Twisted intramolecular charge transfer, aggregation-induced emission, supramolecular self-assembly and the optical waveguide of barbituric acid-functionalized tetraphenylethene

Article abstract of DOI:10.1039/c3tc32161d

A red-emissive barbituric acid-functionalized tetraphenylethene derivative (TPE-HPh-Bar) was designed and synthesized. TPE-HPh-Bar exhibits the effect of twisted intramolecular charge transfer due to the interaction of its donor and acceptor units. Whereas TPE-HPh-Bar emits faintly in solution, it becomes a strong emitter in the aggregated state, demonstrating a phenomenon of aggregation-induced emission. TPE-HPh-Bar can self-assemble into nanospheres upon natural evaporation of its solutions. In the presence of melamine, nanorods and (un)sealed nanotubes are formed, the content of which depends on the melamine amount. The crystalline nanorods of TPE-HPh-Bar grown from diethyl ether/hexane solution exhibit a good optical waveguiding effect with a low optical loss (0.137 dB μm^-1). Such attributes make the material to find wide applications in many areas such as biological imaging and optoelectronic nano-devices.

Full text of DOI:10.1039/c3tc32161d

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Materials Chemistry C
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Twisted intramolecular charge transfer,
aggregation-induced emission, supramolecular
self-assembly and the optical waveguide of
barbituric acid-functionalized tetraphenylethene†
Cite this: J. Mater. Chem. C, 2014, 2,
1801
Erjing Wang,ab Jacky W. Y. Lam,ab Rongrong Hu,ab Chuang Zhang,c Yong Sheng Zhaoc
abd
*
and Ben Zhong Tang
A red-emissive barbituric acid-functionalized tetraphenylethene derivative (TPE-HPh-Bar) was designed
and synthesized. TPE-HPh-Bar exhibits the effect of twisted intramolecular charge transfer due to the
interaction of its donor and acceptor units. Whereas TPE-HPh-Bar emits faintly in solution, it becomes a
strong emitter in the aggregated state, demonstrating a phenomenon of aggregation-induced emission.
TPE-HPh-Bar can self-assemble into nanospheres upon natural evaporation of its solutions. In the
presence of melamine, nanorods and (un)sealed nanotubes are formed, the content of which depends
on the melamine amount. The crystalline nanorods of TPE-HPh-Bar grown from diethyl ether/hexane
solution exhibit a good optical waveguiding effect with a low optical loss (0.137 dB mmꢀ1). Such
attributes make the material to find wide applications in many areas such as biological imaging and
optoelectronic nano-devices.
Received 1st November 2013
Accepted 10th December 2013
DOI: 10.1039/c3tc32161d
www.rsc.org/MaterialsC
research. Their enthusiastic efforts have generated a large
number and a wide variety of AIE luminogens with potential
Introduction
high-technological applications. Most of the AIE luminogens
prepared so far emit blue and green light. For biological
applications, especially in whole animal and deep tissue
imaging,4 dye molecules with longer-wavelength emissions are
preferred because they suffer no interference from the auto-
uorescence of biological tissues. There are two ways to acquire
such dyes. One methodology is the extension of the molecular p
conjugation so as to narrow the energy band gap for electronic
transitions. However, this strategy possesses the disadvantages
of tedious synthesis, high synthetic workload, ease of induction
of emission quenching owing to strong intermolecular inter-
action, susceptible photo-oxidation and poor dye solubility. The
other way is to introduce electron donating (D) and withdrawing
(A) units into the molecular structure. Dye molecules with such
donor–acceptor structures form charge transfer states with a
narrow band gap in polar solvents,5 thus red-shiing the
emission to longer wavelengths.
