Synthesis and self-assembly of tetraphenylethene and biphenyl based AIE-active triazoles

Article abstract of DOI:10.1039/c2jm30620d

Self-assembly of fluorescent functional materials has attracted increasing interest in the fabrication of optoelectronic and biological nanodevices. Tetraphenylethene (TPE) is a typical dye molecule with aggregation-induced- emission (AIE) characteristics. Melding TPE carrying triple-bond functionality with diazide-containing biphenyl through "click" chemistry generates AIE-active luminogens [1,1′-biphenyl]-4,4′-diyl bis(6-(4-(4-(1,2,2- triphenylvinyl)phenyl)-1H-1,2,3-triazol-1-yl) hexanoate) [1(5)] and [1,1′-biphenyl]-4,4′-diyl bis(11-(4-(4-(1,2,2-triphenylvinyl)phenyl) -1H-1,2,3-triazol-1-yl) undecanoate) [1(10)] with solid state efficiencies up to unity. Slow addition of dilute THF solutions of 1(m) (m = 5, 10) into nonsolvents such as n-hexane and water yields self-assembled white wooly solids. TEM and SEM observations reveal the (helical) nanofibrous structure of the aggregates. Upon cooling from their concentrated hot solutions, 1(m) readily precipitate. Meanwhile, they can also form gels at high concentrations. Both precipitates and gels of 1(m) exhibit structures similar to those of the aggregates formed in nonsolvents. These results indicate that 1(m) can facilely self-assemble into high emission efficiency (helical) nanofibers, thus paving the way for their optoelectronic and biological applications. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm30620d

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Volume 22 | Number 21 | 7 June 2012 | Pages 10365–10940
ISSN 0959-9428
PAPER
B. Z. Tang et al.
Synthesis and self-assembly of tetraphenylethene and biphenyl based
AIE-active triazoles
0959-9428(2012)22:21;1-I

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Cite this: J. Mater. Chem., 2012, 22, 10472
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PAPER
Synthesis and self-assembly of tetraphenylethene and biphenyl based
AIE-active triazoles†
Wang Zhang Yuan,ab Faisal Mahtab,a Yongyang Gong,b Zhen-Qiang Yu,c Ping Lu,ad Youhong Tang,a
a
Jacky W. Y. Lam, Caizhen Zhu and Ben Zhong Tang
c
a
*
Received 1st February 2012, Accepted 13th March 2012
DOI: 10.1039/c2jm30620d
Self-assembly of fluorescent functional materials has attracted increasing interest in the fabrication of
optoelectronic and biological nanodevices. Tetraphenylethene (TPE) is a typical dye molecule with
aggregation-induced-emission (AIE) characteristics. Melding TPE carrying triple-bond functionality
with diazide-containing biphenyl through ‘‘click’’ chemistry generates AIE-active luminogens [1,10-
biphenyl]-4,40-diyl bis(6-(4-(4-(1,2,2-triphenylvinyl)phenyl)-1H-1,2,3-triazol-1-yl) hexanoate) [1(5)] and
[1,10-biphenyl]-4,40-diyl bis(11-(4-(4-(1,2,2-triphenylvinyl)phenyl)-1H-1,2,3-triazol-1-yl) undecanoate)
[1(10)] with solid state efficiencies up to unity. Slow addition of dilute THF solutions of 1(m) (m ¼ 5, 10)
into nonsolvents such as n-hexane and water yields self-assembled white wooly solids. TEM and SEM
observations reveal the (helical) nanofibrous structure of the aggregates. Upon cooling from their
concentrated hot solutions, 1(m) readily precipitate. Meanwhile, they can also form gels at high
concentrations. Both precipitates and gels of 1(m) exhibit structures similar to those of the aggregates
formed in nonsolvents. These results indicate that 1(m) can facilely self-assemble into high emission
efficiency (helical) nanofibers, thus paving the way for their optoelectronic and biological applications.
dependent on the intermolecular order and/or emission efficiency
Introduction
in the active layers.15b,20,21 Therefore, development of luminogens
capable of self-assembling with high solid state efficiencies is of
crucial importance.
