Valine-containing silole: Synthesis, aggregation-induced chirality, luminescence enhancement, chiral-polarized luminescence and self-assembled structures

Article abstract of DOI:10.1039/c4tc00432a

The synthesis of valine-containing silole is reported. The introduction of a chiral valine pendant to silole endows the new compound (1 in Scheme 1) with not only circular dichroism (CD) and chiral-polarized luminescence (CPL), but also an aggregation-induced emission (AIE) property. The AIE effect boosts the fluorescence quantum efficiency, Φ_F, from 0.33% in pure THF to a maximum of 18.9% when water is added, which is 57 times higher than that in pure THF. In the thin film state, the Φ_F value measured by a calibrated integrating sphere can reach 80.3%, which is 243 times higher than that in the solution state. The amphiphilic valine attachments enable the compound to aggregate into complex architectures by a self-assembling process as revealed by AFM images. This compound self-assembles into helical fibers on the evaporation of its THF solution, which corresponds well with its CD and CPL properties. The addition of a poor solvent such as water or hexane to the THF solution also leads to the formation of aggregated structures, which exhibit helical enhancement or inversion in handedness to different extents.

Full text of DOI:10.1039/c4tc00432a

Journal of
Materials Chemistry C
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PAPER
Valine-containing silole: synthesis, aggregation-
induced chirality, luminescence enhancement,
chiral-polarized luminescence and self-assembled
structures
Cite this: J. Mater. Chem. C, 2014, 2,
4615
Jason C. Y. Ng,b Hongkun Li,ab Qin Yuan,a Jianzhao Liu,b Chunhua Liu,c Xiaolin Fan,c
a
b
*
*
Bing Shi Li and Ben Zhong Tang
The synthesis of valine-containing silole is reported. The introduction of a chiral valine pendant to silole
endows the new compound (1 in Scheme 1) with not only circular dichroism (CD) and chiral-polarized
luminescence (CPL), but also an aggregation-induced emission (AIE) property. The AIE effect boosts the
fluorescence quantum efficiency, FF, from 0.33% in pure THF to a maximum of 18.9% when water is
added, which is 57 times higher than that in pure THF. In the thin film state, the FF value measured by a
calibrated integrating sphere can reach 80.3%, which is 243 times higher than that in the solution state.
The amphiphilic valine attachments enable the compound to aggregate into complex architectures by a
self-assembling process as revealed by AFM images. This compound self-assembles into helical fibers on
the evaporation of its THF solution, which corresponds well with its CD and CPL properties. The addition
of a poor solvent such as water or hexane to the THF solution also leads to the formation of aggregated
structures, which exhibit helical enhancement or inversion in handedness to different extents.
Received 5th March 2014
Accepted 4th April 2014
DOI: 10.1039/c4tc00432a
www.rsc.org/MaterialsC
underlying mechanism of aggregation and luminescence is now
clearly re-elucidated.8 When the planar p-conjugated molecules
Introduction
aggregate in a face-to-face stacking manner, non-irradiative
energy transfer would take place to make the luminescence
lower or even quenched. However, for the non-planar p-conju-
gated molecules, when aggregation does not occur in a face-to-
face manner, aggregation will efficiently hinder the intermo-
lecular rotation and help enhance the luminescence efficiency.9
Most optical devices are used in the solid state; thus, AIE
molecules are ideal candidates for the fabrication of highly
efficient devices because of their high luminescence efficiency
in the aggregation state. Silole, which is the most famous AIE
molecule, is a propeller-shaped, p-conjugated molecule with
extremely high quantum yield in the solid state; moreover, it
has an important potential application as a material for fabri-
cating optical sensors.10 Fabricating novel silole molecules with
the AIE property into highly ordered nano-architectures would
be desirable because of the increasing demand for the minia-
turization of devices. However, without structural modication,
silole can only self-assemble into nanoparticles due to its lack of
fruitful functional groups that can realize complex self-assem-
bling processes. Thus, it is necessary to modify the silole core
with proper peripheries that are capable of judicious
assembling.
