Triptycene-scaffolded tetraphenylethylenes with irregular temperature-dependence AIE

Article abstract of DOI:10.1016/j.tetlet.2018.12.069

A series of new tetraphenylethylene monomer, dimer and three-dimensional trimer based triptycene-bridged triphenylethenes has been constructed. Their structures were clearly confirmed and the irregular fluorescent behaviors at different temperatures and s

Full text of DOI:10.1016/j.tetlet.2018.12.069

Tetrahedron Letters 60 (2019) 439–443
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Triptycene-scaffolded tetraphenylethylenes with irregular temperature-
dependence AIE
⇑
Lijun Wang, Wenjie Li, Zhaoxia Wang, Qianfu Luo
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science & Technology,
Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new tetraphenylethylene monomer, dimer and three-dimensional trimer based triptycene-
bridged triphenylethenes has been constructed. Their structures were clearly confirmed and the irregular
fluorescent behaviors at different temperatures and solvents were investigated. Results showed the new
tetraphenylethylenes performed remarkable aggregation-induced emission (AIE) properties and the
emission intensities are dependent on temperature and viscosity. Compared with the model tetrapheny-
lethylene without connecting triphenylethylene, the fluorescence quantum yields of the titled triptycene-
scaffolded tetraphenylethylenes in aggregation are greater than tetraphenylethylene and increased as the
increasing of the triphenylethylene units.
Received 25 October 2018
Revised 15 December 2018
Accepted 28 December 2018
Available online 29 December 2018
Keywords:
Triptycene
Triphenylethene
Tetraphenylethylene
AIE
Ó 2018 Elsevier Ltd. All rights reserved.
Introduction
in fluorescent materials are confined because of the non-fluorescent
characteristics whether in solvents or in solid state.
Since aggregation-induced emission (AIE) phenomenon was
reported originally in 2001 by Tang and coworkers [1], AIE behav-
iors have attracted intense interest in many researchers, as the
finding provided a promising method to overcome the limit in
the development of luminogenic devices caused by aggregation-
caused quenching (ACQ) [2]. Tetraphenylethylene is one of the
prototypical AIE molecules. Its excellent emission property in
aggregate state and solid state in contrast to fluorescence quench-
ing in dilute solution is attributed to the restriction of intramolec-
ular rotation in solid and aggregate state. tetraphenylethylene
derivatives has aroused more and more attention due to the versa-
tile abilities to be conveniently incorporated into various multi-
component assemblies [3]. Nowadays, they have been broadly
applied in the self-organization, biosensors [4,5], cellular imaging
In view of the above consideration, we manage to graft different
amount of triphenylethylene units into the molecular scaffold of
triptycene, in which triphenylethylenes share the benzene rings
of the triptycene to construct a series of novel tetraphenylethylene
monomer, dimer and three-dimensional trimer. This design con-
siders the atomic economy and the complimentary characteristics
of the two blocks, including that the rigid structure of triptycene,
which can make intramolecular rotation blocked and enhance
AIE performance of triphenylethylenes. Our results show that the
triptycene-bonded triphenylethylene presents significant AIE phe-
nomenon and the fluorescence intensities depend on temperature
and viscosity. Besides, the fluorescence quantum yields of the new
tetraphenylethylene derivatives increase with the increasing of
triphenylethylene blocks and are all greater than tetraphenylethy-
lene in aggregation. We hope this design could provide a new
choice in the application of solid fluorescent materials based
triptycene.
[
6], bioprobes [7,8], and so on. Similarly, another representative
phenyl building block, triptycene, is a propeller-shaped molecule
composed of three 120°-oriented phenylene blades, owning a
unique three-dimensional rigid structure [9]. It provides a large
free volume around the aromatic skeleton. Due to the arene sub-
units of triptycene are easy to be further functionalized, triptycene
and its derivatives have been studied systematically in the fields of
molecular machines [10,11], material chemistry [12–14],
and supramolecular chemistry [15]. However, their applications
Results and discussion
Synthesis
The synthetic routes of the three target compounds were shown
in Scheme 1. Starting from 1-bromo-1,2,2-triphenylethylene, 4 was
prepared according to modified literature procedures [16]. 2-
⇑
Corresponding author.
E-mail address: qianfu_luo@126.com (Q. Luo).
Iodonetriptycene,2,6-diiodotriptycene
and
2,6,14-triiodotrip-
https://doi.org/10.1016/j.tetlet.2018.12.069
040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
0

