Bis-Silicon-Bridged Stilbene Homologues Synthesized by New Intramolecular Reductive Double Cyclization

Article abstract of DOI:10.1021/ja038487+

A homologous series of bis-silicon-bridged stilbenes has been synthesized on the basis of a new intramolecular reductive cyclization of bis(o-silyl)-diphenylacetylene. Thus, the reaction of bis(o-silyl)-diphenylacetylenes with excess lithium naphthalenide

Full text of DOI:10.1021/ja038487+

Published on Web 10/21/2003
Bis-Silicon-Bridged Stilbene Homologues Synthesized by New Intramolecular
Reductive Double Cyclization
Shigehiro Yamaguchi,*,†,‡ Caihong Xu,‡ and Kohei Tamao*,§
Department of Chemistry, Graduate School of Science, Nagoya UniVersity, PRESTO, Japan Science and Technology
Corporation (JST), Chikusa, Nagoya 464-8602, Japan, and Institute for Chemical Research, Kyoto UniVersity, Uji,
Kyoto 611-0011, Japan
Received September 12, 2003; E-mail: yamaguchi@chem.nagoya-u.ac.jp; tamao@scl.kyoto-u.ac.jp
Ladder-type π-conjugated molecules1,2 are promising materials
Scheme 1
for organic-based devices, including light-emitting diodes,3,4 field
effect transistors,5,6 and optically pumped solid-state lasers.7 In these
molecules, flattening the π-conjugated framework by annelation
eliminates the conformational disorder and effectively enhances the
π-conjugation, which leads to a set of desirable properties such as
high fluorescence efficiency8 and high carrier mobility.9 For
instance, while trans-stilbene only shows a weak fluorescence
(ΦF ≈ 0.05) at ambient temperature, the fluorescence quantum yield
Scheme 2 a
of its bis-methylene-bridged derivative 1 (R ) H) approaches
unity.10 As a new ladder-type π-conjugated system, we are now
interested in its silicon analogue, bis-silicon-bridged stilbene 2. The
silicon-bridges would not only stiffen the skeleton but also
contribute to the electronic structure through orbital interaction, as
seen in the chemistry of silole,11 a substructure of 2. In addition,
this skeleton may be attractive as a model of the all-silicon-bridged
poly(p-phenylenevinylene) 3. While the first example of 2 (R )
Me) was recently synthesized by Barton and co-workers on the
basis of the elegant thermal isomerization of the 5,6-disiladibenzo-
[c,g]cyclooctyne derivative,12 we have been interested in developing
a more facile and general synthetic route. We now disclose the
a Reagents and conditions: (a) (1) t-BuLi (8 mol. amt.), THF, -78 °C,
(2) (Et2N)Me2SiCl (9 mol. amt.), -78 °C to room temperature, (3) EtOH
(excess), NH4Cl (0.5 mol. amt.); (b) (1) LiNaph (8 mol. amt.), THF, room
temperature, (2) I2 (8 mol. amt.), room temperature.
by quenching with iodine, successfully produced the cyclized
product 2a in 71% yield, as shown in Scheme 1. Notably, the
reaction quickly proceeded and was completed within 5 min at room
temperature. As a starting material, the ethoxysilyl analogue 4a′
was also effective and produced 2a in 65% yield. In addition, a
diphenylsilyl derivative 4b similarly gave 2b.
In the present cyclization, the use of excess reductant as well as
the iodine workup are crucial. Because the produced stilbene has
a slightly lower reduction potential than that of the starting
diphenylacetylene,13 the over-reduction of the produced stilbene
skeleton competitively proceeds with the consumption of some
amount of the reductant. Therefore, the use of excess reductant is
essential for the complete conversion of the starting material. In
addition, the iodine workup effectively regenerates the target
stilbene from the “over-reduced” intermediate 5. In fact, while the
hydrolytic workup of the reaction mixture of 4a with LiNaph
(4 molar amounts) resulted in the formation of 6 in 65% yield as
the major product, along with 2a in 15% yield, the iodine workup
improved the yield of 2a up to 71% yield, as mentioned above,
and suppressed the formation of 6 to 9% yield.