Intensive research has focused on the development of functional
soft materials through molecular self-assembly in the past
decades.1–7 Landmark examples of hierarchical assemblies
include micelles,8 vesicles and liposomes,9 microcapsules,10 den-
drimers,11 gels,12 and liquid crystalline materials.3c,5a,13 Along
these lines, self-assembly of p-conjugated molecules into linear
structures has attracted much attention because of their potential
applications in photonics and optoelectronics,3a,6b,14–19 such as
field-effect transistors (FETs),15b,16,17 organic light emitting
diodes (OLEDs),18 organic light emitting transistors (OLETs),17
and solar cells.19 One-dimensional molecular assemblies can be
constructed through weak intermolecular interactions such as
hydrogen bonding, p–p stacking, electrostatic interactions, van
der Waals forces, and solvophobic interactions.4c,6b,14–20 As
a matter of fact, optoelectronic device performances are highly
Fabrication of one-dimensional (1D) nanostructures is
attracting considerable interest in the field of nanotechnology
due to their unique morphology, optical and electrical properties,
and intriguing applications in electronics, sensors, and photonic
materials.3a,15b,20–22 Amongst varying nanostructures of fibers,
wires, rods, belts, tubes, spirals, and rings, nanofibers are pres-
ently attracting increasing attention due to their specific prop-
erties and various applications in medical products, capacitors,
transistors, drug delivery, battery separators, energy storage, fuel
cells, and information technology.4c,23
Numerous methods have been demonstrated for generating
nanofibers,24 among which self-assembly affords an effective way
towards novel materials with molecularly defined properties
which are inaccessible through step-by-step synthesis.6b,4c,15b,15d,20
Self-assembled nanofibers show peculiar, fascinating, and
tunable properties, as well as unique applications in optoelec-
tronically and biologically active materials.7a,7b,16b,20,25–28 There-
fore, it is of both fundamental and technological importance to
craft diverse new functional nanofibers via self-assembly
processes.
aDepartment of Chemistry and State Key Laboratory of Molecular
Neuroscience, The Hong Kong University of Science & Technology,
Clear Water Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk
bSchool of Chemistry and Chemical Engineering, Shanghai Jiao Tong
University, Shanghai 200240, China
cSchool of Chemistry and Chemical Engineering, Shenzhen University,
Shenzhen 518060, China
dState Key Laboratory of Supramolecular Structure and Materials, Jilin
University, Changchun 130023, China
Compared with a variety of counterparts, fluorescent nano-
fibers are of particular interest.25–29 Due to their luminescent
features, fascinating photophysical behavior, and high aspect
ratio, they are widely used in waveguidings,25a frequency
† Electronic supplementary information (ESI) available: Synthetic
procedures, PL spectra of 1(10) in THF and THF–water mixture, DSC
data, and other materials. See DOI: 10.1039/c2jm30620d
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doublers,25b nanolasers,25c chemo-7a,30 and biosensors,28 light-
harvesting nanomaterials,31 and OLEDs.32 Despite their wide
and important applications, fabrication of highly fluorescent
nanofibers with optimal optoelectronic properties has proven to
be a daunting job. It is because the notorious concentration
quenching (CQ) and aggregation-caused quenching (ACQ)
effects, which are usually encountered when luminophores
are condensed and aggregated,33 owing to the formation of
detrimental excimers or exciplexes.
Instrumentations
1H and 13C NMR spectra were measured on a Bruker ARX 400
spectrometer using CDCl3 or DMSO-d6 as solvent and tetra-
methylsilane (TMS) as internal standard. Matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF) high-reso-
lution mass spectra (HRMS) were recorded on a GCT premier
CAB048 mass spectrometer. Absorption spectra were taken on
a Milton Roy Spectronic 3000 Array spectrometer. Emission
spectra were taken on a Perkin-Elmer LS 55 spectrofluorometer.
Transmission electron microscopy (TEM) and scanning electron
microscopy (SEM) images were observed using JOEL 2010 TEM
and JOEL 6700F SEM instruments at accelerating voltages of
200 and 5 kV, respectively. Samples were prepared by drop
casting of dilute aggregate dispersions onto copper 400-mesh
carrier grids covered with carbon-coated Formvar films. The
solvent was evaporated at room temperature in open air.
Thermal transitions were investigated by a TA DSC Q1000
under nitrogen at a heating rate of 10 ꢀC minꢁ1. One-dimensional
wide-angle X-ray diffraction (1D-WAXD) powder experiments
were performed on a Philips X’Pert Pro diffractometer with
a 3 kW ceramic tube as the X-ray source (Cu Ka) and an
X’celerator detector. The sample stage was set horizontally and
the samples were protected by nitrogen gas during the
measurements. The reflection peak positions were calibrated
with silicon powder (2q > 15ꢀ) and silver behenate (2q < 10ꢀ).
