Tetraphenylethene-triphenylene oligomers with an aggregation-induced emission effect and discotic columnar mesophase

Article abstract of DOI:10.1039/c3ra41874j

Tetraphenylethene derivatives not only exhibit "aggregation-induced emission" (AIE) effects, but also can form a mesophase with a turbine-like rigid core. Here we report the synthesis of three tetraphenylethene-triphenylene oligomers 7, 8E and 8Z by a CuS

Full text of DOI:10.1039/c3ra41874j

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Tetraphenylethene–triphenylene oligomers with an
aggregation-induced emission effect and discotic
columnar mesophase3
Cite this: RSC Advances, 2013, 3,
14099
Wen-Hao Yu,a Chao Chen,a Ping Hu,*a Bi-Qin Wang,a Carl Redshawab and Ke-
Qing Zhao*a
Tetraphenylethene derivatives not only exhibit ‘‘aggregation-induced emission’’ (AIE) effects, but also can
form a mesophase with a turbine-like rigid core. Here we report the synthesis of three tetraphenylethene–
triphenylene oligomers 7, 8E and 8Z by a CuSO4/sodium ascorbate catalyzed alkyne–azide cycloaddition
reaction (CuAAC), and the successful separation of the cis- and trans-isomers by column chromatography.
The structure and conformation of the oligomers were fully characterized. The photophysical properties
were studied by using UV-vision and photoluminescent (PL) spectroscopy. The morphology of the
aggregates was investigated by scanning electron microscopy (SEM), and the thermal stability and
mesomorphic properties were investigated using thermal gravimetric analysis (TGA), polarized optical
microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The results revealed
that the oligomers possess an AIE effect, but display no mesomorphism. However, 7 exhibits a hexagonal
columnar mesophase when mixed with 2,4,7-trinitrofluorenone (TNF), forming a charge transfer complex.
Received 18th April 2013,
Accepted 4th June 2013
DOI: 10.1039/c3ra41874j
www.rsc.org/advances
alignment behavior to large size domains, and possess high
charged carrier mobility. Triphenylene discotic oligomers
usually possess both the properties of polymers and small
molecules.5b
1. Introduction
Discotic liquid crystals (DLCs) exhibit the property of self-
organization into a highly ordered columnar mesophase as
well as fast charge carrier mobility, and are emerging as a new
kind of organic semiconductors.1 In particular, a good
solubility in organic solvents makes DLCs very useful materials
for printed electronics, and they have been utilized in organic
photovoltaic cells,2 organic field effect transistors,3 and
organic light emitting diodes.4 Triphenylene derivatives are
one of the most important DLCs,5 and have been commercially
applied in optical compensation films for widening the view
angle of liquid crystal displays.6 In recent years, oligomers of
triphenylenes have attracted considerable interest and much
effort has been paid to the design and synthesis of new types
of such oligomers. In order to get improved optical and
electronic properties, these oligomers are usually designed by
grafting triphenylene discotic unit onto various functional
frameworks, such as perylene,7 crown ether,8 porphyrine,9
DLCs conventionally consist of a large aromatic core and
several peripheral chains. However, molecules devoid of a flat
core can also form mesophases by the micro-segregation of the
polar cores from the lipophilic alkyl chains rather than the
intra-columnar p–p stacking between the core regions, such as
tetraphenylethene (TPE) derivatives,12 triphenylamine deriva-
tives,13 octahedral metal complexes,14 dendrimers and related
compounds.15 The propeller-shaped TPE derivatives consist-
ing of four phenyl rotors and one olefinic stator display very
interesting chemical and physical properties.
Tang et al.16 in 2001 first reported the unusual photo-
physical phenomenon of ‘‘aggregation-induced emission’’
(AIE), for which the propeller-like molecular structure of silole
derivatives was non-emissive in solution, but could turn ‘‘on’’
light emission by aggregate formation, due to the restricted
intramolecular rotation (RIR) in the aggregates.17 Like silole
derivatives, TPE derivatives have also exhibited AIE effects.