In 2001, Tang discovered the phenomenon of aggregation-
induced emission (AIE),1 which is exactly opposite to the
aggregation-caused quenching (ACQ) effect observed in some
conventional uorophores.2,3 For example, hexaphenylsilole
and tetraphenylethene (TPE) are non-luminescent when
molecularly dissolved in solutions, but become highly emissive
in the aggregated state. The restriction of intramolecular
motion (RIM) in the aggregated state is responsible for such a
phenomenon. Such a discovery changes people's way of
thinking about the light emitting process in the aggregated
state and has attracted much interest among scientists. More
than 100 research groups in the world are now doing AIE
aHKUST-Shenzhen Research Institute, No. 9. Yuexing 1st RD, South Area, Hi-tech Park,
Nanshan, Shenzhen, 518057, China. E-mail: tangbenz@ust.hk
bDepartment of Chemistry, Institute of Molecular Functional Materials and Institute
for Advanced Study, The Hong Kong University of Science & Technology (HKUST),
Clear Water Bay, Kowloon, Hong Kong, China
Studies on micro- and nano-materials and related devices are
hot topics in modern nanoscience and nanotechnology. Thanks
to their strong solid-state emission, AIE luminogens are found
to be promising materials for fabricating optoelectronic devices
with micro-dimensions.3,6 Fabrication of micro- and nano-
devices from AIE molecules and studies on their eld emission
and optical waveguide properties, however, are rarely reported.7
Tang and coworkers introduced a mesogenic unit between two
TPE units, and the resulting molecules can self-assemble into
cBeijing National Laboratory for Molecular Sciences, Key Laboratory of
Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China
dGuangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory,
State Key Laboratory of Luminescent Materials and Devices, South China University
of Technology (SCUT), Guangzhou 510640, China
† Electronic supplementary information (ESI) available: 1H NMR and 13C NMR
spectra of TPE-HPH-Bar and its precursor, PL spectra in the
dichloromethane/hexane mixture and SEM images obtained from acetonitrile
solution and the DMSO/ethanol mixture. See DOI: 10.1039/c3tc32161d
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highly uorescent (helical) nanobers.8 They also studied the nanorods and nanotubes from different solvent systems in the
self-assembly behaviours of silole derivatives, which can form presence of melamine. Its microrods exhibit optical wave-
crystalline microbers and further microrods from their THF/ guiding properties with a low optical loss. Such results make
ethanol solutions.9 Tian reported the nano-ring structures of TPE-HPh-Bar have an array of optoelectronic and biological
AIE-active anthracene derivatives formed from THF/water applications.
mixtures.10 While conditions and mechanisms for the forma-
tion of these self-assembly morphologies have been discussed,
their opto-electronic properties have not been studied in detail.
Experimental section
Materials
A previous study has shown that non-covalent interactions
such as hydrogen bonding, p–p stacking, and electrostatic
interactions are the main driving forces for self-assembly to
ordered micro- and nano-structures.11 The mode of self-
assembly, on the other hand, can be realized by tuning the
physical conditions such as temperature, concentration, time,
and addition order of different solutions. Recently, Zheng
published a TPE macrocycle, which can facilely self-assemble
into crystalline and hollow spheres with both intrinsic and
extrinsic pores. Ultrasonic treatment on the hollow spheres can
transform them into bird nests consisting of nanorods.12
Another way to tune the self-assembly behaviour is to utilize
the co-assembly effect of different compositions. In this
strategy, the addition of another composition can induce the
former to self-assemble into a more ordered structure so as to
improve the morphology and modify the physical and chemical
properties.13 One good example is the pair of melamine and
barbituric acid, whose self-assembly behaviours have been
studied for decades due to their strong hydrogen bonding
between molecules.14 Zhang utilized hydrochloric acid and tri-
ethylamine to control the self-assembly behaviour of melamine
and alkylated melamine. The chloride ion connects the mela-
mine molecules by forming hydrogen bonds with their amino
groups and changes the morphologies from 3D networks to 2D
micro-sheets.15 Researchers also utilized the strong interaction
between melamine and barbituric acid to sense melamine in
food and milk.16 Sanji functionalized TPE with cyauric acid, a
counterpart of barbituric acid with a similar structure, and
utilized the resulting luminogen to detect melamine in milk by
virtue of the AIE property of TPE.17
Tetrahydrofuran (THF) was distilled from a sodium benzophe-
none system in a nitrogen atmosphere under normal pressure
immediately prior to use. Triethylamine was dried using potas-
sium hydroxide pellets. Other solvents, such as diethyl ether,
n-hexane, dichloromethane (DCM), methanol and dimethyl
sulfoxide (DMSO), were of high purity and used as received. 2-(4-
Ethynylphenyl)-1,1,2-triphenylethene (1) was synthesized
according to our previously reported procedures.18 4-Bromo-2,5-
bis(hexyloxy)benzaldehyde (2) was synthesized from hydroqui-
none using the literature method.19 Other reagents such as
bis(triphenylphosphine)palladium(II) chloride, triphenylphos-
phine, cuprous iodide, liquid bromine, n-butyllithium and
N-formylpiperidine were purchased from commercial compa-
nies and used directly without further purication.