4
40
L. Wang et al. / Tetrahedron Letters 60 (2019) 439–443
8
7
6
00
00
00
800
700
600
500
400
300
200
100
0
a). compd. 1
a . compd. 1
)
500
00
300
4
2
1
00
00
0
9
8
6
4
0%
0%
0%
0%
0
20
40
60
80
100
Water fraction/vol %
20%
0
%
3
00
400
500
600
700
800
Scheme1. Reagents and conditions: (a) bis(pinacolato)diboron, (PPh
KOPh, toluene, 70 °C, 13 h; (b) 2-iodotriptycene [17], Pd(PPh , K CO
c) 2,6-diiodotriptycene [17], Pd(PPh , K CO , THF, H O; (d) 2,6,14-triiodotrip-
tycene [17], Pd(PPh , K CO , THF, H O.
3
)
2
PdCl
2
, PPh
3
,
)
3 4
2
3
, THF, H
2
O;
wavelength/nm
(
3
)
4
2
3
2
)
3 4
2
3
2
8
00
800
b
). compd. 2
b . compd. 2
)
7
6
5
00
00
00
700
600
500
Water fraction/vol%
400
300
200
tycene were prepared according to previous work [17]. Model tet-
raphenylethylene without any substituents (0) was obtained as the
method reported previously [18]. Target compounds 1, 2 and 3
were prepared via Suzuki coupling reaction, respectively (Support-
ing Information).
9
0%
1
00
0
80%
0
20
40
60
80
100
6
4
0%
0%
400
Water fraction/vol %
20%
300
200
100
0
0%
Aggregation-induced emission properties
We first investigated the AIE properties of compound 1, 2 and 3
in the H O/THF mixed solution, respectively. Results indicated that
2
300
400
500
600
700
800
wavelength/nm
all the three compounds exhibited typical aggregation-induced
emission (AIE) properties, as shown in Fig. 1. The schematic dia-
gram of the aggregation-induced emission is shown in Scheme 2.
Their fluorescent emission could not be observed in pure THF sol-
vent (kex = 320 nm), but gradually increased with the increasing of
the water fraction because of the heavy aggregation in the aqueous
medium which restricting the intramolecular rotation [19]. Here
we choose compound 1 as an example to show their fluorescent
performances. In Fig. 1a, we can see that the fluorescence emission
8
00
8
00
c). compd. 3
c)
. compd. 3
Water fraction/vol%
700
00
500
00
300
00
6
700
4
6
5
4
00
00
00
9
8
0%
0%
60%
0%
0%
2
100
0
0
20
40
60
80
100
4
2
0%
Water fraction/vol%
300
À5
intensity of compound 1 (ꢀ2.5 Â 10 M) in pure THF was close to
2
00
00
0
0
(kex = 320 nm), but increased slowly with the increasing of the
1
water fraction. When the water fraction increased from 20 vol%
to 80 vol%, the fluorescence intensity increased, accordingly. Once
it reached 90 vol%, the fluorescence intensity increased observably.
The other two compounds exhibited the similar emission phe-
300
400
500
600
700
800
wavelength/nm
nomenon in H
The AIE phenomenon can be clearly observed by the fluorescent
images of the three compounds in H O/THF mixed solution with
2
O/THF mixed solution (Fig. 1b and c).
Fig. 1. (a, b, c) Fluorescence emission spectra and images of compounds 1, 2 and 3
in H
2
O/THF mixed solution with different water fractions.
2
different water fractions, as shown in the insert of Fig. 1. The com-
pound 1 was non-fluorescence when it was dissolved in THF solu-
tion excited by 320 nm light. Its emission was turned on when the
water fraction reached to 80 vol%. The fluorescence increased
observably when the water fraction reached to 90 vol% and accom-
panied by blue fluorescence, as a result of heavy aggregation in the
aqueous medium [19]. The test results of laser light scattering
showed that the radius of aggregate particles is around 100 nm,
and the particle size distribution is very narrow, which indicates
that the solution is stable and no insoluble particles are precipi-
tated out directly (Fig. S11). The other two compounds exhibited
the similar performances (Fig. 1b and c). Scanning electron micro-
2
scopy (SEM) image of compound 3 in H O/THF mixtures (90% and
8
0% water content) was performed to provide further evidence for
the aggregate formation, as shown in Fig. 2 and S12. It can be seen
that stacked state has been formed in the mixed solvents and the
size of aggregates in 90 vol% H
The absolute fluorescence quantum yields (
, 3 and the model compound 0 were determined to be 38.7%,
1.3%, 43.8% and 13.9% in H O/THF (90:10) mixed solution,
2
O is larger than that in 80 vol% H
2
O.
U
f
) of compounds 1,
2
4
2
respectively. It indicated that the fluorescence quantum yields of
Scheme 2. Schematic diagram of aggregation-induced emission.

