Silole-core dendrimers: A facile synthesis and photophysical properties

Article abstract of DOI:10.1246/cl.2005.1130

Silole-core dendrimers having benzyl ether-type dendron units, readily synthesized by the Ni-catalyzed reaction of benzyloxy-substituted diphenylacetylenes with 1,1,2,2-tetramethyldisilane, display, upon excitation of the dendron units, efficient energy t

Full text of DOI:10.1246/cl.2005.1130

1130
Chemistry Letters Vol.34, No.8 (2005)
Silole-core Dendrimers: A Facile Synthesis and Photophysical Properties
Takanobu Sanji,ꢀ Hiroyuki Ishiwata, Tomoyoshi Kaizuka, Masato Tanaka,ꢀ Hideki Sakurai,y
Ritsuko Nagahata,yy and Kazuhiko Takeuchiyy
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503
yDepartment of Pure and Applied Chemistry, Tokyo University of Science, Noda 278-8510
yyNational Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565
(Received May 11, 2005; CL-050621)
Silole-core dendrimers having benzyl ether-type dendron
units, readily synthesized by the Ni-catalyzed reaction of benzyl-
oxy-substituted diphenylacetylenes with 1,1,2,2-tetramethyldisi-
lane, display, upon excitation of the dendron units, efficient
energy transfer from the dendron units to the silole core, result-
ing in strong emission from the silole ring.
R
R
NiCl2(PEt3
toluene
)
2
R
C
C
R
+
HMe2SiSiMe2H
R
R
Si
Me2
1a-d
2a (53%)
b (50%)
c (47%)
d (13%)
a: R =
O
Siloles, silacyclopentadienes, have attracted great interest
over the past decade because of their unique photophysical
and electronic properties1 and potential applications as organic
electroluminescent (EL) devices.2 The silole has a relatively
low LUMO energy level due to the ꢀꢀ–ꢁꢀ conjugation in com-
parison with a carbon analogue, cyclopentadiene. Recently, sev-
eral reports have addressed synthesis of siloles with a variety of
substituent groups3 and silole-based polymers.4 In view of the
improvement of the photophysical properties, the aggregation
leading to enhance the emission efficiency of a simple tetraphe-
nylsilole was reported by Tang and co-workers.5,6 The structural
tuning of silole ring was also recently reported by Yamaguchi7
and Pagenkopf.8
O
O
O
O
b: R =
O
O
O
O
O
d: R =
O
O
O
O
O
O
O
O
O
c: R =
O
O
We designed silole-core dendrimers. Dendrimers are a class
of highly ordered, three-dimensional, and tree-like macromole-
cules and potential building blocks for the construction of func-
tional materials.9 In this regard, dendritic macromolecules offer
a special opportunity for improvement of photophysical proper-
ties, for example, a site-isolation of chromophores, a light-
harvesting antenna, and an energy transfer interaction.10 In the
application to EL devices, it is also of interest to develop mate-
rials with improved stability and an alternative class of materials
for solution processing. Recently, several groups have reported
dendrimer-based EL devices.11 We report herein a facile one-
pot synthesis and photophysical properties of silole-core dendri-
mers.
Our synthetic strategy for the silole-core dendrimers is
based on the nickel-catalyzed reaction of 1,1,2,2-tetramethyldi-
silane with disubstituted alkynes,12 which has proven to work
well for dendritic alkynes 1b–1d affording silole-core dendri-
mers 2b–2d. Thus alkynes 1b–1d, prepared by the reaction of
1,2-bis(3,5-dihydroxyphenyl)acetylene with appropriate benzyl
ether-type dendritic bromides,13 were treated with the tetra-
methyldisilane in the presence of NiCl2(PEt3)2 in refluxing tol-
uene (Scheme 1). As the generation grows, the time required
to complete the reaction increased and the reaction became less
clean resulting in a lower yield, especially for 2d. However, si-
lole-core dendrimers 2b–2d could be readily purified by column
chromatography and the structures were characterized by FAB
mass or MALDI-TOF mass spectrometry, GPC, and NMR spec-
O
Scheme 1. Synthesis of silole-core dendrimers.
troscopy.14 This synthetic procedure demonstrates the alterna-
tive convergent approach via one-pot synthesis of the core for
the construction of precise macromolecular structure.15
To the best of our knowledge, these are the first examples of
silole-core dendrimers. The photophysical data in solution,
along with those of the parent compound 2a, are summarized
in Table 1 and the absorption and fluorescence spectra of 2a–
2d are shown in Figure 1. In the absorption spectra, the silole-
core dendrimers (2b–2d) have two absorption bands at about
280 and 360 nm, assignable to the benzyl ether-type dendrons
and the focal silole ring, respectively. The intensity of absorption
of the dendron units increased with increasing the generation of
the dendrons, while the intensity and the position of absorption
of the silole ring did not change significantly. When the silole
ring was excited at 360 nm, the silole dendrimers 2b–2d dis-
played an emission at about 500 nm. The fluorescence quantum
yield (ꢀFL) increased with increasing the generation of the den-
dron units (Table 1). The ꢀFL of 2d was about 200 times greater
than that of the model compound 2a, because the dendron units
prevent the quenching processes for the silole ring effectively.
