Spontaneous chiral self-assembly of achiral AIEgens into AIEgen-silica hybrid nanotubes

Article abstract of DOI:10.1039/c9cc06873b

Antipodal chiral AIEgen-silica hybrid mesostructures were fabricated by spontaneous chiral self-assembly of the achiral amphiphilic AIEgen 2-[4-(1,2,2-triphenylethenyl)phenoxy]-acetic acid via a co-structure directing route in the absence of any symmetry-

Full text of DOI:10.1039/c9cc06873b

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Spontaneous chiral self-assembly of achiral
AIEgens into AIEgen-silica hybrid nanotubes†
Cite this: Chem. Commun., 2019,
5, 14438
5
a
a
b
ab
b
Wanning Zhang,‡ Hui Chang,‡ Jing Ai, Shunai Che,
Yingying Duan * and
b
Received 4th September 2019,
Accepted 4th November 2019
Lu Han
*
DOI: 10.1039/c9cc06873b
rsc.li/chemcomm
6,7
Antipodal chiral AIEgen-silica hybrid mesostructures were fabri- yields and ready functionalization. Notably, chiral TPE deriva-
cated by spontaneous chiral self-assembly of the achiral amphiphilic tives constitute one of the promising candidates for applications in
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AIEgen 2-[4-(1,2,2-triphenylethenyl)phenoxy]-acetic acid via
a
advanced chiral luminescent devices. The aggregation-induced
co-structure directing route in the absence of any symmetry- chirality effect of superstructures based on TPE derivatives has
breaking agent. The produced AIEgen-silica hybrid mesostructured been mostly achieved as a result of the elaborate design of the
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nanotubes showed chiral features with optical activity originating peripheries of the TPE scaffold, the decoration of the TPE core
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from J-aggregation with an excess of one stereochemical hand of with chiral pendants
the helical conformation of the AIEgen.
and introduction of chiral molecules into
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the TPE frameworks. Spontaneous chiral symmetry breaking of
TPE and its derivatives with propeller-shaped structures has been
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Chiral symmetry breaking is a ubiquitous phenomenon through- reported in the crystalline or superstructure states. The chirality
out the universe from the elementary particle scale to the planetary of the TPE-based superstructures can be controlled by adjusting
scale. The enthusiasm and curiosity about the formation of the solvent composition and polarity. The chirality of these TPE-
chiral species have motivated researchers from interdisciplinary core compounds has been demonstrated using a combination of
fields. Symmetry breaking of superstructures by chiral molecules comprehensive solid-state chiroptical spectroscopy and theoretical
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has been widely studied, and chiral inorganic materials have been calculations. However, the synthesis of mesostructured AIEgen-
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induced using chiral molecules as symmetry-breaking molecules.
inorganic hybrid nanomaterials is still challenging, while the
It has been realized that the spontaneous symmetry breaking of a formation of an inorganic framework would lead to the increased
superstructure without a chiral species requires achiral mole- stability and enhanced optical properties of AIE materials.
cules with high rigidity and complexity, such as J-aggregates of
Various highly ordered mesostructured organo-silica hybrid and
pure silica materials have been synthesized through co-operative
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4-sulfonylphenylporphyrin derivatives.
As a typical aggregation-induced emission (AIE) molecule, self-assembly of amphiphilic molecules and a silica source, with
tetraphenylethene (TPE) and its derivatives exhibit a wide range the self-assembly relying on the formation of electrostatic and
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of prospects in applications such as sensors, optoelectronic hydrogen bonding interactions. The co-structure directing
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materials, and catalysts due to their high fluorescence quantum route proposed by Che et al. can immobilize the self-assembled
structures of anionic amphiphilic molecules with silica frameworks
through proper interactions between amphiphilic molecules and
a
School of Chemistry and Chemical Engineering, State Key Laboratory of Composite
Materials, Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs,
inorganic species using cationic organosilanes as co-structure
directing agents (CSDAs). In this work, 2-[4-(1,2,2-triphenylethenyl)-
phenoxy]-acetic acid (TPEA) was used as an anionic amphiphilic
compound, quaternized aminosilane N-trimethoxysilylpropyl-N,N,N-
trimethylammonium chloride (TMAPS) as a CSDA, and tetraethyl
orthosilicate (TEOS) as a silica precursor to self-assemble into
Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240,
P. R. China
b
School of Chemical Science and Engineering, Tongji University,
1
239 Siping Road, Shanghai, 200092, P. R. China. E-mail: luhan@tongji.edu.cn,
yyduan@tongji.edu.cn
1
†
Electronic supplementary information (ESI) available: Experimental section, H
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and C NMR spectra of TPE-1 and TPEA (Fig. S1 and S2), elemental analysis results
of the TPEA (Table S1), SEM images of right-handed ribbon and R-CASN (Fig. S3 and chiral AIEgen-silica hybrid mesostructures. TPEA has been
S4), UV-Vis spectra of TPEA (aq.), TPEA and CASN (Fig. S5), g value of antipodal
shown to display a propeller-shaped set of four aryl blades that
readily stack into a chiral superstructure. In an alkalescent
solution, the negatively charged carboxylate groups of TPEA
have been shown to electrostatically interact with the positively
charged quaternary ammonium groups of TMAPS, while the
CASN (Fig. S6), AIE feature of TPEA (Fig. S7), photophysical data of TPEA and CASN
(
Table S2), photographs of TPEA and CASN under natural light and under 365 nm-
wavelength UV light (Fig. S8), TGA thermograms of TPEA and CASN (Fig. S9), and
lum value of the antipodal CASN (Fig. S10). See DOI: 10.1039/c9cc06873b
g
‡
These authors contributed equally to this work.