Background scattering was recorded and subtracted from the
sample patterns.
Our group is interested in the fabrication of high efficiency
solid emitters, particularly those with aggregation-induced
emission (AIE) characteristics,34 which is contrary to the ACQ
effect. Because highly active intramolecular rotations of the
luminogens effectively consume the exciton energy, AIE-active
molecules are practically nonfluorescent when molecularly
dissolved in solvents; however, they become strong emitters upon
aggregation owing to the restricted intramolecular rotations
(RIRs).34 Utilizing AIE-active molecules to fabricate self-
assembled fluorescent nanofibers will be interesting in terms of
their applications in biological, chemical, and optoelectronic
devices.
Herein, we report the synthesis and self-assembly properties of
tetraphenylethene (TPE) and biphenyl based triazoles 1(5) and
1(10) (Chart 1). Both luminogens are AIE-active, with solid state
emission efficiencies up to unity. They are also capable of self-
assembling into crystalline nanofibers by addition of their solu-
tions into nonsolvents or upon cooling from their condensed hot
ꢀ
solutions to room temperature or 4 C. Moreover, these nano-
fibers are twisted and even helical though they are devoid of any
chiral centers. A comparison experiment indicates that flexible
spacers are of crucial importance in the formation of such helical
structures.
Synthesis
Compounds 1(m) (m ¼ 5, 10) and 11 were prepared through
multistep reactions as shown in Schemes 1 and S1 (ESI†).
Detailed procedures for the syntheses of the reaction intermedi-
ates are described in the ESI†. The final products 1(m) were
Experimental
Materials
THF and toluene were distilled under normal pressure from
sodium benzophenone ketyl under argon immediately prior to
use. Other solvents, such as chloroform, hexane, and dime-
thylsulfoxide (DMSO), were of high purities and used as
received. 4-(2-(Trimethylsilyl)ethynyl)benzophenone (3) was
synthesized according to our previous procedures.35 p-Toluene-
sulfonic acid monohydrate (TsOH), N,N0-dicyclohexy-
lcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP),
6-bromohexanoic acid [7(5)], 11-bromoundecanoic acid [7(10)],
4,40-biphenol (8), sodium azide, and 1 M tetrabutylammonium
fluoride (TBAF) in THF with 5% water were purchased from
Aldrich and used without further purification. Cu(PPh3)3Br
catalyst was prepared according to the literature method.36
Chart 1 Chemical structure of 1(m) (m ¼ 5, 10).
Scheme 1 Synthetic route to 1(m) (m ¼ 5, 10).
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synthesized by ‘‘click chemistry’’ reaction, and the synthetic
procedure of 1(5) is given below as an example.
were then reacted with sodium azide, furnishing the desirable key
intermediates 10(m). The target triazoles 1(m) were obtained
through facile ‘‘click chemistry’’ reaction catalyzed with
Cu(PPh3)3Br in THF at 60 ꢀC. All reactions proceeded smoothly
and the desirable compounds were obtained in high yields. The
compounds were characterized by standard spectroscopic
methods, from which satisfactory analysis data corresponding to
their molecular structures were obtained (ESI†).
Synthesis of [1,10-biphenyl]-4,40-diyl bis(6-(4-(4-(1,2,2-tri-
phenylvinyl)phenyl)-1H-1,2,3-triazol-1-yl) hexanoate) [1(5)].
In a 15 mL Schlenk tube 46.5 mg (0.1 mmol) of 10(5), 78.4 mg
(0.2 mmol) of 6 and 2.8 mg of Cu(PPh3)3Br were placed. 2 mL of
THF was then injected. The reaction mixture was stirred at 60 ꢀC
and found to be gelated 12 h later. The crude product was
purified by a silica gel column using a chloroform–acetone
mixture (15 : 1 by volume). A white solid of 1(5) was obtained in
1(10) shows an absorption profile almost identical to that of
1(5) with an absorption maximum at 322 nm (Fig. 1), which is
corresponded to the p–p* transition of the TPE-based triazoles.
