Synthesis and aggregation-induced emission properties of tetraphenylethylene-based oligomers containing triphenylethylene moiety

Article abstract of DOI:10.1016/j.tetlet.2012.10.023

Three new tetraphenylethylene based oligomers bearing triphenylethylene moiety were synthesized utilizing Suzuki coupling reaction and their aggregation-induced emission properties were studied. They show excellent solubility in common organic solvents and emit light in blue region (440-504 nm). AIE effect is predominant for tetra-substituted TPE (20.1-fold) than mono-substituted one (4.5-fold).

Full text of DOI:10.1016/j.tetlet.2012.10.023

Tetrahedron Letters 53 (2012) 6838–6842
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and aggregation-induced emission properties of
tetraphenylethylene-based oligomers containing triphenylethylene moiety
⇑
Debabrata Jana, Binay K. Ghorai
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new tetraphenylethylene based oligomers bearing triphenylethylene moiety were synthesized uti-
lizing Suzuki coupling reaction and their aggregation-induced emission properties were studied. They
show excellent solubility in common organic solvents and emit light in blue region (440ꢀ504 nm). AIE
effect is predominant for tetra-substituted TPE (20.1-fold) than mono-substituted one (4.5-fold).
Ó 2012 Elsevier Ltd. All rights reserved.
Received 24 August 2012
Revised 3 October 2012
Accepted 5 October 2012
Available online 12 October 2012
Keywords:
Tetraphenylethylene
Triphenylethylene
Suzuki coupling
Aggregation-induced emission
Most conjugated organic molecules have very high lumines-
cence efficiency in dilute solutions, but they exhibit relatively weak
or no emissions when fabricated into thin films due to intra- and
variety of substituents has been attached to phenyl blades of tetra-
phenylethylene moiety to enhance its electronic and optical prop-
erties. Xu et al.11 reported that triphenylethylene derivatives also
show remarkable aggregation-induced emission properties. In this
Letter, we report the synthesis and their aggregation-induced
emission properties of three new oligomers, in which triphenyleth-
ylene units directly connected to tetraphenylethene moiety. These
compounds exhibit remarkable aggregation-induced emission
properties, which make them promising candidates as luminescent
materials for electroluminescence applications.
Our strategy for the synthesis of tetraphenylethylene based core
units and triphenylethylene boronic acid derivatives is outlined in
Scheme 1. The 1-(4-bromophenyl)-1,2,2-triphenylethylene (1)12
was synthesized via the reaction of 4-bromobenzophenone with
diphenylmethyllithium at a lower temperature (0 °C) followed by
acid catalysed dehydration of the resulting alcohol in 85% yield.
The dimethoxy-substituted TPE 2 was obtained from 4,40-dime-
thoxybenzophenone and 4-bromobenzophenone using conven-
tional McMurry reaction in 40% yield. Low yield of 2 is due to
the formation of dibromo-tetraphenylethylene and tetra-methoxy-
tetraphenylethylene derivatives. Tetrakis(4-bromophenyl)ethyl-
ene (3)13 was prepared from 4,40-dibromobenzophenone through
a titanium(0)-catalysed McMurry reaction in 76% yield. Benzophe-
none (4) was converted into 1-bromo-4-(2,2-diphenylvinyl)ben-
zene (6)14 using diethyl 4-bromobenzylphosphonate in the
presence of potassium tert-butoxide as a base in anhydrous THF
at 0 °C temperature. The related methoxy-substituted triphenyl-
ethylene derivative 7 was obtained in a similar manner starting
from 4,40-dimethoxybenzophenone (5) in 82% yield. The desired
boronic acid derivatives 8/9 were prepared from 6/7 as described
intermolecular interactions, such as
p–p interactions, quench the
emission process.1 It is difficult to obtain highly emissive materials
in the solid state and also to make optoelectronic devices because
the organic emissive materials in the devices are normally used
as thin solid films, in which aggregation is inherently accompanied
with film formation.2 It is thus highly desirable to develop electro-
luminescent materials that can emit intense light in the solid state,
which is expected to overcome the aggregation quenching problem.
Tang et al.3 and Park et al.4 have reported several molecules
which show significant enhancement in their light emission upon
aggregation or in the solid state. This intriguing phenomenon
was named aggregation-induced emission (AIE)5 or aggregation-
induced emission enhancement (AIEE).4 Recently, molecules with
AIE properties have drawn more attention, because of their en-
hanced emission in the aggregates. The search for molecules en-
dowed with significant light emission in the solid state has
mainly focused on silole-based compounds, which are complicated
and difficult to prepare. Among the AIE molecules, tetraphenyleth-
ene (TPE) enjoys the advantages of facile synthesis and efficient so-
lid-state emission. TPE and its derivatives have been widely used
as a functional building block in the fabrication of the electrolumi-
nescence materials for organic light-emitting diodes (OLEDs),6 bio-
probes,7 chem-sensors,8 explosive detection9 and latent finger
print10 due to their AIE and electrochemical properties. A wide
⇑
Corresponding author. Tel.: +91 33 26684561; fax: +91 33 26682916.
E-mail address: bkghorai@yahoo.co.in (B.K. Ghorai).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.10.023

