Aldehyde end-capped terthiophene with aggregation-induced emission characteristics

Article abstract of DOI:10.1016/j.tet.2015.06.047

From the viewpoint of practical application, organic materials that are emissive in aggregate or solid state have been a hot research topic. This work demonstrates that 2,2′:5′,2″-terthiophene-5-carbaldehyde (TTA) exhibited aggregation-induced emission (AIE) property. The photophysical properties of the aldehyde in THF/H₂O mixtures were studied, and the aggregates formed with different water fractions were studied by scanning electron microscopy. Our results indicated that the presence of an electron deficient aldehyde group as the end-capped group should endow TTA with AIE characteristic. Furthermore, ordered nanoscale aggregates of TTA exhibited distinct AIE behavior when assembled in solutions with appropriate water content.

Full text of DOI:10.1016/j.tet.2015.06.047

Tetrahedron xxx (2015) 1e6
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Aldehyde end-capped terthiophene with aggregation-induced
emission characteristics
Jun Cheng , Xiaozhong Liang , Yaxiong Cao , Kunpeng Guo ,*, Wai-Yeung Wong a
a
a
a
a
,b,c,
*
a
Ministry of Education Key Laboratory of Interface Science and Engineering in Advanced Materials, Research Center of Advanced Materials Science and
Technology, Taiyuan University of Technology, Taiyuan 030024, PR China
Institute of Molecular Functional Materials, Department of Chemistry and Partner State Key Laboratory of Environmental and Biological Analysis,
b
Hong Kong Baptist University, Waterloo Road, Hong Kong, China
c
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of
Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
From the viewpoint of practical application, organic materials that are emissive in aggregate or solid state
Received 11 April 2015
Received in revised form 9 June 2015
Accepted 11 June 2015
Available online xxx
0
0
00
have been a hot research topic. This work demonstrates that 2,2 :5 ,2 -terthiophene-5-carbaldehyde
TTA) exhibited aggregation-induced emission (AIE) property. The photophysical properties of the al-
dehyde in THF/H O mixtures were studied, and the aggregates formed with different water fractions
(
2
were studied by scanning electron microscopy. Our results indicated that the presence of an electron
deficient aldehyde group as the end-capped group should endow TTA with AIE characteristic. Further-
more, ordered nanoscale aggregates of TTA exhibited distinct AIE behavior when assembled in solutions
with appropriate water content.
Keywords:
Aggregation-induced emission
Nanostructures
Terthiophene aldehyde
Luminescence
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
method to tackle the notorious ACQ.12 Following the concept of AIE,
various aggregated and solid-state emissive organic materials have
Organic materials possessing highly emissive feature in aggre-
gate or solid state have received increasing attention because of the
requirement of these materials in various optoelectronic applica-
tions, such as bioimaging, bioprobes, and organic light-emitting
been prepared through rational coupling of propeller-like AIE
13e18
building blocks.
To satisfy the fundamental and practical de-
mand for optoelectronic active materials, organic materials without
propeller-like conformations exhibiting AIE characteristics present
1
e5
19e22
diodes (OLED), and so on.
While a number of materials are
a great potential.
Therefore, there is still much work to develop
known to be highly fluorescent in dilute solutions, most of them
tend to show a decreased or quenched fluorescence in the aggre-
gate or solid state due to the discouraging non-radiative relaxation
pathways.6 Such aggregation-caused quenching (ACQ) has greatly
limited the practical applications of organic materials because most
of these materials need to be used in the aggregated or solid state.
classical organic materials with AIE features.
Thiophene and its aldehyde derivatives are highly reactive or-
ganic building blocks that provide numerous advantages for syn-
thesizing useful materials such as hole transporters, sensitizers,
,7
2
3e26
0 0 00
By comparison, 2 :5 ,2 -
organic luminophors, and sensors.
terthiophene-5-carbaldehyde (TTA, Fig. 1), a long known com-
pound, which can be regarded as an -aldehyde end-capped olig-
8
e11
To date, various tactics have been developed to tackle ACQ.
a
However, many of them operate at the expense of other useful
properties of the compounds. Development of organic materials to
address this problem is still a challenge to researchers.
omer of thiophene with three thiophene units, is commonly used
as building block to construct light harvesting dyes, medicals,
2
7e32
fluorescence chemosensor, memory device, etc.