Molecules with an AIE effect provide a unique platform for
exploitation as novel optical materials and sensors.18
Luminescent LCs show both a light-emitting property and
self-organization behavior, and have been attracting consider-
able interest over the past decade.19 Recently, Tang et al.20
reported a series of high solid-state efficiency luminescent LCs
which consist of an AIE-active TPE core and four peripheral
calixarene,10 fullerene C60 etc. The polymeric materials can
form a glass-state and exhibit fine mechanic properties, and
low-molecular weight materials tend to show homeotropic
11
aCollege of Chemistry and Materials Science, Sichuan Normal University, Chengdu,
Sichuan, 610066, P. R. China. E-mail: hp_x@163.com; kqzhao@sicnu.edu.cn;
Fax: +86-28-84764743
bDepartment of Chemistry, The University of Hull, Hull, HU6 7RX, UK
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
c3ra41874j
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nylene 1.22a Hydroxytetraphenylenes 3 and 4 were synthesized
by the McMurry coupling reaction.26 The etherification of 3
and 4 with 3-bromopropyne afforded the propargyl-attached
TPE 5 and 6, respectively. A Cu-catalyzed click reaction at room
temperature between 5/6 and azido-functionalized tripheny-
lene 2 then afforded three tetraphenylene-triphenylene oligo-
mers 7/8 (see Scheme 1).
mesogenic substituents. XRD results have shown that the TPE
cores are still capable of forming columnar mesophases.
Since Sharpless first introduced the new synthetic strategy
of click chemistry,21 copper catalyzed alkyne–azide cycloaddi-
tion reactions (CuAAC) have been widely used in the fields of
biological, material, and medicinal chemistry, particularly as
such a strategy has high selectivity and efficiency, functional
group tolerance, and uses simple reaction conditions. In
recent years, much attention has been paid to the synthesis of
DLCs by using the CuAAC reaction,22 and we also have been
focusing on the synthesis of new triphenylene DLCs by using
the click reaction.23 Han et al.24 have using the click reaction
to synthesize novel fluorescent probes based on the TPE motif
for biological studies. Herein, we report the synthesis of three
TPE–triphenylene oligomers by the CuAAC click reaction and
their AIE and liquid crystalline properties; Scheme 1 shows the
structures and synthetic route.
The characterization of the synthesized target molecules 7, 8
was carried out using various spectroscopic methods (see the
ESI ). For example, the formation of the triazole ring was
3
confirmed by the single peak at d 7.60 ppm in the 1H NMR
spectrum and the two peaks at d 122.4 and 144.1 ppm in the
13C NMR spectrum. The IR spectra of the oligomers displayed
a weak band at 3081 cm21 and an extreme sharp band at 1619
cm21 due to the LC–H stretching and CLC, NLN stretching of
the triazole ring. The three oligomers gave satisfactory mass
data by high resolution mass spectrometry (EI, FAB, MALDI-
TOF) and elemental content by elemental analysis.
The four phenyl rings of the TPE were twisted against the
ethene-core and oriented in the same direction.18c,27 8 with
one TPE core and two TP moieties would have Z and E
stereoisomers. Disubstituted TPE derivatives had not been
separated in most of the previous reports, and they have been
simply studied for their chemical and physical properties as a
mixture. In this report, we have successfully separated the
isomers by using column chromatography and carefully
selecting the elution conditions by using a mixed solvent of
dichloromethane, petroleum ether, and ethyl acetate as the
eluent, affording the pure E and Z isomers as pale white solids.
2. Results and discussion
2.1 Synthesis and characterization
2-Hydroxy-3,6,7,10,11-pentakis(hexyloxy)triphenylene as the
starting material was prepared according to the reported
method.25 v-Bromo-substituted triphenylene 1 was prepared
in good yield by the reaction of monohydroxytriphenylene with
excess 1,6-dibromohexane in the presence of K2CO3/acetone.
Subsequently, 2 was prepared in a high yield through the
nucleophilic substitution reaction of NaN3 with bromotriphe-
Scheme 1 Synthesis of tetraphenylethene–triphenylene oligomers.