This methodology is applicable to the synthesis of a more
extended homologue 9, as shown in Scheme 2. Thus, as a starting
material, compound 8 was prepared in 47% yield from the
tetrabromide 7 via tetralithiation with t-BuLi followed by treatment
with (Et2N)Me2SiCl and subsequent ethanolysis. Upon treatment
of 8 with excess LiNaph (8 molar amounts) at room temperature,
the intramolecular reductive double cyclization proceeded at two
acetylene moieties, and the iodine workup afforded the tetrakis-
silicon-bridged derivative 9 in 91% yield as a bright yellow solid.
efficient synthesis of the silicon-bridged systems based on a new
intramolecular reductive cyclization. The structure-property rela-
tionship of a homologous series of the synthesized compounds will
also be reported.
Our new cyclization, outlined in eq 1, employs bis(o-silyl)-
diphenylacetylene derivatives as the starting materials. We antici-
pated that the two-electron reduction of the acetylene moiety
produces a dianion intermediate, which undergoes a double
cyclization in a 5-exo mode to produce the target skeleton. Indeed,
the treatment of bis(o-hydrodimethylsilyl)-diphenylacetylene 4a
with 4 molar amounts of lithium naphthalenide (LiNaph), followed
† Nagoya University.
‡ PRESTO, JST.
§ Kyoto University.
9
13662
J. AM. CHEM. SOC. 2003, 125, 13662-13663
10.1021/ja038487+ CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
π-conjugation from 2a to 9 causes substantial red shifts in the
absorption and emission maxima by 64 and 47 nm, respectively,
without a significant decrease in ΦF. Consequently, 9 exhibits an
intense greenish blue emission, indicative of their potential use as
a new emitting material.
In summary, we have developed a new intramolecular reductive
double cyclization as an efficient synthetic method for the silicon-
bridged stilbene homologues. This methodology will open a new
chemistry of bridged stilbene-based π-conjugated systems, which
have great potentials as new materials for the organic-based
electronic and optoelectronic devices. Further studies on the
electronic properties of the present compounds as well as the
synthesis of more extended ladder-type π-conjugated systems such
as 3 are currently in progress in our laboratory.
Figure 1. ORTEP drawing of 9 (50% probability for thermal ellipsoids).
Table 1. Photophysical Properties of Bridged Stilbene Derivatives
UV−vis absorptiona
fluorescencea
b
c
cmpd
λ
max/nm
log ꢀ
λ
max/nm
ΦF
τS/nsd
Acknowledgment. We thank Prof. Hiroshi Hiratsuka at Gunma
University (Japan) for his helpful discussion. This work was
supported by Grants-in-Aid (No. 12CE2005 for Elements Science
and 15205014) from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan, and PRESTO, Japan Science
and Technology Corporation (JST).
2a
2b
9
360
371
424
447
322
4.08
3.92
4.43
4.36
4.45
426
439
473
0.58
0.61
0.50e
5.5
n.d.
3.5
1a (R ) Me)
367
0.92f
1.6
a In THF. b Emission maxima upon excitation at the absorption maximum
wavelengths. c Determined with 9,10-diphenylanthracene as a standard,
unless otherwise stated. The ΦF is the average value of repeated measure-
ments within (5% error. d Fluorescence lifetimes within (0.5 ns error.
e Determined with perylene as a standard. f Determined with anthracene as
a standard.
Supporting Information Available: Experimental procedures and
spectral data for 2, 4, and 7-9, and crystallographic data of 9 (PDF
and CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
References
To the best of our knowledge, this is the first example of the
tetrakis-bridged bis(styryl)benzene. The X-ray crystallographic
analysis of 9 proved its highly coplanar structure due to the tetrakis-
silicon bridges; the dihedral angle between the central and the outer
benzene rings is 3.9° (Figure 1). This fact suggests that the
π-conjugation is effectively extended over the entire molecule.
