Synthesis and properties of a new luminescent oligoarylsilane dendrimer

Article abstract of DOI:10.1016/j.mencom.2011.03.010

An oligoarylsilane dendrimer containing peripheral 2,2'-bithienyl groups and bis(2,2'-bithien-5-yl)-1,4-phenylene fragments as the central moieties of the molecule has been synthesized; it exhibited efficient intramolecular energy transfer and a high quan

Full text of DOI:10.1016/j.mencom.2011.03.010

Mendeleev
Communications
Mendeleev Commun., 2011, 21, 89–91
Synthesis and properties of a new luminescent
oligoarylsilane dendrimer
Marina S. Polinskaya, Oleg V. Borshchev, Yuriy N. Luponosov,
Nikolay M. Surin, Aziz M. Muzafarov and Sergei A. Ponomarenko*
N. S. Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, 117393 Moscow,
Russian Federation. Fax: +7 495 335 9000; e-mail: ponomarenko@ispm.ru
DOI: 10.1016/j.mencom.2011.03.010
An oligoarylsilane dendrimer containing peripheral 2,2'-bithienyl groups and bis(2,2'-bithien-5-yl)-1,4-phenylene fragments as the
central moieties of the molecule has been synthesized; it exhibited efficient intramolecular energy transfer and a high quantum efficiency
of luminescence.
Over the last years, luminescent dendrimers have been studied
as promising functional materials for various organic-electronic
devices such as organic thin-film transistors, light-emitting diodes
and photovoltaic cells (solar batteries).1,2 These dendrimers exhibit
a so-called molecular antenna effect consisting in the ability of
functional groups to absorb light and then execute nonradiative
energy transfer from the periphery to the core.3,4 Earlier, oligo-
thiophenesilane dendrimers containing various oligothienyl frag-
ments and possessing luminescent properties in the UV-VIS
spectral range (compounds D1 and D2 in Figure 1) were de-
scribed.5–9
The efficiencies of intramolecular energy transfer QETE for
compoundsD1andD2werefoundtobe97and94%, respectively.
However, their luminescence efficiencies were low (10–13%)
because of the low fluorescence quantum yield QF exhibited by
ter- or quaterthiophene8,9 acceptors used in these dendritic struc-
tures. Thiophene-phenylene oligomers demonstrated efficient lumi-
nescence both in solution and in a solid phase.10,11 The aim of this
work was to synthesize an oligoarylsilane dendrimer D3 containing
bithiophenesilane fragments as energy donors and bis(2,2'-bithien-
5-yl)-1,4-phenylene fragments as energy acceptors with a lumi-
nescence quantum yield higher than that exhibited by ter- and
quaterthiophenesilane fragments and to characterize its optical
properties.
The Suzuki reaction of organometallic synthesis was the key
stage of the preparation of the new oligoarylsilane dendrimer.
This reaction, which is well-known in the synthesis of oligo-
thiophenesilane dendrimers,6,8,9 makes it possible to obtain the
target compound almost free from higher molecular weight by-
products (Scheme 1).
Bithiophenesilane monodendron 1, whose synthesis was de-
scribed in detail,7 was used as a monofunctional starting reagent.
Monodendron 2, which was prepared from compound 1 by the
Kumada reaction, contained a bromophenylbithiophene fragment,
whose bromine atom was then replaced by an organoboron group
to yield compound 3. At the same time, for the synthesis of the
C6H13
S
Me
S
S
H13C6
S
Si
S
X
S
Me
S
C6H13
Si
S
S
X
X
S
S
Me
S
Si
S
=
D1 X
Me
S
Si
S
S
H
13C6
S
S
S
=
=
D2 X
D3 X
S
S
S
C6H13
S
C6H13
Figure 1 Organosilicon dendritic molecular antenna.
© 2011 Mendeleev Communications. All rights reserved.
– 89 –