When illuminated with a UV lamp, neither of their solutions
emits any observable light but strong blue light from their solid
powders and thin films, suggesting that aggregation has turned
on their emissions. To verify it, the emission spectra of 1(m) in
THF and THF–water aqueous mixtures were measured. Water is
used since it is a nonsolvent of 1(m): the luminogen molecules
must aggregate in the aqueous mixtures with high water fractions
(fw). Take 1(5) for example, weak PL signals are recorded even
when the fw is as high as 50% because the 1(5) molecules are still
molecularly dissolved in these mixtures (Fig. 2). The PL intensity
starts to rise at fw > 60% and, at much higher fw ($80%), slight
increase in fw gives abrupt promotion in emission intensity,
1
90.1% yield. H NMR (400 MHz, CDCl3), d (TMS, ppm): 7.71
(s, 2H, triazole protons), 7.57 (d, 4H), 7.52 (d, 4H), 7.13, 7.09,
7.07, 7.05, 7.04, 7.02, 4.42 (t, 4H, triaozle-CH2), 2.59 (t, 4H,
OOCCH2), 2.01 (m, 4H, triaozle-CH2CH2), 1.82 (m, 4H,
OOCCH2CH2), 1.48 (m, 4H, OOCCH2CH2CH2). 13C NMR (100
MHz, CDCl3), d (TMS, ppm): 171.83 (C]O), 150.08, 143.73,
143.64, 143.59, 143.53, 141.28, 140.42, 138.08, 131.85, 131.36,
131.31, 128.53, 128.13, 127.76, 127.68, 127.62, 126.52, 126.50,
126.45, 124.98, 121.83, 50.05 (triazole-CH2), 33.97 (OOCCH2),
30.01 (triazole-CH2CH2), 25.90 (OOCH2CH2), 24.15
(OOCH2CH2CH2). HRMS (MALDI-TOF): calcd for
C80H68N6O4: 1176.5302. Found: 1199.4940 [M + Na]+.
Sample preparation for AIE measurement
Stock THF solutions of the luminogens with a concentration of
2 ꢂ 10ꢁ4 M were prepared. Aliquots of the stock solution were
transferred to 10 mL volumetric flasks. After appropriate
amounts of THF were added, water was added dropwise under
vigorous stirring to furnish 2 ꢂ 10ꢁ5 M solutions with different
water fractions (0–90 vol%). PL measurement of the resulting
solutions or suspensions was then performed immediately.
Quantum yield determination
Quantum yields (FF,s) of 1(m) (m ¼ 5, 10) in THF were estimated
using 9,10-diphenylanthracene (FF ¼ 90% in cyclohexane) as
standard (excitation wavelength ¼ 270 nm), while the FF,f values
of solid powders were determined using an integrating sphere.
Visual molecular dynamics simulation
Fig. 1 Normalized absorption spectra of 1(5) and 1(10) in THF.
Concentration ¼ 20 mM.
To obtain theoretical understanding of the molecular interac-
tions and to get insights into the self-assembly mechanism,
molecular dynamic simulation of 1(5) was conducted. The
snapshots for all figures were taken with Visual Molecular
Dynamics (VMD),37 a famous visualization tool to view MD
simulation results, and the structure was modeled by the
self-defined script.
Results and discussion
Scheme 1 illustrates the synthetic routes to the resulting
compounds 1(m). Detailed procedures and characterization data
are given in ESI†. Briefly, important intermediate 6 was prepared
by lithiation of 2, followed by coupling with 4-(2-(trimethylsilyl)
ethynyl)benzophenone (3), dehydration,38 and cleavage of tri-
methylsilyl group in basic medium. Meanwhile, esterification of
4,40-biphenol (8) with u-bromoalkanoic acid 7(m) in the presence
of DCC, DMAP, and TsOH yielded compounds 9(m), which
Fig. 2 (A) Emission spectra of 1(5) in THF and THF–water mixtures.
(B) Plot of emission peak intensity at 480 nm vs. fw of the aqueous
mixture. Concentration ¼ 20 mM; excitation wavelength ¼ 340 nm. Inset:
photographs of 1(5) in THF–water mixtures with fw values of 0% and
90% taken under UV illumination.