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silane site of TMAPS can be co-condensed with TEOS to form the
The acquired diffuse reflection ultraviolet-visible (DRUV-Vis)
inorganic wall. Here, antipodal chiral lamellar mesostructured spectra of the CASN sample are shown in Fig. S5 (ESI†). The
AIEgen-silica hybrid hollow nanotubes (CASNs) displaying optical CASN absorption band from 240 to 410 nm was attributed to
activity were synthesized. The four phenyl rings of TPEA can the overlap of various characteristic TPEA absorption bands.
spontaneously form clockwise or counterclockwise arrangements The TPEA sample yielded dual bands, at 260 nm and 360 nm,
relative to the olefinic plane, resulting in a J-aggregated while the solution of TPEA exhibited dual absorption bands, at
helical ribbon with an excess of one of the stereochemical 244 nm and 318 nm, owing to the p–p* electronic transition in a
conformations of TPEA. The left-handed CASN is denoted as single molecule, while the longer-wavelength band at 360 nm
L-CASN and the right-handed CASN as R-CASN, with the was ascribed to electronic transitions based on intermolecular
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determination of handedness here based on the chiroptical interactions similar to those seen in the case of J-aggregates.
response (vide post). The procedure used to synthesize TPEA Non-racemic molecular or mesoscopic chiral chromophores
and the results of the synthesis are illustrated in (Experimental have been shown to display optical activity (OA). Therefore, the
section and Fig. S1 and S2, ESI†). And the results of the chiral stacking of achiral TPEA should endow the CASN with
elemental analysis of TPEA are provided in Table S1 (ESI†).
OA. The ground-state chirality of CASNs can be detected from a
Analysis of the X-ray diffraction patterns of the CASN indi- diffuse reflection circular dichroism (DRCD) analysis. As shown
cated a typical lamellar organization (Fig. 1a). Two well-resolved in Fig. 2a, mirror-image-related CD spectra curves were
reflections at 2y = 2.51 and 5.01 were observed from both L- and obtained for the antipodal CASN products, and corresponded
R-CASN, and were indexed as first- and second-order reflections to the DRUV-Vis spectra. The DRCD spectrum of L-CASN
of the lamellar structure with a d-spacing of B3.5 nm.
exhibited a negative signal from 200 to 326 nm, attributed to
As shown in Fig. 1b, the L-CASN sample was mostly composed the p–p* electronic transition in a single TPEA molecular unit.
of uniform open-ended tubes with outer diameters of 200–400 nm. In contrast, the assembled structure exhibited a positive–negative
The lengths of the hollow nanotubes ranged widely, from hundreds bisignate signal ranging from 326 to 450 nm due to electronic
of nanometers to dozens of micrometers. As shown in the transitions based on intermolecular interactions of the TPEA
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magnified SEM image (Fig. 1c), very few imperfect nanotubes scaffolds. Notably, a medium with a left-handed helical structure
were observed, and they showed each an incomplete morphology of absorbs right-handed circularly polarized light and exhibits CD
helical ribbons. The pitch length of the left-handed ribbon was signals changing from negative to positive with a shortened
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determined to be B350 nm, (the right-handed ribbon is shown in wavelength. Therefore, it can be inferred that the TPEA surfactant
Fig. S3, ESI†). This phenomenon implied that the chiral hollow formed a left-handed helical arrangement in the L-CASN sample
nanotubes was transformed from helical ribbons by merging edges. and a right-handed helical arrangement in the R-CASN sample.
The R-CASN sample exhibited a hollow nanotube morphology The Kuhn anisotropy factor g was also plotted, as shown in
(
(
R-CASN is shown in Fig. S4, ESI†). The acquired TEM images Fig. S6 (ESI†), for the as-synthesized CASN. Based on the results
Fig. 1d and e) of the L-CASN sample displayed a hollow of 160 experiments (Fig. 2b), spontaneous symmetry breaking
tube structure with inner diameters of 100–180 nm and wall of the CASN was stochastic and unmanageable during the self-
thickness values of 50–110 nm. Lamellar architectures with assembly process.
d-spacings of B3.5 nm were observed in the wall, consistent
with the XRD pattern.
The AIE feature of TPEA is illustrated in Fig. S7 (ESI†). The
fluorescence intensity emerged at a wavelength of B386 nm for
TPEA and showed a greater than 3-fold increase with the aggregation
of TPEA through addition of ether into the TPEA ethanol solution. In
a confocal laser scanning microscopy (CLSM) study, the pure TPEA
showed an irregular shape and emitted weak blue light (Fig. 3a).