Mendeleev Commun., 2011, 21, 89–91
C6H13
C6H13
S
H
13C6
Br
S
S
S
H13C6
S
S
S
C6H13
S
S
S
Si Me
S
Me
S
S
H
13C6
Si
Si Me
S
S
i, BuLi
ii, MgBr2·Et2O
S
S
S
S
S
Br
i, BuLi
ii, IPTMDOB
Br
Br
iii,
S
Br
4
Si Me
S
D3
S
THF
THF
Pd(dppf)Cl2
Pd(PPh3)4
toluene, 2 M Na2CO3
B
O
O
S
Br
1
3
2
Scheme 1 Synthesis of oligoarylsilane dendrimer D3.
dendrimer of interest, methyltris(5'-bromo-2,2'-bithien-5-yl)silane
49 was used as the center of branching.
1.0
0.8
0.6
0.4
absorption dendrimer
absorption 2T-Ph-2T
absorption 2T
luminescence 337 nm
luminescence 407 nm
Monodendron 2 was obtained by the reaction of an organo-
magnesium derivative of compound 1 with 1,4-dibromobenzene in
THF using Pd(dppf)Cl2 as a catalyst.† The Grignard reagent was
prepared in situ by the lithiation of a solution of monodendron 1
in a THF–hexane mixture with a solution of n-butyllithium with
the following replacement of Li by MgBr using the MgBr2·Et2O
complex. This complex was formed in situ from magnesium and
dibromoethane in diethyl ether. This reaction scheme does not
include the stage of preliminary bromination. According to gel-
permeation chromatography (GPC), the yield of compound 2 was
only 68% because of the formation of a disubstitution by-product.
The organoboron derivative of oligoarylsilane monodendron 3
was obtained by the lithiation of compound 2 with BunLi in THF
followed by the reaction of the previously synthesized monolithium
derivative with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane (IPTMDOB) in 78% yield.
0.2
0.0
250 300 350 400 450 500 550 600 650 700
l/nm
Figure 2 Absorption and luminescence spectra of oligoarylsilane dendri-
mer D3.
Dendrimer D3 was synthesized by the Suzuki reaction using
trifunctional branching center 4 and a 30% molar excess of
the organoboron derivative of oligoarylsilane monodendron 3
(Scheme 1). Usually, the synthesis of oligothiophenesilane dendri-
mers under the standard Suzuki conditions is performed using
toluene as a solvent and an aqueous solution of a base (e.g.,
2 m Na2CO3); in addition, the completeness of the process requires
the boiling of the reaction mixture for 48–120 h.5–8 In our
case, the reaction time was 27 h due to the addition of ethanol to
promote the mixing of two phases (toluene–water). The moment
of the termination of the reaction was determined by the cessation
of the growth of the product concentration in the reaction mixture
(GPC control). At the final stage, the mixture contained 58% of
the target compound, which was then purified by preparative
GPC. The yield of dendrimer D3 was 39%. The structures and
purities of the intermediate and final compounds were confirmed
spectra of reference compounds indicated that this spectrum is
the sum of the absorption spectra of bithiophenesilane (peak 2T)
and bis(2,2'-bithien-5-yl)-1,4-phenylene (peak 2T-Ph-2T) frag-
ments. This is consistent with published data for ter- and quater-
‡
The 1H NMR spectra were recorded on a Bruker WP-250 SY spectro-
meter (250.13 MHz) using the signal of CDCl3 (d 7.25 ppm) as the internal
standard. The 13C and 29Si NMR spectra were recorded on a Bruker
DRX500 spectrometer. The GPC analysis was performed using a Shimadzu
chromatography recorder equipped with an RID-10A refractometer and
an SPD-M10AVP diode bar as detectors, a Phenomenex column (USA)
(7.8×300 mm) packed with Phenogel with a pore size of 500 Å; THF was
used as the eluent.
In order to avoid self-absorption, the absorption and luminescence
spectra were measured over the range of 200–600 nm in dilute THF solu-
tions at concentrations of 10–5–10–6 mol dm–3. The absorption spectra
were recorded on a Shimadzu UV-2501PC spectrophotometer. At the same
time, the luminescence measurements were carried out on an ALC01M
multifunctional absorption/luminescence spectrometer, because the three-
channel optical arrangement employed in its principal of functioning makes
it possible to measure electronic absorption, excitation and luminescence
(fluorescence and phosphorescence) spectra. High sensitivity to small
concentrations of the luminescent matter was achieved by the application
of the time-correlated single photon counting technique and the automatic
adjustment of the measured radiation intensity.
1
by GPC, H, 13C and 29Si NMR spectroscopy and elemental
analysis.‡
The absorption and luminescence spectra of the dilute solu-
tions of oligoarylsilane dendrimer D3 in THF are presented in
Figure 2. The absorption spectrum of dendrimer D3 consists of
two bands at 337 and 407 nm. A comparison to the absorption
†
Pd(dppf)Cl2 is [1,1'-bis(diphenylphosphino)ferrocene]dichloropalla-
For synthetic procedures and characteristics of compounds 2, 3 and D3,
see Online Supplementary Materials.
dium(ii).
– 90 –