Intensive research has been performed in p-conjugated
molecular systems in the past few decades owing to their
unique optical properties.1 A notorious problem for p-conju-
gated systems is the formation of excimers or exciplexes via p–p
stacking interactions in the aggregation state, which oen
causes uorescence quenching, namely, the aggregation-caused
quenching (ACQ).2–4 This problem used to be overcome by a
complicated construction strategy until an opposite phenom-
enon, AIE, was discovered.5 The molecules with the AIE property
have weak uorescence in the solution, but their uorescence is
enhanced by several orders on the formation of aggregates.6,7
The discovery of the AIE property is beyond doubt a milestone in
the research on p-conjugated systems because it provides a
brand new view of the relationship between aggregation of
molecules and light-emission properties. With this important
discovery, the previously dominant ACQ phenomenon is now
found to represent only the tip of the iceberg. The whole
aDepartment of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen,
China. E-mail: phbingsl@google.com; Fax: +86-755-26536141; Tel: +86-755-
26558094
bDepartment of Chemistry, Institute for Advanced Study, Institute of Molecular
Functional Materials, Division of Biomedical Engineering, The Hong Kong University
of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China. E-mail:
tangbenz@ust.hk; Fax: +852-2358-1594; Tel: +852-2358-7375
Helical architectures are among the most exciting classes of
self-assembled structures.7 It has been recognized that the
introduction of chiral amino acid to p-conjugated systems can
cDepartment of Chemistry, Gannan Normal University, Ganzhou, China
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spectrophotometer. CD spectra were recorded on a JASCO J-810
spectropolarimeter in a 1 mm quartz cuvette using a step
resolution of 0.1 nm, a scan speed of 100 nm minꢀ1, a sensitivity
of 0.1 nm, and a response time of 0.5 s. Photoluminescence (PL)
spectra were recorded on a Perkin-Elmer LS 55 spectrouo-
rometer. Emission efficiencies of thin lms of the molecules
were measured by a calibrated integrating sphere. High-reso-
lution mass spectra (HRMS) were measured on a GCT Premier
CAB 048 mass spectrometer operating in an electron-ionization
or a MALDI-TOF mode. Circular photoluminescence spectra
were measured with a home-made CPL spectroscopy system.11,13
The system is composed of a 325 nm He–Cd laser as the light
source, a photo-elastic modulator (Hinds, PEM-90, 50 kHz), a
broadband Glan-laser polarizer (Special Optics) oriented at 45ꢁ
to the PEM optical axis, a monochromator (SPEX 500 M) and a
photomultiplier tube (PMT, Hamamatsu, R928). An excitation
laser of 325 nm in wavelength was irradiated on the sample
from the same side of emission collection. A UV depolarizer was
employed on the excitation light to avoid unnatural CPL. The
retardation of the emitted light was controlled by the quarter-
wave 50 kHz modulator PEM-90 and detected by the photo-
multiplier tube aer passing through the linear polarizer. The
DC component of the PMT output was measured by a digital
multimeter (Thurlby 1905a), whereas the AC component with a
frequency of 50 kHz was amplied by a pre-amplier (Stanford
Research Systems, SR560) and analyzed by a lock-in amplier
(Stanford Research Systems, SR510). The CPL dissymmetry
factor, gem ¼ 2(IL ꢀ IR)/(IL + IR), was evaluated from the ratio of
the AC signal to the DC signal.
Scheme 1 Synthetic route to 1.
efficiently induce the formation of helical conformation and
helical aggregates.8 A hybrid of chirality with AIE property might
simultaneously give offspring molecules, which bear helical
sense, luminescence and chiral-polarized luminescence prop-
erty.9 Inspired by this idea, we incorporate valine attachments
in silole and synthesize a chiral silole containing two valine
attachments (Scheme 1). The chiral carbon of valine attach-
ments is expected to exert an asymmetric force on the silole core
in order to induce a helical conformation.10 Hydrogen bonds
between the amino acid attachments and the solvophobic effect
of the amphiphilic molecules help stabilize the helical confor-
mation. Therefore, besides the AIE property, compound 1 is
also expected to possess both CD and CPL properties. Moreover,
the existence of amphiphilic valine attachments also endues the
compound with the ability to accomplish judicious self-
assembling processes.