Instruments
1H and 13C NMR spectra were measured on a Bruker ARX 400
spectrometer using deuterated chloroform as solvent and tetra-
methylsilane (TMS) as an internal standard. High-resolution
mass spectra (HRMS) were recorded on a GCT premier CAB048
mass spectrometer operated in a MALDI-TOF mode. Elemental
analysis was performed on an Elementar Vario EL Elemental
Analyzer. Absorption spectra were taken on a Milton Roy Spec-
tronic 3000 Array spectrometer. Photoluminescence (PL) spectra
were taken on a Perkin-Elmer LS 55 spectrouorometer. The PL
quantum yield of the solid powder was determined using a
calibrated integrating sphere. Scanning electron microscopy
(SEM) images were obtained on a JOEL 6390 SEM instrument.
The SEM samples were prepared by drop-casting the dye solu-
tion onto silicon substrates. The solvent was evaporated at room
temperature in the air. For samples prepared from dye solution
in dimethyl sulfoxide, they were further dried under vacuum
overnight. To measure the microarea PL spectra of the crystal-
line microrods of TPE-HPh-Bar obtained from its diethyl ether/
hexane mixture, the microrods were dispersed on a glass cover-
slip and excited with a UV laser (l ¼ 400 nm, Beamlok, Spec-
traphysics). The excitation laser was ltered with a band-pass
lter (400–450 nm) and then focused to excite the microrod with
a suitable objective. The collected PL emission was coupled to a
grating spectrometer (Acton, SP-2358) with a matched ProEm:
512B EMCCD camera (Princeton Instruments).
Herein, we report the synthesis and self-assembly behaviour
of a TPE derivative functionalized with barbituric acid (TPE-
HPh-Bar; Scheme 1). The luminogen exhibits the characteristics
of both AIE and twisted intramolecular charge transfer (TICT).
The strong D–A effect endows TPE-HPh-Bar with red emission.
TPE-HPh-Bar is also capable of self-assembling into nano-
spheres by slow evaporation of its pure acetonitrile solution or
Synthesis
2,5-Bis(hexyloxy)-4-{[4-(1,2,2-triphenylvinyl)phenyl]ethynyl}-
benzaldehyde (TPE-HPh). To a two-neck round-bottom ask
were added 2-(4-ethynylphenyl)-1,1,2-triphenylethene 1 (1 g,
2.81 mmol), 4-bromo-2,5-bis(hexyloxy)benzaldehyde 2 (1.03 g,
Scheme 1 Synthetic route to TPE-HPh-Bar.
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2.68 mmol), bis(triphenylphosphine)palladium(II) chloride CuI in a THF/Et3N mixture afforded TPE-HPh in a yield of above
(0.089 g, 0.126 mmol), cuprous iodide (0.024 g, 0.121 mmol) and 70%. TPE-HPh has a moderate solubility in common organic
triphenylphosphine (0.066 g, 0.251 mmol). The reaction mixture solvents such as THF, chloroform, dichloromethane and
were degassed and purged with nitrogen three times, aer which acetone. Reaction of TPE-HPh with barbituric acid in methanol
freshly distilled THF (30 mL) and dry triethylamine (10 mL) were under reux produced the nal product TPE-HPh-Bar.20 All the
injected via ꢁsyringes. The reaction mixture was stirred under compounds were characterized by standard spectroscopic
reux at 70 C for 8 h. Aer cooling to room temperature, the methods, from which satisfactory analysis data corresponding
formed precipitate was ltered and the solvent was removed to their molecular structures were obtained. TPE-HPh-Bar was
under vacuum. The crude product was puried on a silica-gel obtained as a red solid and possessed better solubility than TPE-
column using hexane/dichloromethane (4 : 1 v/v) as an eluent to HPh in common solvents. Besides, TPE-HPh-Bar has a good
afford the product as a yellow solid. Yield 73.4% (1.3 g). 1H NMR solubility in diethyl ether.