D. Jana, B. K. Ghorai / Tetrahedron Letters 53 (2012) 6838–6842
6839
Br
Br
a
O
b
85%
40%
Br
MeO
OMe
1
2
Br
Br
Br
Br
O
c
Br
R
Br
76%
3
Br
R
B(OH)2
R
O
e
d
R
R
6, R = H, 72%
R
4, R = H
8, R = H, 60%
5, R = OMe
7, R = OMe, 82%
9,R = OMe,65%
Scheme 1. Reagents and conditions: (a) diphenylmethane, n-BuLi, THF, 0 °C; then PTSA, toluene, reflux, 6 h; (b) Zn (powder), TiCl4, THF, reflux, 4 h; then 5, reflux, 5 h; (c) Zn
(powdered), TiCl4, THF, reflux, 4 h; (d) diethyl 4-bromobenzylphosphonate, t-BuOK, THF, 12 h, rt; (e) n-BuLi, THF, ꢀ78 °C, 2 h; then B(OMe)3, 6 N aq HCl.
in the literature.15 Lithiation of bromo-derivatives 6/7 with n-BuLi
in THF at ꢀ78 °C, followed by treatment with B(OMe)3 and hydro-
lysis with 6 N aq HCl afforded boronic acid derivatives 8/9 in good
yields.
Final construction of the targeted molecule is illustrated in
Scheme 2. To achieve the synthesis of these oligomers Suzuki-type
coupling reactions have been explored. The synthesis of the mono-
substituted TPE compounds 10/11 was achieved by using Suzuki-
type cross-coupling reaction between boronic acid 8 and bromo
derivative 1/2. Products 10 and 11 were obtained in good yields
ranging from 81% to 85%. Using the same cross coupling protocol,
the tetra-substituted TPE oligomer 12 was prepared using tetrakis
(4-bromophenyl)ethylene (3) with 4.5 equiv of boronic acid 9 in
76% yield. These oligomers are readily soluble in common organic
solvents, such as CHCl3, CH2Cl2, THF and toluene. They were puri-
fied conveniently by flash column chromatography rather than
vacuum sublimation usually used for the purification of insoluble
organic semiconductors. 1H and 13C NMR spectra and MS associ-
B(OH)2
2
1
81%
85%
MeO
OMe
10
11
8
MeO
MeO
OMe
MeO
OMe
OMe
B(OH)2
OMe
3
76%
MeO
9
OMe
MeO
12
Scheme 2. Synthesis of TPE-based oligomers 10ꢀ12. Reagents and conditions: Pd(PPh3)4, K2CO3, toluene, H2O, AliquatÒ 336, 90 °C, 16 h.