To the best of
Aggregation-induced emission (AIE) phenomenon, in which
non-luminescent molecules or weak emitters with propeller-like
conformations have been shown to exhibit efficient emission be-
cause of aggregate formation, has been regarded as a promising
S
S
S
S
CHO
CH OH
2
S
S
TTA
TTOH
*
Corresponding authors. E-mail addresses: guokunpeng@tyut.edu.cn (K. Guo),
rwywong@hkbu.edu.hk (W.-Y. Wong).
Fig. 1. Molecular structure of TTA and TTOH.
http://dx.doi.org/10.1016/j.tet.2015.06.047
0
040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
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2
J. Cheng et al. / Tetrahedron xxx (2015) 1e6
our knowledge, the fluorescent properties of TTA in aggregate state
are rarely studied that undoubtedly limit developing its practical
applications much further. Herein, our investigations reveal that
the end-capped group of aldehyde should endow TTA with AIE
characteristic. Furthermore, its AIE mechanism can be mainly
assigned to the ordered nanoscale aggregates formed when as-
sembled in solutions with appropriate water content.
Solvents were removed by rotary evaporation and the residue was
purified by silicon gel column chromatography to afford TTA as
1
a yellow solid (0.84 g, yield 86%). H NMR (600 MHz, CDCl
3
, ppm):
d
¼9.83 (s, 1H), 7.64 (d, 1H), 7.25e7.10 (m, 4H), 7.09 (d, 1H), 7.02 (d,
þ
8 3
1H). MALDI-TOF: m/z [M] calcd C13H OS , 276.3970; found:
276.3968.
0
0
00
2
.2.2. 2,2 :5 ,2 -Terthiophene-5-methanol (TTOH). To a stirred so-
lution of TTA (0.50 g, 1.80 mmol) in 25 mL of dry THF was added
sodium borohydride (0.07 g, 1.80 mmol). After stirring the mixture
for 4 h at room temperature, the THF was evaporated, dilute acid
2
2
. Experimental
.1. Materials and characterization
(1 M HCI) was added. Yellow precipitate was filtered and washed
with water for several times, and then the crude product was
TTA and (5-(5-(thiophen-2-yl)thiophen-2-yl)thiophen-2-yl)
recrystallized in hexane/ethyl acetate mixture to afford TTOH as
methanol (TTOH, Fig. 1), a reduction compound of TTA, were syn-
thesized using synthetic method described in Scheme 1. Solvents
1
a yellow powder (0.49 g, yield 98%). H NMR (600 MHz, CDCl
3
,
ppm):
d
¼7.23 (d, 1H), 7.18 (d, 1H), 7.07e7.05 (q, 2H), 7.04e7.02 (q,
1
were purified according to the standard procedures. H NMR
þ
2H), 6.92 (d, 1H), 4.81 (s, 2H). MALDI-TOF: m/z [M]
spectra were recorded at room temperature on Bruker AV 600 MHz.
Data are listed in parts per million (ppm) on the delta scale (d). The
calcd C13
H10OS
3
, 278.4129; found: 278.4126.
S
S
S
S
POCl
3
, DMF, CH
2
Cl
2
S
S
CHO
S
S
0oC to rt
TTA
NaBH , THF
4
S
S
CHO
CH OH
2
S
S
TTA
TTOH
Scheme 1. Synthesis of TTA and TTOH.
splitting patterns are designated as follows: s (singlet); d (doublet);
t (triplet); q (quartet); m (multiplet) and br (broad). Mass spectra
measured on Microflex MALDI-TOF MS. UVevis spectra were
recorded on a HITIACH U-3900 spectrometer. Photoluminescent
3. Results and discussion
3.1. Photophysical properties of TTA
(
PL) spectra were recorded on a HORIBA FluoroMax-4 spectrome-
TTA is soluble in common organic solvents such as acetone,
dichloromethane, and tetrahydrofuran (THF). The absorption and
ter. The absolute fluorescence quantum yields of solution (10 M)
m
and solid powder were measured on HORIBA FluoroMax-4 (excited
at 365 nm) by using a calibrated integrating sphere. The quartz
cuvettes used were of 1 cm in path length. The scanning electron
microscopy (SEM) studies were performed using a JEOL JEM-6700F
scanning electron microscope. One drop of the solution was placed
on a silicon slice, which was then dried in air. Dynamic light scat-
tering (DLS) measurements were conducted on a Delsa PN
A54412AB Nano Submicron Grain Particle Size Analyzer.
emission spectra of TTA in dilute THF (10 mM) at room temperature
are shown in Fig. 2. TTA shows relatively strong absorption in the
350e440 nm region, with the absorption maximum located at
397 nm, which corresponds to the
emission of TTA in dilute THF solution (10
UV (365 nm) irradiation. The peak emission wavelength of TTA was
p
e
p
*
transitions. A weak blue
mM) was observed under
F
observed at 479 nm, exhibiting a fluorescence quantum yield (F )
of 7.59%.