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Very recently, Tang et al.28 also successfully separated E and Z
disubstituted TPE derivatives.
The polarity and melting point (m.p.) of a compound is
related to its molecular symmetry: the higher the molecular
symmetry, the lower the polarity and higher the m.p. In the
present case, the E isomer should have lower polarity than the
Z isomer, and with regard to the column chromatography, the
first eluted compound is 8E with lower polarity. Furthermore,
we confirmed their m.p. by the use of DSC measurements, and
the first eluted compound displayed a m.p. of 150 uC, whilst
the second elutant had a m.p. of 92 uC. The first compound
was relatively easy to crystallize and showed a thermal
crystallization peak in the second heating run of DSC curves.
We assigned the first eluted compound to be the trans-
structure 8E given its lower polarity and higher m.p.; the
second eluted compound was the cis-structure 8Z. The m.p. of
7, 8E and 8Z measured by DSC were 112 uC, 150 uC and 92 uC,
respectively.
Fig. 2 UV-Vis absorption spectra of oligomers in THF.
Z and E isomers can be distinguished by NMR analysis.18c,28
So we tested 8E, 8Z and a mixture consisting of an equal
of 7, 8E and 8Z. All three compounds displayed high thermal
stability with 5% weight loss occurring above 300 uC.
1
amount of the 8Z/8E isomers. The full H NMR spectra of the
isomers are similar, but careful observation can find the
differences. The most obvious spectral difference lies in the
tetraphenylethene aromatic region at the chemical shift area
of 6.65–7.15 ppm. The partially magnified spectra in the
aromatic region is depicted in Fig. 1. It reveals that some of the
resonance peaks of the 8Z isomer are downfield-shifted from
those of its 8E counterpart and some of the resonance peaks
are opposite. The 8Z isomer resonates at d #6.73 and 6.95
(Fig. 1A), where the 8E isomer does not (Fig. 1C). On the other
hand, the 8E isomer exhibits a big resonance peak at d #6.68
and 6.89. However, all of the resonance absorption peaks of
the mixture 8Z/8E are present (Fig. 1B). Obviously, these
differences make it possible to monitor conformation changes
of the isomers by NMR spectroscopy.
2.2 Aggregation-induced emission behaviour
The UV-vis absorption spectra and fluorescent spectra of the
oligomers 7, 8E, 8Z, 2,4,7-trinitrofluoren-9-one (TNF) and 7–
TNF complex were measured. The oligomers in THF exhibited
two absorption bands at 270 nm and 290 nm, as shown in
Fig. 2. The stronger band at 270 nm was attributed to the p–p*
transition of the aryl groups and the weaker band at 290 nm
was due to the n–p* transition of the triazole ring. For the 7
and 7–TNF complex, the lower concentrated solutions showed
red-shifted absorption bands. The red shifts of UV-vis spectra
might be attributed to the concentration-dependent micro-
environment of the triphenylene cores of 7 and 7–TNF, lead to
the flat aromatic core effective stacking through p–p interac-
tions. In a lower concentrated solution, the intermolecular p–p
stacking of triphenylene resulted in the higher HOMO level,
and the energy gap between the LUMO–HOMO is narrowed
and the absorption peaks at the lower concentration showed a
red shifted absorption band compared to that in the higher
concentrated solution. TNF is well known to form solid-state
charge-transfer complexes with various electron donors such
as triphenylenes, and these complexes exhibit charge-transfer
absorption bands in the visible spectral region. But the charge-
transfer band of a 7–TNF complex in diluted solution was not
observed in its absorption spectrum in Fig. 2, due to the low
molar absorption coefficients in solution.29
The thermal stability of the TPE–TP oligomers was
measured by using TGA. Fig. S2, ESI shows the TGA curves
3
Fig. 3 shows the photoluminescent (PL) spectra of 7.
Oligomers 7, 8E, and 8Z displayed good solubility in common
organic solvents, such as CH2Cl2, CHCl3, THF and DMF, but
poor solubility in water. Almost no PL signals were detected for
their dilute solutions in THF, revealing that the dyes were
practically non-luminescent when dissolved in good solvents.