Herein, we report the synthesis, optical properties and self-
assembling behavior of 1. Compound 1 showed AIE, CD and
CPL properties, and it also had the capacity to self-assemble
into helical architectures on evaporation of solvent or by addi-
tion of a poor solvent such as water and hexane. These results
help to broaden the view of AIE molecular design and also
provide important insight for using AIE molecules as the
building blocks for the fabrication of novel devices with CPL
and luminescence properties.
The AFM samples were prepared by dropping a small drop of
a diluted solution of 1 on the surface of newly cleaved mica
under ambient conditions. Aer 5 min of incubation, the
sample was dried with nitrogen gas. The images were collected
with a Nanoscope atomic force microscope (Icon) operating in a
tapping mode using hard silicon tips with a spring constant of
ꢂ40 N mꢀ1
.
Synthesis
Experimental section
Materials
Synthesis of 1. In a 50 mL round-bottom ask equipped with
a reux condenser, 0.105 g (0.2 mmol) of 2 and 0.192 g (0.5
mmol) of 3 were added to 15 mL of water–THF–ethanol mixture
(1/1/1, v/v/v). Then, 0.5 mL of freshly prepared 1 M aqueous
solution of sodium ascorbate was added, followed by 7.5 mg
(0.03 mmol) of copper(II) sulfate in 0.1 mL of water. The solution
Tetrahydrofuran (THF) was distilled from sodium benzophe-
none ketyl in an atmosphere of nitrogen immediately prior to
use. Copper(I) iodide, ZnCl2$TMEDA, triphenylphosphine,
dichlorobis(triphenylphosphine)palladium(II),
and
other
ꢁ
was stirred vigorously at 70 C overnight. Aer being cooled to
chemicals and solvents were all purchased from Aldrich and
used as received without further purication. 1,1-Dimethyl-2,5-
bis[4-(azidomethyl)phenyl]-3,4-diphenylsilole (2) and (4-ethy-
nylbenzoyl-L-valine (1S,2R,5S)-(+)-menthyl ester) (3) were
prepared according our previous papers.11,12
room temperature, 50 mL of water was added, and the reaction
mixture was extracted with DCM. The combined organic layer
was washed with brine and water. Aer solvent evaporation
under reduced pressure, the residue was puried by a silica gel
column using ethyl acetate as the eluent. A yellow solid was
1
obtained in 95.6% yield. H NMR (400 MHz, CDCl3), d (TMS,
ppm): 7.86 (m, 8H), 7.71 (s, 2H), 7.06 (d, 4H), 7.00 (m, 6H), 6.93
Instrumental analysis
1H and 13C NMR spectra were measured on a Bruker ARX 400 (d, 4H), 6.77 (m, 4H), 6.67 (d, 2H), 5.48 (s, 4H), 4.79 (m, 2H), 4.73
NMR spectrometer using chloroform-d as the solvent and tet- (d, 2H), 2.33 (d, 2H), 2.03 (d, 2H), 1.92 (m, 2H), 1.72 (m, 4H),
ramethylsilane (TMS) as the internal reference. UV absorption 1.45 (m, 2H), 1.05 (d, 10H), 0.96 (d, 7H), 0.90 (d, 15H), 0.75 (d,
spectra were obtained on a Milton Ray Spectronic 3000 array 6H), 0.46 (s, 6H). 13C NMR (100 MHz, CDCl3), d (TMS, ppm):
4616 | J. Mater. Chem. C, 2014, 2, 4615–4621
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171.9, 166.9, 141.2, 129.9, 129.6, 127.9, 127.8, 127.6, 125.8, 76.1, pendants to the nonchiral luminophoric cores. Furthermore, in
57.8, 46.9, 40.9, 34.2, 31.8, 31.5, 26.1, 22.9, 22.0, 21.0, 19.6, 17.4, the solution, almost no CD signals are observed; however, on
15.8, 2.2, 0.1, ꢀ3.7. HRMS (MALDI-TOF): m/z 1290.7086 molecular aggregation (lm state), strong CD signals appear
(M+, calcd 1290.7066).
and display aggregation-induced circular dichroism (AICD).11
Results and discussion
Aggregation-induced emission
Material preparation and structural characterization
It is known that conventional conjugated molecules generally
suffer from the ACQ effect in the solid state, which has become
a thorny problem in the development and fabrication of effi-
cient organic light-emitting diodes and biological uorescent
kits.16 The p–p stacking interaction between the conjugated
chiral molecules normally serves as a dominant force that drives
the self-assembling process to form hierarchical structures.