L. Wang et al. / Tetrahedron Letters 60 (2019) 439–443
441
presented strong blue fluorescence in solid under the irradiation
of UV light (kex = 320 nm).
It is noteworthy that the fluorescence emission changes irregu-
larly at different temperatures. The 90 vol% H
2
O/THF mixed solu-
À5
tions of the three compounds (2.0 Â 10 M) were chosen to
investigate the temperature effects, respectively (Fig. 4). In general,
the lower the temperature, the stronger the fluorescence for tetra-
phenylethylene [20]. Fig. 4 shows that their emission intensities
below room temperature are stronger overall than those at 25 °C,
owning to the fact that the intramolecular rotation is limited at
low temperature [19]. However, the changes fluctuated as the tem-
perature further drops. From Fig. 4 we can see that the emission
intensity first increases gradually with the decreasing of the tem-
perature from 25 °C to 0 °C until it reaches the maximum intensity
at 0 °C. After that, their fluorescence intensities decrease dramati-
cally when the temperature below zero and finally keep constant
below À5 °C. This irregular temperature-dependence phenomenon
was rarely reported in the performance of AIE. There might be sev-
eral reasons: first is attributed to the reduction of solubility and the
2
Fig. 2. SEM image of compound 3 in H O/THF mixtures (90% water content).
the target compounds increased as the increasing of triph-
enylethylene units and were all greater than model compound tet-
raphenylethylene (0) without triptycenes (Table 1). This is mainly
because the triptycene and tetraphenylethylene are both rigid. As
the number of triphenylethylenes increases, rigid structure
increases and the molecular size becomes larger. Intramolecular
rotation is more restricted; thus, the fluorescence intensity
becomes stronger in the order of compound 0–3.
The solid fluorescence images of compound 1, 2 and 3 were
showed in Fig. 3. In general, triptycene itself has no fluorescence
in solution, aggregation and solid. The fluorescence of triph-
enylethylene is also relatively weak. However, these tetrapheny-
lethylene derivatives based triptycene and triphenylethylene
Table 1
Photophysical properties of compounds 1–3 and the model compound tetrapheny-
lethylene (0) (T: 25 °C).
em
max
Compound
k
ex/nm
k
/nm
f
U (%)
1
2
3
0
320
320
320
320
482
479
478
481
38.7
41.3
43.8
13.9
(k
ex: excitation wavelength; k em ma x: maximum emission wavelength).
Fig. 3. Solid fluorescence images of compounds 1, 2 and 3 before and after
2
Fig. 4. The fluorescence emission spectra of 90% (V/V) H O/THF mixed solution of
excitation of UV light (320 nm).
compounds 1, 2 and 3 at different temperature.