As shown in Fig. 3A, the emission from the THF solution of 7
is so weak that its PL spectrum is virtually a flat line parallel to
the abscissa. However, when the water fraction was increased
(.40%) in the solution, the PL of 7 was switched on. When the
water fraction was further increased, the light emission
continuously intensified, and was accompanied by a blue-
Fig. 1 1H NMR spectra of (A) 8E, (B) mixture of 8E and 8Z (1 : 1), (C) 8Z in
chloroform-d.
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Fig. 4 SEM images of particles of 7 , prepared in a THF/water mixture with 40%
(A), 70% (B), 80% (C) and 90% (D) water fraction.
solutions, we investigated them by scanning electron micro-
scopy (SEM). The samples were prepared by adding water into
the THF solution of 7 and the aqueous mixtures were stood for
12 h to allow the molecules to be well self-assembled. Then,
the aqueous mixture was dropped on a silica grid and the
solvents were allowed to evaporate. As shown in Fig. 4,
oligomer 7 aggregated into spherical particles in the aqueous
mixtures. More interestingly, the aggregates of 7 were nano-
dimensional and their average diameters decreased from y1
mm, 620 nm, 500 nm and 310 nm when the water fraction in
the mixture was increased from 40 to 90%. The molecules
might agglomerate quickly to form small aggregates in
mixtures with higher water contents, while the molecules of
7 might slowly assemble in an ordered fashion to form larger
clusters at lower water content.31
Fig. 3 (A) Fluorescent spectra of 7 in water–THF (1025 mol L21) (percentages
are volume fractions of water) and (B) peak intensities with the volume fractions
of water; insets are the fluorescence emission images of 7 from 0% to 90%
volume fractions of water under UV light, excitation wavelength: 365 nm.
shift from 488 nm to 455 nm for the PL peak. This was due to
the aggregation morphology change of 7.18a,30 In a 90%
aqueous mixture, the emission intensity was about 500-fold
higher than that in pure THF solution (Fig. 3B). Similar effects
were observed for the dyes 8E and 8Z, but their fluorescence
was turned on when the water fraction increased to 70%. The
fluorescent intensity only increased by about a 100-fold when
2.3 Mesomorphic behavior
The liquid crystalline property of an organic compound is
determined by its molecular shape and intermolecular
interactions. Amongst the specific species available, charge-
transfer complexes have opened up the prospect of new
research. In order to increase intermolecular interactions and
thus induce the mesophase, we have doped the electron-
donating oligomers with electron-accepting TNF in different
molar ratios. The results are summarized in Table 1 and are
shown in Fig. 5.
the water fraction comprised 90% of the solvent (Fig. S3, ESI ).
3
The differences of fluorescence excitation and fluorescent
intensity of 7 and 8 is caused by the different degree of
molecular size and rigidity which resulted in the rotation
barrier. As water was a non-solvent for the TPE–TP dyes, the
dye molecules thus must have aggregated in the water–THF
mixtures with high water content. Apparently, the emissions of
the TPE–TP dyes were induced by aggregate formation; in
other words, they were AIE active. The RIR process is a
plausible mechanism for the AIE activity of such TPE–TP
oligomers.17 That is, in dilute solution, the active rotations of
the four phenyl rings against the ethylene core may have
effectively non-radioactively deactivated the excited states. The
enhanced light emission in the aggregate state is caused by the
restricted motions of the aromatic rotors.