However, the accompanying ACQ effect induced by the inter-
molecular excitonic coupling between the adjacent lumino-
phores plays a role in remarkably weakening the light emission.
Thus, developing luminophores with high efficiency in the solid
state are highly demanded. The AIE luminogens may be ideal
candidates for the fabrication of highly efficient devices with
both CD and CPL properties because they exhibit enhanced
luminescent efficiency and high spectral stability in the aggre-
gate state.
To check whether 1 is AIE-active, we investigated the PL
spectra of 1 in THF–water mixtures with different water frac-
tions (fw). When excited at 360 nm, the PL curve of 1 in THF
solution shows almost a at line parallel to the abscissa
(Fig. 2A), suggesting that it is nonemissive when the molecules
dissolve in the solvent. In contrast, when water is added to its
THF solution, the emission peaking at 487 nm, gradually
intensies and increases swily when fw is larger than 50% due
to the nanoaggregate formation, which is indicative of the AIE
feature of 1.
In order to synthesize p-conjugated molecules with chiroptical
properties and strong solid-state light emission, we chose silole
as the AIE luminophore and valine as the chiral attachment,
which was synthesized with an azide–alkyne click route. Our
rich experiences in click14 and silole15 chemistries allowed us to
readily carry out this synthesis. Diazide-functionalized silole (2)
and amino acid derivative (3) were synthesized following our
previously published procedures.11,12 The click reaction of two
equivalents of 2 and 3 was catalyzed by CuSO4/sodium ascor-
bate (Scheme 1), affording 1 in excellent yield (see Experimental
section for detailed procedures).
The resultant 1 was characterized by standard spectroscopic
techniques, including 1H NMR, 13C NMR, and HRMS, from
which satisfactory analysis data corresponding to its expected
chemical structure were obtained (see Experimental section for
detailed spectroscopic data).
UV absorption and CD spectra
The UV absorption spectrum of 1 is shown in Fig. 1A. In THF,
compound 1 exhibits two absorption bands located at 350 and
279 nm, corresponding to the absorptions of the silole core and
the peripheral triazolylphenyl rings, respectively.11,12 We then
investigated its optical activity by CD spectroscopy. As can be
seen in Fig. 1B, the isolated molecules of 1 in the THF solution
show only very weak CD signals in the absorption region of the
triazolylphenyl ring. However, lm casting by solvent evapora-
tion leads to the emergence of Cotton effect at 249, 280 and
375 nm. Since the chiral unit (3) is CD silent at any wavelength
longer than 300 nm,12 the peak at 375 nm should be associated
with the absorption transition of the silole core, unambiguously
proving that chiral pendants induce the silole cores to be
arranged helically with a preferred screw sense. This shows an
interesting chiral transcription process from the chiral
The changes in the FF values of 1 further verify its AIE
activity. Fig. 2B shows a very small FF value (0.33%) in pure
THF; however, when water is added, the FF values increase
gradually and reach the highest value (18.9%) at the fw of 90%,
Fig. 2 (A) PL spectra of 1 in THF–water mixtures with different volume
fractions of water (fw). Concentration: 1 ꢃ 10ꢀ5 M; excitation wave-
length: 360 nm. (B) Fluorescence quantum yield (FF) of 1 versus
solvent composition of the THF–water mixture. The FF values were
Fig. 1 (A) Normalized UV absorption spectrum of 1 in THF. (B) CD estimated using quinine sulfate in 0.1 M H2SO4 (FF ¼ 54.6%) as stan-
spectra of 1 in 1,2-dichloroethane solution with a concentration of dard. Inset in panel (B): solutions of 1 in THF and a THF–water mixture
10ꢀ4 M and drop cast film from a 1,2-dichloroethane solution of 1 with 90% water; photographs taken under illumination of a handheld
(2 mg mLꢀ1).