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suggesting that the luminogen molecules are seriously aggregated
in such aqueous media. Fig. 2B shows the emission peak inten-
sities at varying water fractions. Clearly, from the molecular
solution in THF to the aggregate suspensions in 90% aqueous
mixture, the emission intensity of 1(5) at 480 nm is increased by
ꢃ5944 times. Similar behaviors are also observed for 1(10),
whose peak emission enhancement upon adding 90% water is as
high as ꢃ7819 times (Fig. S1, ESI†). Evidently, both luminogens
of 1(5) and 1(10) are AIE active. Moreover, such high emission
enhancement values are quite impressive among those reported
for other AIE molecules.39
To further validate the AIE activity of 1(m), their emission
quantum efficiencies (FF’s) in THF solutions and solid states
were determined. The FF,s values of 1(5) and 1(10) in THF are as
low as 0.54% and 0.52%, whereas those of their solid powders
(FF,f) are both boosted to 100% by aggregate formation,
resulting in AIE factors (aAIE ¼ FF,f/FF,s) as high as 185.2 and
192.3, respectively. Similar to previous systems, the AIE
behavior of 1(m) should be ascribed to RIR mechanism of the
chromophores.34,39–41 Namely, the molecularly dissolved species
of 1(m) in solvents undergo active intramolecular rotations
owing to their highly twisted configurations of TPE based tri-
azoles. These rotations effectively dissipate the excitons energy,
making the luminogens nonemissive in solutions. However, when
they are aggregated as nanosuspensions, powders, or solid thin
films, the intramolecular rotations are strikingly impeded;
meanwhile, their propeller-like structure prevents the formation
of detrimental excimers or exciplexes, thus endowing the aggre-
gates with intense emissions.
Fig. 3 (A and B) Photographs for flocculent precipitates of 1(10) in
n-hexane. (C and D) TEM and (E and F) SEM morphologies of the
flocculent precipitates of (C and E) 1(5) and (D and F) 1(10) formed by
dropping their THF solutions (1 mg mLꢁ1) into n-hexane.
We are interested in fabrication of high efficiency luminescent
liquid crystals.34g,34h To check whether 1(m) are liquid crystals,
POM observation was conducted but only a negative result was
obtained. However, accidentally, when THF solutions of 1(m)
were dropped into n-hexane, flocculent precipitates were
instantaneously formed (Fig. 3A and B). Morphologies of the
precipitates were characterized by TEM and SEM, and charac-
teristic images are presented in Fig. 3C–F. TEM image of 1(5)
reveals the formation of ribbon-like nanofibers, with lengths of
several micrometres and widths ranging from 30 to 100 nm
(Fig. 3C). Similar morphologies with well developed fibrillar
structures are also observed for 1(10) (Fig. 3D). SEM images
show the entangled 3-dimensional networks consisting of fibrous
nanostructures for the flocculent precipitates. The formation of
such elongated nanofibers suggests that self-assembly of 1(m) is
driven by the strong directional intermolecular interactions.
Generally, nanofibers are taken as an intermediate state
between molecularly dissolved species and organogels upon
increasing the concentration.42 The strong tendency of 1(m) to
self-assemble into nanofibers inspired us to investigate their
gelation capability. It is known that gelation behavior of
a compound is strikingly dependent on its inherent properties,
solvent, temperature, concentration, cooling rate, etc. Several
solvents, such as cyclohexane, THF, DCM, and DMSO, were
tested. Both compounds were insoluble in cyclohexane even at
elevated temperatures. They were soluble in DCM and DMSO at
low concentrations, but would precipitate upon cooling from
their concentrated hot solutions. Differently, they showed high
solubility in THF at ambient conditions, in which their gelation
abilities were tested. The concentrated solutions of 1(m) were
obtained at elevated temperatures. However, no gel but a small
amount of precipitates were formed for 20 mg mLꢁ1 1(10) when
cooled from 60 to 4 ꢀC. Further increasing the concentration to
50 mg mLꢁ1, still no gelation occurred. However, typical gels
were formed at a much higher concentration of 60 mg mLꢁ1. For
1(5), whose hydrophobic chain is much shorter than 1(10), only
when the concentration was as high as 90 mg mLꢁ1, gels could be
formed, indicating that a much longer spacer length is favored
for the gelation process.
Fig. 4 shows the TEM and SEM images of the gel and
precipitates formed in THF. As shown in Fig. 4A and D, fibrous
helical nanostructures are formed for the gel of 1(10). 1(5) and
1(10) precipitates exhibit similar fibrous structures (Fig. 4B, E, C
and F), suggesting the strong tendency of 1(m) to form ordered
(helical) nanofibers, which might be resulted from the hydro-
phobic interactions between flexible spacers and effective inter-
actions between intermolecular biphenyl and TPE units.
