microscopy (SEM) images (Fig. S4, ESI†). This result suggested
that the layer-by-layer assembly of micro-loops. Hence, for L,L-
G1, once elementary nanofibers coiled to form a superhelix in a
hierarchical manner, it is thought that these nanofibers pack
into significantly large supercoiled structures. Meanwhile, D,D-
G1 possibly assembled “layer by layer,” thereby forming layers
of periodically stacked patterns. Previously, π–π stacking
interactions among phenyl groups have been reported to be
one of the key factors responsible for periodically stacking
λ
ex: 370 nm
Surprisingly, both the microstructures emitted dissimilar
fluorescence signals. As shown in the dark-field image in Fig. 3a,
L,L-G1 aggregates did not exhibit any remarkable fluorescence.
However, a network of fiber bundles or ropes composed of thin
fibers was clearly observed in the SEM images. By contrast, in
D,D-G1 aggregates, several micro-loops exhibited bright blue
fluorescence. The diameter of those loops ranged from 5
m, because of the dynamic stages of aggregation, as shown
in the SEM images. Interestingly, non-conjugated molecules
such as D,D-G1) exhibited typical AIE behavior (Fig. 3c).
μm to
40 μ
11
micropatterns.
The mechanical properties of aggregates were examined by
AFM in the PeakForce Quantitative Nanomechanical Mapping
(
Furthermore, the micropatterns switched from non-emissive
fibers to emissive loop-like aggregates because of chiral
inversion. The nanostructures and PL emissions of the mixed
L,L-G1 and D,D-G1 in various ratios were also investigated (Fig.
S5, ESI†). With the increase of D,D-G1 percentage, PL intensity
increased with the D,D-G1 ratio rising and entangled nanofibers
gathered. First, those loops appeared above the fibers’ surface
with an average height of 64 nm. Then the quantity of fibers
decreased and micro-loop increased as the typical patterns to
be observed. When D,D-G1 ratio was 100%, no fibers existed
except for loops of varying sizes. These results further
demonstrates the “layer by layer” assembled way for D,D-G1
and reflects the immense potential of this AIE material for
applications for biological detection via specific recognition.
Furthermore, substituent replacement was carried out to
investigate the relationship between peptide sequences and
fluorescence discrimination during aggregate formation. Eight
peptide sequences were examined (Fig. 3d, e): L-Phe-L-Phe (L,L-
G2); D-Phe-D-Phe (D,D-G2); L-Asp-L-Val (L,L-G3); D-Asp-D-Val
(
QNM) mode, which allows for quantitative nanomechanical
mapping of the Young’s modulus. The elastic modulus can be
calculated from the Derjaguin, Muller, Toropov model, which is
a direct measure of the surface hardness of samples.12 The
average Young’s modulus value for the micro-loops and
nanofibers were approximately 190 MPa and 160 MPa,
respectively (Fig. 2c). These results indicated that as compared
to fiber-like aggregates, the loop-like aggregates in D,D-G1 are
assembled more compactly. To further explore this difference
in assembly, dynamic light scattering (DLS) measurements were
conducted to monitor the kinetic process at room temperature.
The apparent hydrodynamic radius was calculated using the
1
3
Stokes-Einstein equation,
several measurements were
performed for each sample to obtain an average hydrodynamic
radius. The initiation time of micro-loops was twice that of the
fibers (Fig. 2d), implying that the construction of fine D,D-G1
superstructures takes a longer time than that required to
construct random nanofibers.
2
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(
(
D,D-G3); L-Asp (L-G4); D-Asp (D-G4); L-Phe (L-G5); and D-Phe Hence, the attenuated total reflection (ATR)-FTIR spectrum of
View Article Online
DOI: 10.1039/C6CC08808B
D-G5). First, Phe was used to replace the Asp residue, affording monomeric samples is recorded (Fig. S7a, ESI†) to measure the
-
1
a rigid sequence with strong π–π interactions. Neither L,L-G2 amide I and II bands in the range 1400—1700 cm . Similar peaks
nor D,D-G2 exhibited clear emissions, indicating that excessive were observed for both monomeric L,L-G1 and D,D-G1. From
π–π interactions possibly inhibit radiative decay; this inhibition the nano-FTIR spectrum of their aggregates in the
can result in aggregation-caused quenching (ACQ). Next, the supramolecular state (Fig. S7b, ESI†), strong peaks were
Phe residue was replaced with Val (Valine), to obtain a flexible observed at 1530 and 1657cm in the spectrum of D,D-G1
sequence. Interestingly, the fluorescence discrimination ratio aggregates. The peak at 1530 cm could be ascribed to NH
was slightly greater than that of a Phe-Phe sequence, indicative deformation and CN stretching vibrations (the amide band II
of the involvement of hydrogen bonding. Control experiments region), while the peak at 1657 cm could be attributed to CO
were conducted using a single-arm peptide based on Asp-Phe. stretching vibrations (the amide band I region). However, peaks
The discrimination ratio of L(D)-G4 was less than that of in the L,L-G1 aggregates (only 1657 cm ) were weaker than
L,L(D,D)-G3, indicating that multiple hydrogen bonds can those observed for D,D-G1, demonstrating that only CO
promote dissimilar light emission behavior. The discrimination stretching vibrations participate in fiber-like aggregation.