UV lamp.
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which is 57 times higher than that in pure THF. It is amazing
that in the thin lm state the FF value measured by a calibrated
integrating sphere can reach 80.3%, which is 243 times higher
than that in the solution state.
Supramolecular assembly
Compound 1 possesses two chiral amino acid attachments,
which are anticipated to exert an asymmetric force eld on the
silole core and induce the formation of helical conformation in
the molecule. The CD and CPL spectra have revealed that the
helical sense is obvious only when 1 is in a solid state, which is
indicative of the formation of characteristic chiral aggregation
that forms due to the evaporation of the solvent. Thus, the
chiral aggregating behavior of 1 is pivotal for understanding the
enhanced helical conformation and chirality-related properties.
We then turned to AFM to investigate the morphological
structures of the chiral aggregation of 1. We rst investigated
the morphology of the aggregation of 1 on the evaporation of its
good solvent, i.e., the THF solution. Helical bers with consis-
tent le-handedness were formed by 1, as shown in Fig. 4. The
bers were arranged in an extended way, and thinner bers
were found to helically rotate at different levels to construct
thicker ones, which still retained the le-handedness. Because
of the hierarchical construction, the helical bers have a broad
distribution of width and helical pitches. This formation of
helical bers corresponds well with the CD spectra in Fig. 1,
suggesting that helical aggregation with a preferred sense has
been successfully created by the introduction of chiral attach-
ment to silole.
In addition to evaporation-induced aggregation, aggregates
can also be formed by the addition of a poor solvent to the
solution of the compound.18 Thus, we carried out a series of
experiments to induce the formation of aggregates by adding
different fractions of the poor solvents of compound 1. We rst
studied the CD spectra of the mixture by adding increasing
amounts of poor solvents, such as water and hexane, to the THF
solution. Note that precipitates appeared for the typical
concentration of 10ꢀ4 M of 1, and further dilution could effi-
ciently avoid the formation of the precipitates; however, the
Circularly polarized luminescence
Compound 1, possessing both CD and AIE characteristics, is
anticipated to possess circularly polarized luminescence (CPL)
property. We studied the CPL behavior of 1 in two states: in
solution in a good solvent and as a solid thin lm by evaporation
of its solution on a quartz substrate. An epi-illumination (reec-
tion mode) optical system was home-built to evaluate the CPL
activity by recording the differential spontaneous emission,
DI(l) ¼ IL(l) ꢀ IR(l). This setup is suitable for investigating the
chirality of opaque or nontransparent samples with strong light
scattering and low transmittance. Fig. 3A reveals the dependen-
cies of DI and the emission dissymmetry factor, gem, on the
wavelength. The sample in the pure solution state was also
measured for comparison. No CPL signals were detected in the
solution of good solvent because 1 is nonluminescent, and its
conformation is randomized by free intramolecular rotation.
Once the molecules aggregated as a cast thin lm, all DI signals
become negative with dominant RCP over LCP light emission in
the whole monitored spectral region. This demonstrates that the
arrangement of luminophores in the aggregates has preferred
one-handed helical structures (vide infra). The emission dissym-
metry factor, gem, is about ꢀ0.05 on average in the detected
spectral window of 420–620 nm, which is almost the highest
value in comparison to the commonly reported conventional
pure organic molecules (ꢂ10ꢀ5 to 10ꢀ2).17 In terms of both
emission deciency and dissymmetry factor, the compound
presents good results among conjugated organic molecules
without introduction of an external chiral eld, e.g., a chiral-
nematic liquid crystalline host, to enhance the helical arrange-
ment of luminophores. Furthermore, the solid sample can
preserve the CPL performances even aer more than half a year
under ambient conditions, which demonstrates that such a
characteristic has high stability. This compound is also a prom-
ising candidate for fabrication of micrometer- to nanometer-
sized patterns with potential applications such as generation of
CPL lasing and information processing and storage.