(400 MHz, CDCl3): d (TMS, ppm) 10.44 (s, 1H), 7.30 (d, 2H), 7.27
(s, 1H), 7.15–7.08 (m, 9H), 7.06 (s, 1H), 7.05–7.00 (m, 8H), 4.05–
4.00 (m, 4H), 1.87–1.77 (m, 4H), 1.50–1.46 (m, 4H), 1.37–1.31 (m,
8H), 0.93–0.85 (m, 6H). 13C NMR (100 MHz, CDCl3): d (TMS,
ppm) 189.42, 155.73, 153.82, 144.74, 143.63, 143.58, 143.47,
142.07, 140.38, 131.61, 131.58, 131.53, 131.50, 131.32, 128.04,
127.98, 127.88, 126.92, 126.85, 124.90, 120.92, 120.81, 117.48,
110.06, 97.80, 85.98, 69.53, 69.41, 31.74, 31.70, 29.32, 25.92,
25.88, 22.81, 22.79, 14.25, 14.23. HRMS (MALDI-TOF): m/z
660.3596 (M+, calcd 660.3603). M.p.: 99.3–100.2 ꢁC.
TPE-HPh-Bar. To a round-bottom ask were added TPE-HPh
(500 mg, 0.758 mmol) and barbituric acid (106 mg, 0.828 mmol)