Self-assembly of TPE derivatives and other AIE-active lumi-
nogens into needle-like microcrystals,40a fibrils,26 nanorods,43 and
nanorings44 was previously reported; however, helical nanofibers
were scarcely present. Herein, the nanofibrous structure of gels
and precipitates of 1(m) should be highly related to the internal
alkyl chain substituted biphenyl units; however, the ordered
nanostructures also strongly suggest the regular packing of TPE
luminogens.
1(m) can form nanofibers upon precipitation and gelation.
Such strong tendency to self-assemble into ordered 1D nano-
structures implies the effective molecular association, which
might be an inherent attribute of 1(m). To verify it, THF solution
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nanofibers or even helical fibrous structures, thereby confirming
the above conjecture.
It is also notable that helical structures may endow the
aggregates with optical active properties; however, no obvious
signal was recorded in circular dichroism (CD) measurement for
the nanofibrous suspensions, indicating statistically equal
ensemble of left and right handed conformations.
X-ray diffraction measurement was also conducted to evaluate
the supramolecular nanoassemblies. Fig. 6 presents the X-ray
powder diffractograms for varying precipitates of 1(m) obtained
in n-hexane, THF, and water. Evidently, all spectra display
similar crystal profiles, suggesting the crystalline nature of the
precipitates. However, while peaks at 2q of ꢃ1.76 and 1.73ꢀ for
1(5) and 1(10) suggest long-range-ordered arrangements of the
aggregates, the absence of sharp and narrow peaks indicates that
the molecular packing of 1(m) is not as good as that in single
crystals. According to Bragg’s equation, d-spacings of the
aggregates for 1(5) and 1(10) in n-hexane are estimated to be
ꢀ
51.07 and 51.67 A, respectively, which are comparable to their
ꢀ
extended molecular lengths (50.13 and 62.54 A for 1(5) and 1(10),
Chart S1, ESI†), suggesting the monolayer packing of the
molecules. In the wide-angle region, broad peaks with corre-
ꢀ
sponding d-spacings of ꢃ4.5 A are assignable to molecular
stacking between adjacent TPE units. Meanwhile, signals of the
precipitates obtained in n-hexane are relatively well reserved,
thereby indicating a much better molecular packing in it.
Fig. 4 (A–C) TEM and (D–F) SEM morphologies for (A and D) the gel
of 1(10) formed in THF and precipitates of (B and E) 1(5) and (C and F)
1(10) generated from their concentrated hot THF solutions upon cooling.
Generally, self-assembly processes can be controlled both
kinetically and thermodynamically. THF and water are typical
solvent and nonsolvent for 1(m), respectively. While 1(m) can be
molecularly dissolved in THF, they must aggregate in THF–
water aqueous mixtures with high fw due to the decreased
solvating power. Therefore, in aqueous media with gradually
changed THF and water fractions, 1(m) should be present in
varying forms. For example, during the AIE measurement
experiment, in the 10/90 THF–water system, 1(m) molecules
must immediately aggregate when a large amount of water is
added into the THF stock solution, forming kinetically stable
nanosuspensions. Specifically, in the systems with inflexional
solvating power for 1(m), though the molecules started to
aggregate, giving slight increase in PL intensity, there was still
a considerable amount of molecules dispersed in the mixture.
These molecules can steadily assemble into more stable and
more ordered agglomerations. Indeed, during our experiment,
the 10/90 THF–water system remained unchanged for days once
of 1(10) was slowly dropped into water. Once the solution was
diffused into water, white turbid mixture was formed, then these
small white precipitates attached together to form wooly solids
when further solution drops were added, finally giving a clear
aqueous mixture with white flocculent precipitates in it. Such
phenomenon is quite similar to that observed in n-hexane–THF
system. The SEM images of the precipitates are given in Fig. 5.
Apparently, sea chestnut-like spheres consisting of numbers of
nanofibers were formed in water. It is reasonable to speculate
that 1(10) initially assembled into nanofibers due to its inherent
insolubility in water, then emerged together to form spherical
particles to minimize their surface energies. The result indicates
that it is a general phenomenon that 1(m) can self-assemble into
Fig. 5 SEM morphologies of the flocculent precipitates of 1(10) formed
by dropping its THF solution (1 mg mLꢁ1) into water.
Fig. 6 WAXD powder diffractograms for varying precipitates of (A)
1(5) and (B) 1(10) generated in n-hexane, THF, and water.