From polarizing optical microscopy we observed that the 7–
TNF complex displayed a typical fan-shaped texture of the
columnar mesophase. From optical textures given in Fig. S4,
ESI, it is clear that the average domain size decreases and
3
becomes less defined on increasing the oligomer or TNF
concentration in the binary mixture. The mesophase to
isotropic transition temperature is affected remarkably by
the TNF proportion in the binary mixture. Only when the TNF
concentration is between 67% to 80%, can the complex obtain
To prove that aggregates of the oligomers 7, 8E and 8Z were
formed when large amounts of water was present in their THF
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Table 1 Thermal and thermodynamic properties of the synthesized oligomers and their composites by DSC (10 uC min21) or POM
Second heating scan
First cooling scan
Transition T (uC) and enthalpy change (DH, kJ mol21
)
Transition T (uC) and enthalpy
Compound/mixture [mole ratio]
change (DH, kJ mol21
)
7
Cr 112 (86.68) Isoa
Colh 134 Isob
—
b
7 : TNF [2 : 1]
7 : TNF [1 : 1]
7 : TNF [1 : 2]
7 : TNF [1 : 3]
7 : TNF [1 : 4]
7 : TNF [1 : 5]
8E
Iso 130 Colhb
Iso 172 Colhb
Iso 184 Colhb
Iso 184 Colhb
Iso 184 Colhb
Iso 118 Colh
—
Colh 174 Isob
Colh 186 Isob
Colh 186 Isob
Colh 186 Isob
Colh 120 Isob
Cr 150 (88.29) Isoa
Cr 92 (65.41) Isoa
8Z
—
a
b
From DSC and POM. From POM. Cr: Crystal phase; Colh: Hexagonal columnar phase; Iso: Isotropic phase.
the highest clearing point temperature, i.e. a suitable inducing
action can be obtain a stable mesophase. When the molar
ratio of donor to acceptor is 1 : 5, excess TNF will interfere
with the formation of the mesophase; when less than 1 : 1, the
of 100, 200, 210, 300, 410 respectively. In the wide-angle
region, a diffuse halo reflection at 2h = 15–24u was attributed
to the molten state alkyl-chain distances in the columnar
phase. At the 2h value of 26.37u, there was a broad peak
corresponding to ordered disc–disc stacking in the column.
The lattice parameter of the Colh phase was calculated to be
4.52 nm.1b When the molecules most extended conformation
was assumed, the molecular diameter is about 4.52 nm, which
was calculated by the MM2 method. This value is consistent
with the XRD result. Based on the XRD result and the MM2
calculation, we propose that the 7–TNF complex possessed a
hexagonal columnar phase, as schematically illustrated in
Fig. 7.
complex cannot effectively form a mesophase (Fig. S4, ESI ). It
3
is also notable that the disubstituted TPE–TP derivatives 8Z
and 8E doped with TNF did not show any mesomorphic
behavior. This may be associated with the anchor effect of the
triphenylene discogen.29,32 Oligomer 7 with four triphenylenes
discogen can more easily self-assemble than 8 with two
triphenylenes discogen when their formed charge-transfer
complex is doped with TNF. As we know, TNF is known to be
an efficient fluorescence quencher. So we tested the AIE effect
of the charge-transfer complex 7–TNF (mole ratio 1 : 4), and
found that it did not show any AIE effect. Such molecules may
be used as a fluorescent probe for detecting electron-deficient
aromatic compounds.
3. Conclusions
In summary, we have synthesized tetraphenylethene–triphe-
nylene oligomers 7, 8E and 8Z by the CuAAC click reaction,
and successful separated the E and Z isomers of 8 by using
column chromatography. The oligomers displayed aggrega-
tion-induced emission (AIE) behavior, specifically the tetra-
phenylethylene derivatives in a mixed solvent of THF–water.
We further used wide angle X-ray diffraction (XRD) to
confirm the mesophase structure. The result of the XRD
measurements of the charge-transfer complexes are shown in
Fig. 6. In the small-angle region, we noted that the reflections
at 2h = 2.27, 4.55, 5.95, 6.81 and 10.29u are characteristic of the
hexagonal columnar mesophase (Colh) with a d-spacing ratio
of 1, 1/!4, 1/!7, 1/!9, 1/!21 corresponding to the Miller indices
Fig. 5 Optical textures of the complex 7–TNF (1 : 4 mole ratio) at 120 uC,
Fig. 6 X-Ray diffraction pattern of 7–TNF (mole ratio 1 : 4) at 25 uC, sample
crossed polarizers.
slowly cooled from isotropic liquid.
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Financial support from the National Natural Science
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