in 40 mL of methanol. The mixture was stirred and heated to
reux under nitrogen for 24 h. The formed red precipitates were
ltered while hot and washed with methanol three times. The
red solid was dissolved in diethyl ether, aer which an appro-
priate amount of hexane was added. The resulting mixture was
le for recrystallization to acquire needle-like crystals in 61%
yield (0.356 g) 1H NMR (400 MHz, CDCl3): d (TMS, ppm) 9.06 (s,
1H), 8.30 (s, 1H), 8.18 (s, 1H), 8.11 (s, 1H), 7.29 (d, 2H), 7.13–7.08
(m, 9H), 7.05–7.01 (m, 8H), 6.98 (s, 1H), 4.06–4.02 (m, 4H), 1.86–
1.81 (m, 4H), 1.58–1.43 (m, 4H), 1.37–1.29 (m, 8H), 0.92–0.84 (m,
6H). 13C NMR (100 MHz, CDCl3): d (TMS, ppm) 163.06, 161.00,
155.25, 154.63, 152.73, 148.88, 144.92, 143.59, 143.48, 142.17,
140.40, 131.65, 131.59, 131.54, 131.50, 131.44, 128.06, 128.00,
127.90, 126.95, 126.88, 122.15, 116.88, 115.68, 114.89, 98.85,
86.66, 69.80, 69.69, 31.80, 31.67, 29.38, 29.24, 25.96, 25.85, 22.82,
22.78, 14.26, 14.21. HRMS (MALDI-TOF): m/z 770.3722 (M+,
calcd 770.3720). Anal. calcd for C51H50N2O5: C, 79.45; H, 6.54; N,
3.63. Found: C, 79.60; H, 6.606; N, 3.59. M.p.: 189.0–190.1 ꢁC.
Optical properties
Fig. 1 shows the UV spectra of TPE-HPh and TPE-HPh-Bar in
pure THF solutions. The absorption maxima of TPE-HPh and
TPE-HPh-Bar are located at 388 nm and 447 nm with molar
absorptivities of 2.63 ꢂ 105 L molꢀ1 cmꢀ1 and 2.99 ꢂ 105
L
molꢀ1 cmꢀ1, respectively, which are 63 nm and 122 nm red
shied from that of TPE owing to the extended conjugation as
well as the TICT effect.21 The formation of the D–A structure will
narrow the energy band gap for electronic transitions, as
reected by the red-shi UV absorption and PL emission. Since
the polarity of water is much higher than that of THF,
increasing the water content leads to higher polarity of the
solvent system and thus red-shis the emission due to the
TICT attribute.5 Owing to that the spectroscopic property of
TICT molecules is sensitive to environmental polarity, the TICT
portrait of molecules can be veried through spectroscopic
measurement. From Fig. S5,† it can be seen that with increased
solvent polarity, both UV absorption and PL emission red-shi.
The emission changes from about 545 nm in diethyl ether to
590 nm in methanol. Because of the higher electron-with-
drawing character of the barbituric acid unit than the aldehyde
group, the D–A interaction in TPE-HPh-Bar is stronger than that
in TPE-HPh, thus leading to larger red-shi in the absorption
maximum. The dilute THF solution (10 mM) of TPE-HPh-Bar
Preparation of nanoaggregates
Stock THF and dichloromethane solutions of TPE-HPh-Bar with
a concentration of 200 mM were prepared. Aliquots of the stock
solution were transferred to 10 mL volumetric asks. Aer
appropriate amounts of THF or dichloromethane were added,
water or hexane was added dropwise under vigorous stirring to
furnish 10 mM solutions with different water or hexane fractions
(0–95 vol%). PL measurements of the resulting solutions were
then carried out immediately.
Results and discussion
Scheme 1 shows the synthetic route to TPE-HPh-Bar. Sonoga-
Fig. 1 UV-vis spectra of TPE-HPh and TPE-HPh-Bar in pure THF
shira cross-coupling of 1 and 2 catalysed by Pd(PPh3)2Cl2 and solutions.
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shows a yellow emission at 545 nm upon UV irradiation. To characteristics of TPE-HPh-Bar. Therefore, TPE-HPh-Bar shows
check whether TPE-HPh-Bar is AIE-active, we added water, a TICT plus AIE characteristics in the highly polar THF/H2O
non-solvent for TPE-HPh-Bar, into its THF solution and exam- mixture and typical AIE characteristics in the less polar
ined the PL change. When the water fraction was gradually dichloromethane/hexane mixture. The solid-state quantum
increased to 60 vol%, the emission from the solution was yield of TPE-HPh-Bar determined by an integrating sphere is
weakened progressively. Aerwards, the PL intensity starts to 37.4%, which is quite high among the red-emissive semi-
rise and reaches its maximum at 80% water fraction (Fig. 2). conductors. This makes TPE-HPh-Bar promising for the fabri-
Below 60% water fraction, the molecules do not form sufficient cation of optoelectronic devices.