6840
D. Jana, B. K. Ghorai / Tetrahedron Letters 53 (2012) 6838–6842
ated with elemental analysis were employed to confirm the struc-
ture and purity of all new compounds.
electron donating ability of the oligomers. This results in the
enhancement of the HOMO energy level and lowers the barrier
to hole injection from the most widely used anode material, in-
dium tin oxide (ITO), which has a work function of 5.0 eV.
The absorption and emission spectra of the oligomers 10–12 in
toluene and tetrahydrofuran solutions are shown in Figure 1 and
the corresponding absorption kabs and kem maxima, as well as the
fluorescence quantum yields Ufl are presented in Table 1. All com-
pounds absorb light (kmax) in spectral region 346–357 nm, which is
attributed to the triphenylethylene substituted TPE moiety and
emit light in the blue region with peaks ranging from 440 to
504 nm. The absorption and emission spectra are nearly indepen-
dent of solvent polarity, except for a slight, insignificant blue shift
observed with increasing the solvent polarity along with a succes-
sive decrease in the fluorescence quantum yields. The quantum
efficiencies (Ufl) were determined to be 13%, 4% and 14% in toluene
and 6%, 7% and 9% in tetrahydrofuran for 10–12, respectively, using
pyrene (Ufl = 0.32, in CH2Cl2) as a reference.
To determine whether these three new compounds are AIE ac-
tive, the fluorescence behaviour of their diluted mixtures was stud-
ied in a mixture of water/THF under different water fractions. Since
the compounds were insoluble in water, increasing the water frac-
tion in the mixed solvent could change their existing forms from a
solution or well-dispersed state in THF to the aggregated particles
in the aq THF with high water content. Changes in the PL peak
intensities versus water fraction of the mixture for compound 12
were plotted, as shown in Figure 2(a) as an example. When the
water fraction was increased from 0% to 90%, the fluorescence
intensity of 12 was correspondingly enhanced 20.1-fold. Similar
enhancement was observed in the behaviour of the remaining
two compounds. When the water fraction was increased from 0%
to 90%, the fluorescence intensities of 10 and 11 were enhanced
6.5-fold and 4.5-fold, respectively. This increase in fluorescence
intensity was considered to be a result of the AIE effect. As aggre-
gates formed the restriction of intermolecular rotation increased
which led to increased fluorescence emission. The results indicate
that the AIE effects of the compounds increase along with an in-
crease in the number of triphenylethylene groups that were pres-
ent in the molecules. Initially PL intensity increases gradually up to
70% H2O content, then decreases at 80% H2O content, again reaches
maximum intensity at 90% H2O content. This phenomenon was of-
ten observed in some compounds with AIEE properties, but the
reasons remain unclear. There are two possible explanations for
this phenomenon: first after the aggregation, only the surface mol-
ecule of the nanoparticles emitted light and contributed to the
fluorescent intensity upon excitation, leading to a decrease in fluo-
rescent intensity. However, the restriction of intramolecular rota-
tions of the aromatic rings around the carbon–carbon single
bonds in the aggregation state could enhance light emission. The
net outcome of these antagonistic processes depends on which
process plays a predominant role in affecting the fluorescent
behaviour of the aggregated molecules.17 Second when water is
added, the solute molecules can aggregate into two kinds of
nano-particle suspensions: crystal particles and amorphous parti-
cles. The former leads to an enhancement in the PL intensity, while
the latter leads to a reduction in intensity.18 Thus, the measured
overall PL intensity data depend on the combined actions of the
two kinds of nanoparticles. However, it is hard to control the for-
mation of nanoparticles in high water content. Thus, the measured
PL intensity often shows no regularity in high water content.
The absorption spectra of the compound 12 in the THF/water
mixtures (ꢁ1 ꢂ 10ꢀ5 M) are shown in Figure 2(b). In the solvent
mixture, the spectral profile remains almost unchanged up to addi-
tion of 50% water. The spectrum showed a decrease in absorption
band at 60% water content indicating the formation of dimer or tri-
mer.19 When the water content is 80%, the increase in absorbance
(at 360 nm) suggests that the formation of nanoscopic aggregates
of the compound. The nanoaggregate suspensions in the solvent
mixtures effectively decrease light transmission in the mixture
and cause apparent high absorbance (Mie scattering effect).20 At
90% water content again decreases in absorbance (at 363 nm) that
may be due to the number of molecules in an aggregate increase.19
When the water fraction in the solvent increased from 0% to 90%, the
UV absorption wavelength of 12 red-shifted from 354 to 365 nm (an
11-nm red shift), indicating the formation of J-aggregates.21
Cyclic voltammetry (CV) analyses were carried out to measure
the HOMO values of the synthesized compounds. On the basis of
ox
onset
the roughly evaluated onset oxidation potentials (E
), the HOMO
energy levels of 10, 11 and 12 are estimated as ꢀ5.91, ꢀ5.64 and
ox
onset
ꢀ5.63 eV, respectively (HOMO = ꢀe(E
+ 4.66)).16 The LUMO en-
ergy levels of 10, 11 and 12 are ꢀ2.34, ꢀ2.14 and ꢀ2.16 eV, respec-
tively, calculated from the HOMO energy level and HOMO–LUMO
gap (Eg), estimated from the onset of the absorption spectra (LU-
MO = HOMO + Eg). The electrochemical properties as well as the
energy level parameters of the oligomers are listed in Table 1.
These results indicate that the HOMO energy levels of 12 and 11
are nearly same but for 10, the HOMO value is decreased. The pres-
ence of methoxy group in the oligomers 11 and 12 increases the
9.0x104
(a)
10
11
12
0.2
6.0x104
0.1
3.0x104
0.0
650
0.0
300
350
400
450
500
550
600
Wavelength (nm)
(b)
9.0x104
6.0x104
0.3
10
11
12
0.2
0.1
0.0
3.0x104
0.0
In conclusion, we have synthesized three new tetraphenylethyl-
ene based AIE compounds containing triphenylethylene moiety.
The compounds exhibit a strongly enhanced emission in the aggre-
gated state and it is predominant for tetra-substituted TPE (20.1-
fold) than mono-substituted one (4.5-fold). All compounds are blue
300
350
400
450
500
550
600
650
Wavelength (nm)
Figure 1. Absorption (solid line) and photoluminescence spectra (dotted line) of
10ꢀ12 (1 ꢂ 10ꢀ6 M) in toluene (a) and in THF (b).