Mendeleev Commun., 2011, 21, 89–91
Table 1 Luminescent spectral characteristics of dilute solutions of D1, D2
and D3 in THF.a
fragments in the peripheral and central parts of the molecule,
respectively. The thermal and thermal oxidative stability of the
compound and its optical properties were characterized. The new
oligoarylsilane dendrimer is a promising material for photo- and
electrooptical applications in organic electronics and photonics.
Com-
pound
labs
nm
/
Kext
/
QF (%)
l
lum/nm
QETE (%)
dm3 mol–1 cm–1
338
388
334
416
337
407
147000
114000
126000
138000
136000
187000
10
10
12
13
41
46
D1
D2
D3
427, 450
97 3
The authors are grateful to Dr. A. P. Pleshkova for MALDI-
TOF measurements and Dr. M. I. Buzin for TGA measurements.
This work was supported by the Ministry of Education and
Science of the Russian Federation (contract no. 16.740.11.0337),
and the Council on Grants of the President of the Russian
Federation (Program for State Support of Young Candidates of
Sciences, grant nos. MK-1528.2011.3 and MK-1567.2011.3).
470, 496, 537 94 3
456, 486, 523 88 3
a labs is the absorption maximum, llum is the luminescence maximum, Kext is
the molar extinction coefficient, QF is the luminescence quantum yield, and
QETE is the intramolecular energy transfer efficiency.
thiophene systems with no conjugation between oligothiophene
fragments connected via a silicon atom.8,9 The luminescence
spectra of dendrimer D3 excited at both 337 (absorption exhibited
by the bithiophenesilane fragments) and 407 nm [absorption exhi-
bited by the bis(2,2'-bithien-5-yl)-1,4-phenylene fragments] cor-
respond to that of bis(2,2'-bithien-5-yl)-1,4-phenylene. At the same
time, no luminescence of the bithiophenesilane fragments with a
maximum at 380–400 nm was observed even during excitation in
the region of the maximal absorption exhibited by bithiophene-
silane (337 nm), which is indicative of the high efficiency of intra-
molecular energy transfer from bithiophenesilane to bis(2,2'-bithien-
5-yl)-1,4-phenylene fragments.
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2011.03.010.
References
1 S.-C. Lo and P. L. Burn, Chem. Rev., 2007, 107, 1097.
2 D. Astruc, E. Boisselier and C. Ornelas, Chem. Rev., 2010, 110, 1857.
3 C. Devadoss, P. Bharathi and J. S. Moore, J. Am. Chem. Soc., 1996, 118,
9635.
4 A. Andronov and J. M. J. Frechet, Chem. Commun., 2000, 1701.
5 S.A. Ponomarenko,A. M. Muzafarov, O.V. Borshchev, E.A.Vodopyanov,
N. V. Demchenko and V. D. Myakushev, Izv. Akad. Nauk, Ser. Khim.,
2005, 673 (Russ. Chem. Bull., Int. Ed., 2005, 54, 684).
The efficiency of intramolecular energy transfer QETE cal-
culated from experimental data was 88%; at the same time, the
quantum yield of luminescence QF was 46%.
6 O. V. Borshchev, S. A. Ponomarenko, N. M. Surin, M. M. Kaptyug,
M. I. Buzin, A. P. Pleshkova, N. V. Demchenko, V. D. Myakushev and
A. M. Muzafarov, Organometallics, 2007, 26, 5165.
7 Y. N. Luponosov, S. A. Ponomarenko, N. M Surin andA. M. Muzafarov,
Table 1 demonstrates an increase in luminescence efficiency
(from 10 to 13%) from ter- to quaterthiophenesilane molecules.
However, in the case of the new oligoarylsilane dendrimer, a
fourfold increase is observed. Meantime, the energy transfer
efficiency was maximal for dendrimer D1, in which the donor–
acceptor distance is minimal, and expectedly decreased with the
length of the acceptor, since it leads to a longer distance between
the donor and acceptor. The experimental data are consistent
with the theoretical model, according to which energy transfer in
such systems occurs by the dipole-dipole mechanism charac-
terized by a significant dependence of QETE on the donor–acceptor
distance.9,12
Org. Lett., 2008, 10, 2753.
8 Y. N. Luponosov, S. A. Ponomarenko, N. M. Surin, O. V. Borshchev,
E. A. Shumilkina and A. M. Muzafarov, Chem. Mater., 2009, 21, 447.
9 O. V. Borshchev, S. A. Ponomarenko, E. A. Kleymyuk, Yu. N. Luponosov,
N. M. Surin and A. M. Muzafarov, Izv. Akad. Nauk, Ser. Khim., 2010,
781 (Russ. Chem. Bull., Int. Ed., 2010, 59, 797).
10 S. Hotta and S. A. Lee, Synth. Met., 1999, 101, 551.
11 M. Ichikawa, R. Hibino, M. Inoue, T. Haritani, S. Hotta, T. Koyama and
Y. Taniguchi, Adv. Mater., 2003, 15, 213.
12 N. M. Surin, O. V. Borshchev,Y. N. Luponosov, S. A. Ponomarenko and
A. M. Muzafarov, Zh. Fiz. Khim., 2010, 84, 2168 (Russ. J. Phys. Chem.,
2010, 84, 1979).
13 O. V. Borshchev, Ph.D. Thesis, Moscow, ISPM RAS, 2007.
As found by TGA, the test compound is stable up to 275°C
in the presence of oxygen or up to 390°C in argon, which is 90°C
higher than in the case of the quaterthiophenesilane dendrimer.13
Thus, we synthesized a new oligoarylsilane dendrimer con-
taining 2,2'-bithienyl and bis(2,2'-bithien-5-yl)-1,4-phenylene
Received: 8th October 2010; Com. 10/3608
– 91 –