After the formation of the CASNs, the cylindrical objects emitted
stronger cyan light (Fig. 3b), occurring with an obvious red-shift,
which was consistent with the emission spectra of the TPEA and
CASN samples. Photographs of the TPEA and CASN samples
Fig. 1 XRD patterns (a) of as-made L- (black) and R-CASN (red), SEM
images (b and c) and TEM images (d and e) of L-CASN. The synthetic molar
composition of each CASN was TPEA : NaOH : TMAPS : TEOS : H
: 1 : 1.5 : 7 : 24 000.
2
O
=
Fig. 2 DRCD spectra (a) of as-made L- (black) and R-CASN (red). CD
values (b) at 363 nm for each of the 160 experiments.
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Scheme 1 Schematic drawing of the formation of L-CASN.
From the above data, a mechanism was derived to explain the
determination and amplification of chirality from the molecular
level to the mesoscopic level according to majority-rules principle
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(
Scheme 1). In the solution state, the four phenyls of TPEA are free
to rotate and the TPEA is achiral. However, in the co-structure
directing route, the stereochemical conformation is immobilized as
a result of silica condensation, which consequently leads to either
an anticlockwise (M) or clockwise (P) arrangement. In the early
stages, once the formation of the P-conformer TPEA predominates
over that of the M-conformer, the achiral TPEA of the solution could
Fig. 3 CLSM images of pure TPEA surfactant (a) and L-CASN (b) and
fluorescence spectra (c) of the pure TPEA surfactant (blue) and the CASN
(cyan). CPL spectra (d) of antipodal samples. lex = 366 nm.
under natural light and 365 nm-wavelength UV light are shown transform into P-conformer TPEA as a result of p–p stacking and the
in Fig. S8 (ESI†). The TPEA and CASN samples showed fluores- steric repulsion of the adjacent phenyl units and the reverse process
cence lifetimes of 4.41 ns and 5.58 ns, and emission quantum forming achiral TPEA could also occur. The transformation in the
yields of 0.41 and 0.57, respectively (Table S2, ESI†). These preferential configuration further propagates into the formation of
results indicated the ordered arrangement of TPEA in the silica helical ribbon J-aggregate. With the silica polymerization, the multi-
wall can give rise to fluorescence aggregation-induced emission lamellar CASN hybrids are finally formed. On the other hand, if P-
with relatively high efficiency, which can be ascribed to the conformer and M-conformer TPEA were formed in equal amounts,
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0
intramolecular motions of the four phenyl rings of TPE being racemic CASN would be fabricated with the absence of CD signals.
restricted by the rigid inorganic silica wall. Besides, the CASN The symmetry-breaking process is stochastic and unmanageable.
showed higher thermal stability than the pure TPEA (Fig. S9,
ESI†), reflecting the silica wall of AIEgen-silica hybrid materials tubes from precursors without any symmetry-breaking agent.
showing a certain ability to protect the TPEA.
The CD and CPL signals of the samples demonstrated the
In summary, we fabricated chiral CASN hybrid hollow nano-
Fig. 3c depicts the emission behaviors of the TPEA and CASN chirality and circularly polarized AIE properties of antipodal
samples in aggregated states. The TPEA sample emitted light at a CASNs. Our results may provide a new, convenient and efficient
wavelength of 443 nm. Compared to this emission, that of the CASN method for immobilizing a particular stereoisomer of a mole-
sample was red-shifted, to 453 nm, indicating that the close stacking cular structure, especially with a focus on propeller-shaped
21
of the TPEA molecules increased p–p conjugation. When the structures with a central stator and surrounding rotor. This
molecules formed a lamellar micelle, the TPE core may have become research may also help explain the origin of chirality and extend
more coplanar and the surfactant may have possessed better the applications of AIE molecules to those involving the for-
electronic conjugation and experienced stronger intermolecular mation of inorganic–organic nanomaterials.
interactions, causing the red-shifted emission of the CASN sample.
This work was supported by the National Natural Science
Furthermore, OA of the CASN sample in the electronic Foundation of China (21873072, 21571128, 21601123) and
excited state involved in the emission process was indicated Shanghai Rising-Star Program (17QA1401700). The authors
according to the results of a circularly polarized luminescence thank the Instrumental Analysis Centre of Shanghai Jiao Tong
(CPL) study. Left-handed helical structured materials prefer to University for CD measurements.
emit left-handed circularly polarized light and display positive
2
2
CPL signals. CPL intensities of antipodal CASNs as strong as
1
50 mdeg and a glum value as large as 0.02 with an excitation Conflicts of interest
wavelength lex of 366 nm were measured (Fig. S10, ESI†). The
positive CPL signal from the L-CASN sample at this excitation
wavelength confirmed the left-handed helical arrangement of
the TPEA surfactant. The slight differences between the CPL
signals of different CASNs may be attributed to the variation of
the helicity of TPEA.
There are no conflicts to declare.
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