Single-crystal X-ray diffraction of TTA was performed on
a Bruker SMART APEX II diffractometer with Mo K
a
radiation
¼0.71000 A). The structure was solved with direct method
SHELX-97) and refined with full-matrix least-squares technique.
ꢀ
(l
(
All non-hydrogen atoms were refined anistropically and hydrogen
atoms were geometrically placed.
2
.2. Synthesis of TTA and TTOH
0
0
00
2
2
.2.1. 2,2 :5 ,2 -Terthiophene-5-carboxaldehyde (TTA). A solution of
0
0
00
,2 :5 ,2 -terthiophene (0.87 g, 3.50 mmol) in CH
cooled to 0 C, then POCl
.02 mmol) and CH
2
Cl
2
(10 mL) was
ꢀ
3
(0.76 g, 5.02 mmol) in dry DMF (0.39 mL,
(15 mL) were added dropwise. After stir-
5
2
Cl
2
ring the resulting mixture overnight at room temperature, the
reaction mixture was added with 1 M sodium acetate (30 mL), and
then stirred for about 5 h. The resulting solution was extracted
250 300 350 400 450 500 550 600 650
Wavelength / nm
with CH
2 2
Cl , and the combined organic layer was washed with
water, brine, sequentially, and dried on anhydrous MgSO
4
.
Fig. 2. UVevis spectra and PL spectra of TTA in dilute THF (10 mM).
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J. Cheng et al. / Tetrahedron xxx (2015) 1e6
3
3
.2. AIE phenomenon of TTA
synthesized and studied. TTOH is similar in molecular structure
with TTA but differs in an electron rich eCH OH replacing
2
We examined the emission behavior of TTA aggregates formed
in THF/water mixtures because TTA is soluble in common organic
solvents but insoluble in water, and TTA molecules must aggregate
the electron deficient aldehyde on the end-capped group. Op-
posite to the emission behavior of TTA aggregates formed in THF/
water mixtures, the emission behavior of TTOH shows
typical ACQ, as illustrated in Fig. 4. The PL intensity of TTOH is
evidently weakened with increasing the water fractions in the
in THF/water mixtures with appropriate water fraction (f
w
, the
volume percentage of water in THF/H
the changes in PL spectra.
2
O mixtures), which leads to
mixture. When f
w
reaches 90%, the PL curve of TTOH is almost
The recorded photographs and spectra are displayed in Fig. 3A
and B, respectively. When TTA is molecularly dissolved in THF, it
emits very faint blue light. As illustrated in Fig. 3A and B, the PL
intensity of TTA is evidently intensified with increasing the water
fractions in the mixture. The resultant emission is followed by
a red-shift of the maximum emission wavelength of TTA by about
flat line parallel to the abscissa, confirming that it is nearly non-
emissive. Thus far, we can propose is that the presence of an
electron deficient aldehyde group as the end-capped group of
terthiophene is
phenomenon.
a key structural unit respect to the AIE
6
0 nm. This shift can be attributed to typical twisted intermolecular
charge transfer (TICT) effects arising from increased solvent po-
larity through an increase in the water fraction. When f reaches
0%, the PL intensity is four times higher than that in pure THF. A
3.4. AIE mechanism
w
It has been revealed that the AIE effect in propeller-like con-
formations is mainly attributed to the restriction of intermolecular
7
12
further increase of the water fraction above 80%, leads to a decrease
of the emission intensity. This up-down phenomenon has been
observed in some compounds with AIE properties, although the
reasons behind this phenomenon are still under debate.
better understand the diverse photoluminescence behavior and
mechanism of TTA in solvent mixtures, further testing was carried
rotations (RIR) when forming molecular aggregates. TTA, how-
ever, neither shows a propeller-like conformation nor possesses
a sterically hindered bulky group in its molecular structure. To gain
insight into the AIE mechanism of TTA, the following measure-
ments were carried out.