D. Jana, B. K. Ghorai / Tetrahedron Letters 53 (2012) 6838–6842
6841
Table 1
Photophysical properties of compounds 10ꢀ12
a
d
HOMO/LUMO (eV)c
Eg
em
ox
onset
k
ðnmÞ
E
(V)b
abs
Compd
Ufl (%)
k
ðnmÞ
max
max
Tol/THF
Tol/THF
Tol/THF
10
11
12
347/346
354/353
357/355
442/440
453/452
504/502
13/6
4/7
14/9
1.25
0.98
0.97
ꢀ5.91/ꢀ2.34
ꢀ5.64/ꢀ2.14
ꢀ5.63/ꢀ2.16
3.57
3.50
3.47
a
b
c
Fluorescence quantum yields (U), measured in solution using pyrene (Ufl = 0.32, in CH2Cl2) as standard, excited at 275 nm.
ox
E
: onset oxidation potential; potentials versus Ag/Ag+, working electrode glassy carbon, 0.1 M Bu4NPF6/CH2Cl2, scan rate 100 mVsꢀ1
.
onset
ox
onset
HOMO (eV) = ꢀe(E
+ 4.66); LUMO (eV) = HOMO + Eg.
d
Estimated from the onset of the absorption spectra: 1240/konset
.
Supplementary data
3.0x106
250
(a)
200
150
100
50
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.10.
023.
2.5x106 Water fraction / %
0
2.0x106
10
20
30
40
50
60
0
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thanked for the award of Senior Research Fellowship to D.J.

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