-
1
-
1
-
1
-
1
ratio of L(D)-G5 with a single Phe arm was two times as high as
To visualize the nanoscale distribution of functional groups,
that of L,L(D,D)-G2, suggesting that appropriate π–π stacking s-SNOM imaging was employed within a range of frequencies
can effectively increase the optical emission gap. The Phe using a tunable quantum cascade laser (QCL). The IR near-field
residue is confirmed to be crucial for light-emission, and the images of the loop-like aggregates and thin fibrils were clearly
presence of Asp promotes this process. Hence, when coupled different at the two wavenumbers. The micro-loop was clearly
-
1
with limited π–π stacking, multiple hydrogen bonding can observed at 1530 cm (Fig. 4b). However, it was difficult to
-
1
cooperatively contribute to the contrasting fluorescence observe the nanofibers (Fig. 4a). At 1657 cm , a fiber structure
emission characteristics, similar to that observed for L,L-G1 and was observed (Fig. S7c, ESI†), and micro-loops exhibited
D,D-G1. The self-assembled nanostructures of these eight relatively strong signals (Fig. S7d, ESI†). This observation
molecules were also investigated (Fig. S6, ESI†). These results indicated that only the hydrogen bonding motif of the C=O
not only confirmed the significant contribution of Asp and Phe group participates in the fiber-like assembly. However, the
residues to the AIE phenomenon, but also revealed that loop- micro-loop aggregates were associated with multiple hydrogen
3
like, optically active aggregates of D,D-G1 are possibly related bonds. Assuming that C -symmetric molecules can form
to stereoselective interactions. This assumption can be further supramolecular stacks, the central aromatic bridge with three
explained with the use of restricted intramolecular motion phenyl rings is hypothesized to exhibit strong π–π interactions.
1
4
(
RIM). In D,D-G1 assemblies, congested phenyl-packing can Both multiple hydrogen bonding and π–π interactions
block the motion of intramolecular bonds, leading to cooperatively contributed to the loop-like aggregates to some
1
5
remarkable enhancement of luminescence. Recently, Liu et degree. The presence of C=O hydrogen bonds can possibly
1
6
al. have reported a C
3
-symmetric molecule self-assembled explain the self-association of fibrils.
In summary, an interesting AIE phenomenon was
into optically active supramolecular gels. The results obtained
herein are in agreement with their results in terms of the discovered, confirming that discrimination by chiral self-
growth of the hierarchical aggregates exhibiting chiral assembly affects fluorescence emission. Novel luminescent
conformation, generating different optical effects with micro-loops were constructed by the assembly of the D-Asp-D-
macroscopic chirality.
3
Phe methyl-ester-modified C -symmetric molecule. This
exquisite bottom-up fabrication via non-covalent molecular
interactions of molecules is efficient for constructing novel
nanomaterials with specific superstructures and enhanced
emission characteristics. Loop-like, fiber-like aggregates exhibit
different alignment modes, based on π–π stacking interactions
and intermolecular hydrogen bonds. The optically active
aggregates are possibly formed by steric hindrance, resulting
from overcrowded packing of molecules. These results provide
interesting insight into the combination of AIE characteristics
and chiral self-assembled structures. Exploring the chiral factor
and the involved non-covalent interactions may help in better
understanding the fundamental mechanism during
supramolecular self-assembly, as well as their resultant optical
capabilities. This may lead to innovations, such as improvement
-
1
Fig. 4 Infrared near-field images of aggregates at 1530 cm : (a) L,L-G1; (b)
D,D-G1.
To further explore this phenomenon, Fourier transform
infrared nanospectroscopy (nano-FTIR) using a scattering-type
scanning near-field optical microscope (s-SNOM) was employed
to investigate the involvement of functional groups in these two
aggregates. The analysis of amide bands provides notable
insights into the secondary structures of peptides or proteins.17
in the rational molecular design, thereby expanding the
18
application
electroluminescence devices.
areas
of
biological
19
detection
and
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This work was sponsored by the Major State Basic 11 (a) A. Kuzyk, R. Schreiber, Z. Fan, G. PardatscherV,iEew. MArt.icRleoOlnlelinre,
DOI: 10.1039/C6CC08808B
Research Development Program of China (973 Program) (No.
013CB933002), the China National Funds for Distinguished
Young Scientists (No. 51325302) and the National Natural
A. Högele, F. C. Simmel, A. O. Govorov and T. Liedl, Nature,
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