Increased chromophore rigidity often leads to improved lumi-
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.8 (2005)
1131
10
2c, than that displayed by direct excitation of the silole core
(360 nm). This ‘‘antenna effect’’ is the result of the larger extinc-
tion of the dendrimers at 280 nm compared to 360 nm.9
In conclusion, we have prepared the silole-core dendrimers
by the nickel-catalyzed reaction of the tetramethyldisilane and
dendritic acetylenes having poly(benzyl ether) units through
the third generation. The findings herein reported would be
important in view of optoelectronic applications. Further study
along the line is currently in progress.
2a
b
c
FLU
ABS
d
5
0
This work was partially supported by CREST-JST. We also
thank the Ministry of Education, Culture, Sports, Science and
Technology of Japan (No. 16685004).
200
300
400
Wavelength/nm
500
600
References and Notes
Figure 1. Absorption and fluorescence spectra of 2a–2d in
CH2Cl2.
1
a) J. Dubac, C. Guerin, and P. Meunier, ‘‘The Chemistry of Organic
Silicon Compounds,’’ ed. by Z. Rappoport and Y. Apeloig, John Wiley
& Sons, New York (1999), Vol. 2, Chap. 34. b) S. Yamaguchi and K.
Tamao, Bull. Chem. Soc. Jpn., 69, 2327 (1996). c) S. Yamaguchi and
K. Tamao, Chem. Lett., 34, 2 (2005). d) M. Hissler, P. W. Dyer, and R.
Table 1. Photophysical properties of silole-core dendrimers
2a–2d
´
Reau, Coord. Chem. Rev., 244, 1 (2003).
2
3
K. Tamao, M. Uchida, T. Izumizawa, K. Furukawa, and S. Yamaguchi,
J. Am. Chem. Soc., 118, 11974 (1996).
Absorption
Fluorescence
Cmpd
(Gn)
b,c
ꢂ
max/nm "=104 cmꢂ1 Mꢂ1
ꢂ
FLmax/nma
ꢀFL
ꢀET/%d
a) S. Yamaguchi, T. Endo, M. Uchida, T. Izumizawa, K. Furukawa, and
K. Tamao, Chem.—Eur. J., 6, 1683 (2000). b) S. Yamaguchi and K.
Tamao, J. Chem. Soc., Dalton Trans., 1998, 3693. c) J. Ohshita, H.
Kai, A. Takata, T. Iida, A. Kunai, N. Ohta, K. Komaguchi, M. Shiotani,
A. Adachi, K. Sakamaki, and K. Okita, Organometallics, 20, 4800
(2000). d) K.-H. Lee, J. Ohshita, and A. Kunai, Organometallics, 23,
5365 (2004). e) T. Lee, I. Jung, K.-H. Song, H. Lee, J. Choi, K. Lee,
B. J. Lee, J. Park, C. Lee, S. O. Kang, and J. Ko, Organometallics, 23,
5280 (2004).
a) S. Yamaguchi, R.-Z. Jin, and K. Tamao, J. Am. Chem. Soc., 121, 2937
(1999). b) H. Sohn, R. R. Huddleston, D. P. Powell, and R. West, J. Am.
Chem. Soc., 121, 2935 (1999). c) T. Sanji, T. Sakai, C. Kabuto, and
H. Sakurai, J. Am. Chem. Soc., 120, 4552 (1998). d) J. Chen, Z. Xie,
J. W. Y. Lam, C. C. W. Law, and B. Z. Tang, Macromolecules, 36,
1108 (2003).
J. Luo, Z. Xie, W. Y. Lam, L. Chang, H. Chen, C. Qiu, H. S. Kwok, X.
Zhan, Y. Zhu, and B. Z. Tang, Chem. Commun., 2001, 1740.
The very high photoluminescence quantum yield of tetraphenylsilole
derivatives in the solid state (ꢁ97%) was reported: H. Murata, Z. H.