aggregates, and the TICT effect dominates the emission
behaviour: since water is more polar than THF, with increase in
water content, the increased polarity will induce intensity-
Self-assembly
reduced and red shied emission. Above 60% water fraction,
the AIE effect overwhelms the TICT effect, and as a whole, the
emission begins to increase and TPE-HPh-Bar mainly shows the
AIE effect. Further increment of the water fraction has, however,
decreased slightly the light emission. On the other hand, the PL
spectrum shis progressively to the redder region with
increasing the water content. At 90% water fraction, the PL
maximum is located at 630 nm, which is 100 nm red-shied
from that in pure THF solution. Such PL change is not unusual
and has been observed in some AIE-active luminogens with D–A
structures. With a gradual increase in the water content, the
polarity of the THF/water mixture becomes higher. This
strengthens the TICT effect and thus weakens and red-shis the
light emission of TPE-HPh-Bar. At water content higher than
60%, the solving power of the solvent mixture becomes so poor
that it induces the TPE-HPh-Bar molecules to aggregate. This
activates the AIE process and hence enhances the light emis-
sion. The slight drop in PL intensity at high water content is due
to the precipitation of the large-sized aggregates, which
decreases the effective dye concentration in the solution. To
diminish the TICT effect on the AIE process, we repeated the PL
measurement in DCM with a less polar solvent of hexane. The
addition of hexane has little inuence on the polarity of the
whole system, thus eliminating the polarity effect. As shown in
Fig. S6,† the PL intensity gradually increases with increasing the
hexane fraction. Meanwhile, the emission maximum blue shis
from 587 nm in DCM to 538 nm in the DCM/hexane mixture
with 95% hexane fraction. This result truly veries the AIE
Self-assembly is an efficient bottom-up method to form mate-
rials with ordered micro- and nano-structures or even supra-
molecular structures. Interestingly, upon natural evaporation of
a few drops of acetonitrile solution of TPE-HPh-Bar on the
silicon substrate, nanospheres are formed. The nanospheres
are uniform with size tunable by the solution concentration
(Fig. 3 and Fig. S7†). The higher the concentration, the larger is
the size of the formed nanospheres. Addition of ethanol or
water into the acetonitrile solution causes no change in the self-
assembly morphology, but the size of the nanoparticles
becomes smaller at the same experimental conditions. For
example, to achieve the same particle size of 500 nm, an
acetonitrile/ethanol mixture (1 : 1 v/v) with a concentration of
50 mM should be used, which is ve times more concentrated
than that of the acetonitrile solution. Even smaller-sized
nanospheres are obtained from the acetonitrile/water mixture
(1 : 1 v/v). Since ethanol and water are both poor solvents for
TPE-HPh-Bar, addition of these solvents into its acetonitrile
solution will aggregate its molecules quickly, leading to the
formation of nanospheres with small sizes. We then investi-
gated the effect of melamine on the self-assembly behavior of
TPE-HPh-Bar. Instead of nanospheres, nanorods are formed
from acetonitrile solution containing melamine (Fig. 4), whose
fraction increases with increasing the amount of melamine in
the solution. Some tubular structures are also formed at the
Fig. 2 (A) PL spectra of TPE-HPh-Bar in THF/water mixtures with
different water fractions (fw). Concentration: 10 mM; excitation wave- Fig. 3 (A and B) SEM images of micro- and nano-spheres formed by
length: 447 nm. (B) Plot of relative PL intensity (I/I0) and the emission slow evaporation of TPE-HPh-Bar solution in acetonitrile with
maximum versus the composition of the THF/water mixture of TPE- concentrations of (A) 100 mM and (B) 10 mM. (C and D) SEM image of
HPh-Bar. I0 ¼ emission intensity in pure THF solution. Inset: fluores- nanospheres formed by slow evaporation of TPE-HPh-Bar solution
cent photographs of TPE-HPh-Bar in THF/water mixtures with water (50 mM) in (C) acetonitrile/ethanol and (D) acetonitrile/water mixtures
fractions of 0% and 95% taken under UV irradiation.
(1 : 1 v/v) at room temperature.
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same time. Some papers proposed the transition mechanism
from nanorods and nano-sheets to nanotubes,22 in which the
melamine molecules induce an ordered arrangement of TPE-
HPh-Bar through intermolecular hydrogen bonding in certain
dimensions.
Since acetonitrile is not a good solvent for melamine, only
part of the TPE-HPh-Bar molecules can interact with the mela-
mine molecules. Thus, we used DMSO as solvent for the
morphology study as melamine has good solubility in this
solvent. However, its low volatility requires us to modify the self-
assembly process. First, a DMSO solution of TPE-HPh-Bar was
prepared and an appropriate amount of melamine solution in
DMSO was then added. The resulting mixture was le at room
temperature for some times, aer which ethanol was added.