4
42
L. Wang et al. / Tetrahedron Letters 60 (2019) 439–443
further limitation of intramolecular rotation as the temperature
drops, which makes the fluorescence intensity increases; second
is that the relative content of the good solvent, THF increases while
Table 3
The fluorescence emission inten-
sities of compounds 1, 2, 3 and 0
under different viscosity (glycerol
fractions = 80%, 90%).
2
the poor solvent, H O decreases gradually in the mixed solution
due to the formation of solid ‘‘ice”, which increases solubility and
decreases the fluorescent intensity. In addition, the formation of
Compound
F
1
F
2
‘
‘ice” may lead to scattering, which make the determining fluores-
0
1
2
3
45
95
290
350
440
490
580
690
cence intensity decreases, too. Finally, these effects achieve a rela-
tive equilibrium and the fluorescence intensity remains unchanged
F
1
(glycerol fractions = 80%),
F
2
Table 2
(
glycerol fractions = 90%).
The fluorescence emission intensities of compounds 1, 2, 3 and tetraphenylethylene
(
0) under different temperature (25 °C, 0 °C) in 90% (V/V) H
2
O/THF mixed solution.
at a certain temperature [21]. The fluorescence emission intensities
(F) of the three target compounds and the model tetraphenylethy-
lene (0) with the same concentration under different temperature
Compound
F (25 °C)
F (0 °C)
0
1
2
3
50
400
430
480
130
630
740
920
in 90% (V/V) H
2
O/THF mixed solution were further compared
(Table 2). On the whole, the fluorescence emission intensities of
the three target compounds increased as the increasing of the
triphenylethylene units under the same temperature and were
stronger than that of monomer 0.
The viscosity effect on emission in the glycerol/methanol (V/V)
mixed solution was further checked (Fig. 5). Results showed the
emissions of the three target compounds became stronger upon
increasing the solution viscosity (glycerol fraction). Their fluores-
À5
cence emissions (3.0 Â 10 M) first increased gradually with the
increasing of the glycerol fraction from 10 vol% to 50 vol%, then
increased dramatically from 50 vol% to 60 vol%, and finally reached
the most fluorescence emissions at around 90 vol% of glycerol frac-
tions (kex = 320 nm). The fluorescent images of the three com-
pounds in glycerol/methanol mixed solution with different
glycerol fractions were shown in the inserts of Fig. 5. Their fluores-
cence intensities were very weak when the glycerol fraction was
less than 50 vol% but presented obvious blue fluorescence when
the glycerol fraction exceeded 50 vol% under the excitation of UV
light (kex = 320 nm). Besides, we further studied the fluorescence
emission intensities of target compounds and tetraphenylethylene
(0) with the same concentration under different viscosity in glyc-
erol/methanol (V/V) mixed solution (Table 3). It is also found that
the fluorescence emission intensities of target compounds
increased as the increasing of the triphenylethylene units under
the same viscosity and were all higher than that of monomer 0.
Conclusions
In summary, we have designed and constructed a series of new
tetraphenylethylene monomer, dimer and 3D trimer based pro-
peller-shaped triptycene and triphenylethenes. Their various fluo-
rescence performances in solutions, solid state and aggregation
were investigated. Results showed that they presented remarkable
fluorescence in solid but no fluorescence in THF solvent. Their
emissions were turned on when added aqueous medium in the
solution, which exhibited the significant AIE characteristics. Their
irregular fluorescence behaviors at different temperatures were
further examined. The fluorescence quantum yields in aggregation
increased as the increasing of the triphenylethylene blocks and
were all greater than model compound tetraphenylethylene; the
fluorescence emission intensities in solutions became stronger
upon increasing the solution viscosity. We hope this research could
provide a new choice in the application of solid fluorescent mate-
rials based triptycene and triphenylethenes.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2018.12.069.
Fig. 5. Fluorescence emission spectra and images of compounds 1, 2 and 3,
respectively, in glycerol/methanol mixed solution with different glycerol fractions.

L. Wang et al. / Tetrahedron Letters 60 (2019) 439–443
443
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