3
3,34
To
2
The recorded UVevis spectra of TTA (10 mM) in fresh THF/H O
out. The absolute
the emission enhancement quantitatively using a calibrated in-
tegrating sphere. As shown in Fig. 3C, increasing the water faction
F
F
of solutions (10
m
M) was measured to evaluate
mixtures are displayed in Fig. 5. When the water fractions are in the
range of 0e70%, the mixtures are homogeneous and visually
transparent, suggesting that the size of the aggregates is in the
nanoscale. The red-shift of absorption maximum from 398 nm to
404 nm can be assigned to the TICT effect. The absorption spectra of
the mixtures with fw above 80% appear to have tails that level-off in
from pure THF to 70% increases the
7.96%. These results clearly indicate that TTA possesses typical AIE
behavior.
F
F value of TTA from 7.59% to
5
Fig. 3. (A) Photographs of TTA in different water fractions taken under UV illumination (
l
ex¼365 nm); (B) PL spectra of TTA(10
).
mM) in fresh THF/water mixtures with different water
fractions (f ); (C) quantum yield changes of TTA(10 M) in THF/water mixtures with different water fractions (f
w
m
w
3
.3. Molecular structureeproperty relationship
To explore the molecular structureeproperty relationship of
the long wavelength region, which can be attributed to the well-
3
3
known Mie effect induced light scattering. It is reasonable to
speculate that the nanostructures of TTA formed in the mixtures
with higher fraction of water should be different from that at lower
water content.
TTA, especially the role of aldehyde group in AIE, a referenced
terthiophene derivative TTOH (Fig. 1 and Scheme 1) was
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4
J. Cheng et al. / Tetrahedron xxx (2015) 1e6
forms fine suspensions despite their clear appearance. SEM images
in Fig. 7 support our hypothesis that aggregates in THF and the
aqueous mixtures form differently with variation in the water
content. In THF, molecules of TTA cannot form well nanostructured
aggregates (Fig. 7A). In the mixtures with ‘low’ water contents of
60% or 70%, molecules of TTA form ordered nanoscale aggregates
akin to the packed nanoribbons (Fig. 7B and C). At a water fraction
of 80%, TTA molecules may abruptly form microcrystalline struc-
tures, implying that a recrystallization process might take place at
water fractions above 80% (Fig. 7D), which causes emission
quenching. The test results not only give direct evidence of mo-
lecular aggregation during emission enhancement but also reveal
that ordered nanoscale aggregates of TTA formed in THF/water
mixtures can efficiently prevent non-radiative relaxation pathways,
which leads to AIE.
fw (vol%)
0%
1
2
3
0%
0%
0%
0
90%
40%
50%
60%
70%
80%
90%
400
450
500
550
600
650
Wavelength / nm
Fig. 4. PL spectra of TTOH (10
fractions (f ).
mM) in fresh THF/water mixtures with different water
According to the AIE-active molecules reported, distinct emis-
sions probably depend on different conformations and packing
w
3
4e36
modes of the molecules in the aggregates.
We thus speculate
that: (1) In the pure THF solution, TTA is molecularly isolated
without any interaction between adjacent molecules, showing
weak fluorescence. (2) The molecules of TTA in the aggregated
states associated with the lower water content are arranged in
ordered clusters that restrained the non-radiative decay process,
leading to an enhancement of the fluorescence. (3) The crystalli-
zation of TTA leads to more dense molecular packing that may
quench the emission.
To understand how the molecular packing affects the emission
behavior of TTA, a single crystal of TTA (CCDC 1006636) was grown
by slow evaporation of its hexane solution and the crystallographic
data are summarized in Table 1. The crystals exhibit relatively weak
0.8
0.6
0.4
0.2
0.0
0
%
10%
f
(vol%)
20%
30%
40%
50%
90
0
60%
7
8
0%
0%
90%
fluorescence with a solid-state
F
F of 5.98%, in accordance with the
2
50 300 350 400 450 500 550 600 650 700
above hypothesis of the fluorescence decrease during the re-
Wavelength (nm)
crystallization process. In Fig. 8A, we can see that the dihedral angle
ꢀ
between T1 plane and the adjacent thienyl T2 plane is 168.4 ,
Fig. 5. Absorption spectra of TTA (10
water fractions (f ).