Fig. 3 Plots of (A) (IL ꢀ IR) and (B) CPL dissymmetry factor, gem, versus
wavelength for 1 in THF solution with a concentration of 10ꢀ4 M and Fig. 4 AFM images of the helical assemblies formed by 1 on the
drop cast film from 1,2-dichloroethane solution (2 mg mLꢀ1).
evaporation of its THF solution (c ¼ 0.001 mg mLꢀ1).
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concentration of the diluted solution could not meet the became much thicker with more bers joined together, and all
requirement for CD measurement. Therefore, the CD spectrum the bres exhibited le-handedness. Thus, we have revealed
is not suitable to explore the induced chiral aggregation of the that 1 formed chiral aggregates on the addition of water. The
compound due to the poor solvent because of the concentration formation of loops adds additional helical rotation to helical
requirement. Hence, by AFM, we studied the aggregation of 1 on bers, providing direct evidence that the helical sense is
addition of its poor solvent, i.e., water. At a concentration of enhanced at different levels in the aggregation process. The
ꢂ10ꢀ6 M, no precipitate appeared; however, the introduction of clearly observed helical pitches correspond well with the AICD
the poor solvent, even as small as a 5% fraction of water, caused property of the compound as mentioned above. Thus, the
a dramatic morphological transition, as shown in Fig. 5A and B. aggregation process is pivotal for the transition and amplica-
Helical bers were still formed, but only part of them tion of the chirality from the pendant to the silole core. The
retained the extended arrangement while the rest associated morphologies of the aggregated structures varied with the
into loops. Along the circular contour of the loops, braids of conditions under which they were formed. A thorough investi-
helical bers were clearly observed, indicating that the loops gation of the aggregated structures and their corresponding
were formed by winding of the helical bers. The loops were in aggregating conditions helps to better understand the critical
different dimensions due to the winding of the helical bers to factors involved in the aggregation.
different extents. Further increase in the water content (to 20%)
We further studied the aggregating behavior of 1 on the
caused the majority of bers to wind into loops with only a addition hexane (also a poor solvent for 1), as shown in Fig. 6.
small fraction retaining the extended arrangement. In fact, the When blending with 10% of hexane, the previously formed
extended bers still retained le-handedness, but they were helical bers were found to convert into networked structures.
much thicker than those that formed with low water content, as When the hexane content further increased to 50%, compound
shown by the labels in Fig. 5C. The loops were also much thicker
1 formed braids of bers arranged in a nebular-like
and more compact and showed a donut-like shape. When more morphology, showing an inclination to wind clockwise. When
loops accumulated together, they formed a network, as shown hexane content increased to 80%, right-handed helical bers
in Fig. 5D. When the water content was as high as 80%, were formed. The helical bers must have undergone hierar-
networks of extended bers were dominant. Moreover, when chical assembly because the thick helical bers were helically
the water content was as high as 90%, the network of bers twisted by multiple thinner helical bers and their dimensions
were distributed in a wide range. The helical bers at different
assembling levels all showed a consistent right-handedness.
Thus, the AFM images clearly showed that the addition of poor
solvent such as hexane to the THF solution caused an inversion
in the handedness of the helical bers. The poor solvent effect
induced by hexane is different from that induced by water. This
difference is expected because water and hexane have opposite
polarities. Thus, in a mildly polar solvent like THF, the occur-
rence of water leads to a gradual increment in the polarity of the
solution and nally creates a strong polar environment at
higher water contents. The addition of hexane to THF, on the
contrary, caused the mixture to undergo a gradual polarity
decrease until it nally reached nonpolarity. To balance these
diverse external environments, the amphiphilic 1 requires
proper adjustment. For instance, the nonpolar silole cores
dislike the polar environment, whereas the peripheral amino
acids have high affinity with the polar solvent. Therefore, the
molecules have to assume an appropriate arrangement that
minimizes the contact of silole cores with water and exposes the
polar amino acid attachments to the polar environment. In the
Fig. 5 AFM images of the helical assemblies formed by 1 on the Fig. 6 AFM images of the manipulated assemblies formed by 1 on the
evaporation of its different THF–water solution. The water content is evaporation of its different THF–hexane mixtures. The hexane content
5% (A and B), 20% (C and D), 80% (E) and 90% (F).
is 10% (A), 50% (B), 80% (C).