The mixture was then dropped on the silicon substrate to allow
solvent evaporation under ambient conditions. As the molec-
ular size of TPE-HPh-Bar is much larger than melamine, a
higher relative amount of melamine was used to enable
complete interaction between the two molecules. In the pres-
ence of 10 equivalents of melamine, nanotubes with lengths of
about 30 mm and diameters of about 50 nm are formed from the
DMSO/ethanol mixture (1 : 1 v/v) (Fig. 5). Most of the ends of the
tubes are not sealed, presumably due to the etching effect of
DMSO. Such morphology is rarely observed in adducts of bar-
bituric acid and melamine. Increasing the melamine amount to
25 equivalents exerts little change on the thickness of nanorods
but has sealed the tube ends. Besides, other solvent systems
including 1,4-dioxane, THF, CH3CN, CH3CN/H2O, and CH3CN/
ethanol were used to study the self-assembly behaviour. In most
cases, nanosphere structures are obtained in the absence of
melamine. But with the addition of melamine, rods will form in
the solvents (see Fig. S9†).
Fig. 5 SEM images of micro- and nano-aggregates formed by slow
evaporation of TPE-HPh-Bar solution (10 mM) in the DMSO/ethanol
mixture (1 : 1 v/v) at room temperature in the presence of (A and B)
10 and (C and D) 25 equivalents of melamine.
effect on the formation of opened nanotubes. The good self-
assembly capability of TPE-HPh-Bar makes it to nd wide
applications in micro- and nano-devices such as light trans-
mission and eld emission.
Optical waveguide
TPE-HPh-Bar can facilely self-assemble into crystalline micro-
rods from the diethyl ether/hexane mixture, as revealed by the
X-ray diffraction and the uorescent photograph shown in
Fig. 6. However, their sizes are too small to be analysed by
single crystal X-ray diffraction. We tried to grow better samples
in other solvent systems such as chloroform, acetonitrile, and
chloroform/pentane and chloroform/hexane mixtures but
failed to obtain any suitable specimen. Nevertheless, the
appropriate length and regular shape of the microrods
encourage us to study their optical wave-guiding properties.23
As suggested by the uorescent photograph shown in Fig. 7A,
the nanorod surface shows no obvious defect. When the
We also studied the self-assembly morphology by TEM but
observed no such micro- and nano-structures, probably due to
the different sample preparation procedure. When the
volume ratio of DMSO to ethanol is changed from 1 : 1 to 1 : 4
v/v, the micro- and nano-structures become smaller and
irregular (Fig. S8†). The TPE-HPh-Bar molecules will aggre-
gate in the solvent mixture with a large amount of ethanol,
thus impeding their co-assembly with the melamine mole-
cules. Lowering the DMSO content, on the other hand, has
diminished the etching effect, which may have an adverse
Fig. 4 (A) SEM image of micro-aggregates formed by slow evapora-
tion of TPE-HPh-Bar solution in acetonitrile (10 mM) in the presence of Fig. 6 X-ray diffractogram of microrods of TPE-HPh-Bar formed from
1 equivalent of melamine at room temperature. (B) Chemical structure the diethyl ether/hexane mixture. Inset: fluorescent photograph of
of melamine.
TPE-HPh-Bar microrods taken under UV irradiation.
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Acknowledgements
The work reported in this paper was partially supported by the
National Basic Research Program of China (973 program,
2013CB834701), the Research Grants Council of Hong Kong
(HKUST2/CRF/10 and N_HKUST620/11), and the University
Grants Committee of Hong Kong (AoE/P-03/08 and T23-713111-
1), and B. Z. Tang thanks the support of the Guangdong Inno-
vative Research Team Program (201101C0105067115).
Notes and references
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2 (a) Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun.,
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Chem. Soc. Rev., 2011, 40, 5361.
Fig. 7 (A) Microarea fluorescent images of the TPE-HPh-Bar microrod
obtained by exciting at seven different positions with a focused UV
laser (400 nm); (B) spatially resolved PL spectra of the TPE-HPh-Bar
microrod recorded at the right rod ends labelled with a–g in 7A; (C)
plot of peak intensity versus the distance between the excited sites and
the emitted ends.
3 Z. Zhao, J. W. Y. Lam and B. Z. Tang, J. Mater. Chem., 2012,
22, 23726.
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Chem. Mater., 2012, 24, 812.