mM) in fresh THF/water mixtures with different
ꢀ
w
whereas the dihedral angle between the T2 and T3 is 169.5 . This
indicates that the TTA molecule adopts a certain planar confor-
mations in the crystal. Fig. 8B shows that the two TTA molecules
stack together in a face-to-face shape. The intermolecular distance
To get more information about the aggregates of TTA formed in
THF/water mixtures, dynamic light scattering (DLS) measurements
and scanning electron microscope (SEM) were used for morpho-
logical examination. From dynamic light scattering (DLS) mea-
surements, the average diameter of the particles of TTA in water
fractions below 80% was less than 3 nm (Fig. 6A and B). However,
ꢀ
from C6 to C10 in Fig. 8B is 3.443 A, and the angle between the line
C6eC10 and the plane of the molecule, which consists of the line
ꢀ
C9eC10eC12 is 87.03 . The vertical distance between the two
ꢀ
planes of the whole molecule is 3.438 A, which is smaller than
ꢀ
3
.6 A. It shows clearly that it has strong
pe
p
stacking between the
two TTA molecules, which promote the decay of the excited species
through non-radiative pathways, resulting in the weak fluores-
cence in TTA crystals.
when f
w
reaches 80%, larger particles are formed with an average
diameter of approximately 200 nm (Fig. 6C). DLS measurements
allow us to make the speculation that in THF/water mixtures, TTA
Fig. 6. Particles size distributions of TTA in THF/water mixtures with different water fractions: (A) TTA in THF/H
20:80, v/v).
2 2 2
O (40:60, v/v); (B) TTA in THF/H O (30:70, v/v); (C) TTA in THF/H O
(
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J. Cheng et al. / Tetrahedron xxx (2015) 1e6
5
Fig. 7. SEM images of TTA (10
mM) in THF/water mixtures with different water fractions: (A) TTA in THF/H
2
O (100:0, v/v); (B) TTA in THF/H
2
O (40:60, v/v); (C) TTA in THF/H
2
O (30:70,
v/v); (D) TTA in THF/H
2
O (20:80, v/v).
Table 1
Crystallographic data for TTA
Compound
TTA
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
C
13
H
8
OS
3
276.37
296.21(10)
Triclinic
P-1
6.4043(5)
13.8631(13)
14.4086(13)
77.021(8)
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
ꢀ
a
b
g
( )
ꢀ
( )
79.759(7)
89.528(7)
1226.03(19)
ꢀ
( )
ꢀ
3
V (A )
Fig. 8. (A) ORTEP diagram of TTA. (B) Two molecular packing diagrams of TTA.
Z
D
m
4
3
calcd (mg/mm )
1.497
0.582
(mm 1
ꢁ
)
Reflections collected
8863
5.9e52.042
4809
behavior. This work expands the study of AIE-active chromophores
to aldehyde end-capped oligomer of thiophene.
ꢀ
q
Range ( )
Independent reflections
Index ranges
ꢁ7ꢂhꢂ7, ꢁ16ꢂkꢂ17, ꢁ17ꢂlꢂ17
R
int
0.0301
wR
2
0.1167
4809/0/307
1.036
Acknowledgements
Data/restraints/parameters
GOF
This work was supported by the National Natural Science
Foundation of China (No. 21303117), the Shanxi University Scien-
tific and Technical Innovation Project (No. 2013114), the Natural
Science Foundation for Young Scientists of Shanxi Province (No.
2014021014-3), the Qualified Personal Foundation of Taiyuan Uni-
versity of Technology (No. tyut-rc201269a). We also thank the
National Basic Research Program of China (973 Project
4
. Conclusions
0
0
00
In summary, the work here demonstrates that 2,2 :5 ,2 -ter-
thiophene-5-carbaldehydeshowed prominent AIE characteristics.
Molecular structureeproperty relationship shows the electron
deficient aldehyde group endows TTA with AIE characteristic. SEM
analyses revealed that ordered nanoscale aggregates of the alde-
2013CB834702),
Hong
Kong
Research
Grants
Council
(HKBU12302114), Areas of Excellence Scheme of HKSAR (AoE/P-03/
08), National Natural Science Foundation of China (project num-
bers: 91222201) and Hong Kong Baptist University (FRG2/13-14/
083). The work was also supported by Partner State Key Laboratory
2
hyde formed in THF/H O mixtures are responsible for the AIE
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of Environmental and Biological Analysis (SKLP-14-15-P011) and
Strategic Development Fund of HKBU.
18. Wang, C.; Wang, K.; Fu, Q.; Zhang, J.; Ma, D.; Wang, Y. J. Mater. Chem. C 2013, 1,
10e413.
9. Gao, M.; Hu, Q.; Feng, G.; Tang, B.; Liu, B. J. Mater. Chem. B 2014, 2, 3438e3442.
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1
20. Xiao, H.; Chen, K.; Cui, D.; Jiang, N.; Yin, G.; Wang, J.; Wang, R. New J. Chem.
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Please cite this article in press as: Cheng, J.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.06.047