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nonpolar environment of hexane, however, the silole core and compound formed helical bers on evaporation of its THF
amino acid attachments would assume the opposite arrange- solution. On the addition of poor solvent, such as water or
ment by placing the polar groups inside and leaving the hexane (as low as 5%), 1 underwent dramatic morphological
nonpolar silole outside the associated structures. As a conse- transitions accompanied by helical enhancement and inversion
quence of the divergent assembling manners, their assemblies in handedness to different extents.
show opposite helical senses.
The addition of poor solvents is the main strategy to induce
the AIE property in molecules, and it works well for 1. The AIE
Acknowledgements
effect is usually obvious at very high content of poor solvents This work is supported by the National Natural Science foun-
(above 70%) in the mixture. Below this percentage, they caused dation of China (21104046), the Outstanding Youth foundation
almost no change to the uorescence emission. The poor of Shenzhen (JC201005250038A), the National Basic Research
solvents exerted their inuence by inducing the formation of Program of China (973 program, 2013CB834701), the Research
aggregates, as revealed by particle size analysis, SEM and TEM.19 Grants Council of Hong Kong (HKUST2/CRF/10 and
A high percentage of poor solvents usually caused the formation N_HKUST620/11), and the University Grants Committee of
of smaller nanoparticles compared with that formed at a low Hong Kong (AoE/P-03/08 and T23-713111-1).
percentage of poor solvents. Because of the simplicity of the
aggregates, few reports have focused on the aggregate trans-
formation. By the introduction of amphiphilicity and chirality to
Notes and references
the well-known AIE molecule, silole, we have provided a novel AIE
system to study the formation of aggregates and their trans-
formation with low to high percentage of solvent. Our AFM
images showed that a poor solvent, having fraction as low as 5%,
actually led to a dramatic morphological change. They suggested
that the addition of poor solvent induced the aggregation of 1 at
much lower volume fraction than that detected by uorescence
spectra. The lag in uorescence detection is due to the different
resolutions of AFM and uorescence spectrometer. AFM reveals
the aggregates of molecules on the molecular or supramolecular
levels;20 thus, it is more sensitive and able to capture the changes
of individual aggregates. However, uorescence spectrum
collects statistical information of the aggregates and is able to
show the difference only when the collective contribution of the
aggregates is signicant enough to reect their optical properties.
In contrast to the typical silole derivatives, the introduction
of chirality to the silole periphery endowed the molecules with
the capacity to aggregate into complex architectures. The
nanobers formed by 1, bearing CD, CPL and AIE properties,
have important potential applications in various areas,
including electronics and optics. The formation of various
aggregates also proves the possibility of fabricating intricate
architectures by the incorporation of chirality to the AIE system
and manipulating the architectures by changing the content of
the poor solvent.
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We have synthesized and characterized an amino acid con-
taining silole, which bore both chirality and the AIE properties.
The AIE effect boosted the uorescence quantum efficiency, FF,
from 0.33% in pure THF to a maximum of 18.9% when water 11 J. Liu, H. Su, L. Meng, Y. Zhao, C. Deng, J. C. Y. Ng, P. Lu,
was added, which was 57 times higher than that in pure THF. In
M. Faisal, J. W. Y. Lam, X. Huang, H. Wu, K. S. Wong and
the thin lm state, the FF value measured by a calibrated inte-
B. Z. Tang, Chem. Sci., 2012, 3, 2737.
grating sphere can reach 80.3%, which is 243 times higher than 12 L. M. Lai, J. W. Y. Lam and B. Z. Tang, J. Polym. Sci., Part A:
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assembly as helical aggregates. AFM images show that the
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J. Mater. Chem. C, 2014, 2, 4615–4621 | 4621