Kafafi, and M. Uchida, Appl. Phys. Lett., 80, 189 (2002).
a) S. Yamaguchi, C. Xu, and K. Tamao, J. Am. Chem. Soc., 125, 13662
(2003). b) C. Xu, A. Wakamiya, and S. Yamaguchi, J. Am. Chem. Soc.,
127, 1638 (2005).
e
2a
(G0)
2b
249
356
278f
360
277
366
278
358
2.34
0.98
1.59
0.93
3.85
0.88
7.64
0.72
485
514
514
503
5:0 ꢃ 10ꢂ4
—
2:5 ꢃ 10ꢂ2 ꢁ100
(G1)
2c
6:8 ꢃ 10ꢂ2
51
28
(G2)
2d
4
0.12
(G3)
aExcited at 360 nm. bDetermined with quinine sulfate as a standard.
cFluorescence quantum yield, when excited at the core silole rings.
dEnergy transfer quantum efficiency from the dendron units to the core
silole within the dendrimers, when excited at the dendron units (280
nm). eNot estimated. fObserved as a shoulder.
5
6
nescence by dually facilitating electron delocalization and min-
imizing vibrational–rotational events in the excited state. Inter-
estingly, excitation of the benzyl ether-type dendron units at
280 nm also displayed an emission around 500 nm from the si-
lole ring unit, but almost no emission from the benzyl ether-type
dendron units was observed (310 nm),9 indicating that the exten-
sive energy transfer (ET) from the dendron units to the focal si-
lole ring occurred within the dendrimers. The ET quantum effi-
ciency (ꢀET), estimated by a comparison of the absorption spec-
trum and the excitation spectrum of the silole-core dendrimers
by monitoring the emission of the acceptor, i.e., the silole, are
also listed in Table 1. The efficiency was essentially quantitative
for 2b. However, the energy transfer efficiency to the core de-
creased with the generation growth of the dendrons and dropped
7
8
9
a) A. J. Boydston, Y. Yin, and B. J. Pagenkopf, J. Am. Chem. Soc., 126,
3724 (2004). b) A. J. Boysdston and B. L. Pagenkopf, Angew. Chem., Int.
Ed., 43, 6336 (2004).
´
S. M. Grayson and J. M. J. Frechet, Chem. Rev., 101, 3819 (2001).
10 a) D.-L. Jiang and T. Aida, J. Am. Chem. Soc., 120, 10895 (1998). b)
M. S. Choi, T. Aida, T. Yamazaki, and I. Yamazaki, Chem.—Eur. J., 8,
2667 (2002). c) C. Devadoss, P. Bharathi, and J. S. Moore, J. Am. Chem.
Soc., 118, 9635 (1996).
´
11 a) A. Adronov and J. M. J. Frechet, Chem. Commun., 2000, 1701. b)
P.-W. Wang, Y.-J. Liu, C. Devadoss, P. Bharathi, and J. S. Moore,
Adv. Mater., 8, 237 (1996). c) A. W. Freeman, S. C. Koene, P. R. L.
´
Malenfant, M. E. Thompson, and J. M. J. Frechet, J. Am. Chem. Soc.,
122, 12385 (2000).
12 H. Okinoshima, K. Yamamoto, and M. Kumada, J. Am. Chem. Soc., 94,
9263 (1972).
13 C. Hawker and J. M. J. Frechet, J. Am. Chem. Soc., 112, 7638 (1990).
14 The syntheses will be described in detail: T. Sanji, H. Ishiwata, T.
Kaizuka, M. Tanaka, H. Sakurai, R. Nagahata, and K. Takeuchi, Can.
J. Chem., in press.
´
15 a) S. Hecht and J. M. J. Frechet, J. Am. Chem. Soc., 121, 4084 (1999).
b) K. H. Duchene and F. Vogtle, Synthesis, 1986, 659. c) G. V.
dramatically to 28% for 2d.16 The Forster-type ET is favored by
¨
´
a) the large spectral overlap between donor emission and accep-
tor absorption, b) the high molar absorption coefficient of the ac-
cepter, c) the high fluorescence quantum yield of the donor, and
d) the short interchromophoric distance.11 The present system
was, however, not very high, especially for the higher genera-
tion,9 probably because all of the requirements for the efficient
Forster ET were not met. Further, the emission from the silole
¨
core observed upon excitation at the dendron units (280 nm)
was more intense, approximately three times more for 2b and
ˆ
¨
Wuytswinkel, B. Verheyde, F. Compernolle, S. Toppet, and W. Dehaen,
J. Chem. Soc., Perkin Trans. 1, 2000, 1337.
16 When 2d was excited at 280 nm, an emission around 390 nm was
observed along with the emission at 500 nm from the silole ring. A
detailed photophysical property of the dendrimers needs further study.
Published on the web (Advance View) July 9, 2005; DOI 10.1246/cl.2005.1130