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4452; (b) A. Maliakal, G. Lem, N. J. Turro, R. Ravichandran,
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6 (a) W. Z. Yuan, P. Lu, S. Chen, J. W. Y. Lam, Z. Wang, Y. Liu,
H. S. Kwok, Y. Ma and B. Z. Tang, Adv. Mater., 2010, 22, 2159;
(b) Y. Liu, S. Chen, J. W. Y. Lam, P. Lu, R. T. K. Kwok,
F. Mahtab, H. S. Kwok and B. Z. Tang, Chem. Mater., 2011,
23, 2536; (c) B.-K. An, S. H. Gihm, J. W. Chung, C. R. Park,
S.-K. Kwon and S. Y. Park, J. Am. Chem. Soc., 2009, 131,
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4564; (b) A. Patra and U. Scherf, Chem.–Eur. J., 2012, 18,
10074.
microrod was excited by 400 nm UV light produced by an 800
nm laser device at different positions, the emission was
detected at the rod end, whose intensity becomes lower when
the excitation spot moves gradually to the opposite rod end
(Fig. 7B). By plotting its intensity (Iend) and its distance to the
irradiated area (x), the optical loss coefficient (a) was calculated
by using a single exponential tting [Iend/Iex ¼ Aexpꢀax, where
Iex is the PL intensity at the excited spot and A is the ratio of the
light escaped from the excited area and that propagated along
the ber]24 and was determined to be 0.137 dB mmꢀ1 (Fig. 7C).
This value is quite low among the red-emissive AIE-active
luminogens reported so far.25 The partial overlapping between
the absorption and emission spectra is one of the factors to
diminish the optical wave-guiding effect.26 The large Stokes
shi (ꢃ100 nm) and the smooth and at end facets of TPE-
HPh-Bar crystals help in minimizing the optical loss, thus
contributing to their good optical wave-guiding behaviours.
Applications of TPE-HPh-Bar in micro/nano-laser27 and spec-
tral modulation28 are currently being carried out in our
laboratory.
8 W. Z. Yuan, F. Mahtab, Y. Gong, Z.-Q. Yu, P. Lu, Y. Tang,
J. W. Y. Lam, C. Zhu and B. Z. Tang, J. Mater. Chem., 2012,
22, 10472.
9 Z. Zhao, S. Chen, X. Shen, F. Mahtab, Y. Yu, P. Lu,
J. W. Y. Lam, H. S. Kwok and B. Z. Tang, Chem. Commun.,
2010, 46, 686.
10 B. Xu, J. He, Y. Dong, F. Chen, W. Yu and W. Tian, Chem.
Commun., 2011, 47, 6602.
Conclusion
A red-emissive barbituric acid-functionalized TPE derivative, 11 L. Maggini and D. Bonifazi, Chem. Soc. Rev., 2012, 41, 211.
TPE-HPh-Bar, was designed and synthesized. TPE-HPh-Bar 12 S. Song and Y.-S. Zheng, Org. Lett., 2013, 15, 820.
exhibits both the TICT effect and the AIE phenomenon. It can 13 (a) H. Y. Lee, S. R. Nam and J.-I. Hong, J. Am. Chem. Soc.,
¨
self-assemble into nanospheres through slow evaporation of its
acetonitrile solution. In the presence of melamine, nanorods
and (un)sealed nanotubes are generated. The crystalline
microrods of TPE-HPh-Bar grown from diethyl ether/hexane
2007, 129, 1040; (b) W. Yang, Y. Jiang, J. Zhuang, N. Lu,
S. Chen and T. Li, Sci. China, Ser. B: Chem., 2001, 44, 478;
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X. Wang, Chem. Mater., 2007, 19, 14.
solution show a good optical waveguiding effect with a low 14 (a) Q. Feng, M. Wang, B. Dong, C. Xu, J. Zhao and H. Zhang,
optical loss. Such attributes make TPE-HPh-Bar have wide
applications in many areas such as biological imaging and
optoelectronic nano-devices.
CrystEngComm, 2013, 15, 3623; (b) H. Koyano, K. Yoshihara,
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