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the nanosuspensions were formed, whereas the 40/60 system
changed from an initially transparent mixture to a white solid-
containing heterogeneous admixture upon standing for 6 h.
Meanwhile, the emission intensity of 1(m) was also remarkably
enhanced with the formation of white solids due to the AIE
feature of the molecules.
(Fig. 8A), without alkyl chains was designed and successfully
synthesized (Scheme S1, ESI†). The solubility of 11 is low in
THF, chloroform, and DCM, and moderate in DMF due to its
rigid skeleton and strong intermolecular interactions. For
comparison, slow evaporation of the dilute THF solution of 11
and a precipitation experiment by dropping its THF–DMF (8/2,
v/v) solution into n-hexane nonsolvent were conducted. The
morphologies of the resulting precipitates were observed by
TEM, and the results are shown in Fig. 8B–D. Apparently,
different to what has been observed in the systems of 1(m),
crystalline particles (Fig. 8B) or nanosheets (Fig. 8C and D)
rather than helical nanofibers were generated during the aggre-
gation processes, thus proving the crucial role of flexible spacers
in the formation of helical aggregations.
TEM observation discloses the morphology of the aggregates
in 10/90 and 40/60 THF–water aqueous media. As depicted in
Fig. 7A and B, fine nanoparticles with diameters ranging from
several tens to ꢃ200 nm were formed in the 10/90 mixtures for
both molecules. However, well reserved helical nanofibers with
the length of several micrometres and the width of ꢃ100 nm were
formed for the 40/60 system of 1(5) upon standing (Fig. 7C and
D). These results clearly suggest that the helical nanofibers are
thermodynamically stable aggregates for 1(m) during the self-
assembly process, whereas the nanoparticles in 10/90 THF–water
mixture are only kinetically controlled agglomerations, whose
further morphology developing is highly restricted due to the
freezing of the molecules.
To get further insights into the self-assembly processes and
mechanism, visual molecular dynamics simulation (VMDS) of
1(5) was conducted. As shown in Fig. 9, adjacent 1(5) molecules
were packed together through effective intermolecular
Emission spectra were also obtained for the nanoparticles and
helical nanofibers. As shown in Fig. 7E and F, PL spectrum for
the nanoparticles of 1(5) exhibits an emission maximum at
480 nm, whereas that of the helical fibers peaks at 455 nm, giving
a large blue shift of 25 nm. Further excitation of such aggregates
with different wavelengths gives almost identical emission peaks
(ESI†). Therefore, the emission peak difference of 1(5) in 10/90
and 40/60 THF–water aqueous mixtures might be ascribed to the
change in the packing mode of the dye molecules in aggregates.
In 40/60 mixture, the dye molecules steadily assembled into
ordered crystalline nanofibers. In 10/90 mixture, however, the
dye molecules quickly agglomerated in a random way to form
amorphous particles. The amorphous TPE molecules may adopt
more planar conformations and hence exhibit redder emissions
than those in the crystalline state.45
Fig. 8 (A) Chemical structure of compound 11 and TEM morphologies
of the aggregates of 11 obtained by (C) slow evaporation of its dilute
THF solution (0.1 mg mLꢁ1) and (C and D) dropping its THF–DMF (8/
2, v/v) (1 mg mLꢁ1) solution into n-hexane.
To evaluate the effect of flexible spacers on the formation of
helical nanofibers, a congenerous compound of 1(m), namely 11
Fig. 7 TEM images for the aggregates of (A) 1(5), (B) 1(10) formed in 10/90 THF–water aqueous media and (C and D) the precipitates of 1(5) in THF–
water mixture (40/60) formed upon standing for 6 h. Normalized emission spectra for (E) the nanosuspensions and (F) helical nanofibers of 1(5) in 10/90
and 40/60 THF–water aqueous media. Excitation wavelength ¼ 340 nm.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 10472–10479 | 10477

NSFC/RGC grant (N_HKUST620/11), the University Grants
Committee of Hong Kong (AoE/P-03/08), the grant of HKUST
(SRFI11SC03PG, RPC11SC09), and the National Science
Foundation of China (21104044, 20974028). W. Z. Yuan thanks
the support from Ph.D. Programs Foundation of Ministry of
Education of China (20110073120040) and the Start-up Foun-
dation for New Faculties of Shanghai Jiao Tong University.
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