The photophysical data of the silicon-bridged stilbene homo-
logues are summarized in Table 1, together with the data of 1a
(R ) Me),14 for comparison. There are a couple of notable points:
(1) The silicon-bridge significantly shifts the absorption and
emission maxima to a longer wavelength. As compared with the
carbon analogue 1a, the absorption and emission maxima of 2a
are longer by about 40 and 60 nm, respectively. These significant
red shifts are attributable to the electronic contribution of the silicon
bridges. Thus, the orbital interaction between the σ* orbital of the
Me2Si moiety and the π* orbital of the stilbene framework
effectively decreases the LUMO level.11a Preliminary calculations
at the HF/6-31G(d) level showed that the LUMO of 2a is about
0.55 eV lower than that of 1a, while the decrease in the HOMO
level from 1a to 2a is only 0.19 eV. As a consequence, the silicon
analogue 2a has a smaller HOMO-LUMO energy gap and its
emission color reaches the blue region, despite its rather short
π-conjugation length. (2) The fluorescence quantum yield of the
silicon analogues tends to be slightly lower than that of the carbon
analogue 1a (R ) Me). Fluorescence lifetime measurements showed
that this is mainly due to the slower radiative decay process from
the excited state in 2a. Thus, the radiative rate constant of 2a (kr )
1.1 × 108 s-1), calculated on the basis of ΦF and τs, is about one-
fifth of that of 1a (kr ) 5.8 × 108 s-1), while the nonradiative rate
constants are only slightly different from each other (knr ) 7.6 ×
107 s-1 for 2a vs 5.0 × 107 s-1 for 1a). (3) The extension of the
(1) For recent reviews: (a) Martin, R. E.; Diederich, F. Angew. Chem., Int.
Ed. 1998, 37, 402. (b) Scherf, U. J. Mater. Chem. 1999, 9, 1853. (c)
Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV. 2001, 101,
1267.
(2) (a) Zhang, Q. T.; Tour, J. M. J. Am. Chem. Soc. 1997, 119, 9624. (b)
Haryono, A.; Miyatake, K.; Natori, J.; Tsuchida, E. Macromolecules 1999,
32, 3146. (c) Seayesh, S.; Marsitzky, D.; Mu¨llen, K. Macromolecules 2000,
33, 2016. (d) Xia, C.; Advincula, R. C. Macromolecules 2001, 34, 6922.
(e) Yamaguchi, S.; Swager, T. M. J. Am. Chem. Soc. 2001, 123, 12087.
(f) Oyaizu, K.; Mikami, T.; Mitsuhashi, F.; Tsuchida, E. Macromolecules
2002, 35, 67. (g) Former, C.; Becker, S.; Grimsdale, C.; Mu¨llen, K.
Macromolecules 2002, 35, 1576.
(3) Stampfl, J.; Granupner, W.; Leising, G.; Scherf, U. J. Lumin. 1995, 63,
117. Tasch, S.; Niko, A.; Leising, G.; Scherf, U. Appl. Phys. Lett. 1996,
68, 1090.
(4) (a) Bernius, M. T.; Inbasekaran, M.; O’Brien, J.; Wu, W. AdV. Mater.
2000, 12, 1737. (b) Wu, C.-c.; Liu, T.-L.; Hung, W.-Y.; Lin, Y.-T.; Wong.
K.-T.; Chen, R.-T.; Chen, Y.-M.; Chien, Y.-Y. J. Am. Chem. Soc. 2003,
125, 3710.
(5) Babel, A.; Jenekhe, S. A. AdV. Mater. 2002, 14, 371.
(6) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359.
(7) (a) Zenz, C.; Graupner, W.; Tasch, S.; Leising, G.; Mu¨llen, K.; Scherf,
U. Appl. Phys. Lett. 1997, 71, 2566. (b) Kallinger, C.; Hilmer, M.;
Haugeneder, A.; Perner, M.; Spirkl, W.; Lemmer, U.; Feldmann, J.; Scherf,
U.; Mu¨llen, K.; Gombert, A.; Wittwer, V. AdV. Mater. 1998, 10, 920.
(8) Lemmer, U.; Heun, S.; Mahrt, R. F.; Scherf, U.; Hopmeier, M.; Siegner,
U.; Go¨bel, E. O.; Mu¨llen, K.; Ba¨ssler, H. Chem. Phys. Lett. 1995, 240,
373.
(9) Hertel, D.; Scherf, U.; Ba¨ssler, H. AdV. Mater. 1998, 10, 1119.
(10) (a) Saltiel, J.; Zafiriou, O. C.; Megarity, E. D.; Lamola, A. A. J. Am.
Chem. Soc. 1968, 90, 4759. (b) Saltiel, J.; Marinari, A.; Chang, D. W. L.;
Mitchener, J. C.; Megarity, E. D. J. Am. Chem. Soc. 1979, 101, 2982.
(11) (a) Yamaguchi S.; Tamao, K. Bull. Chem. Soc. Jpn. 1996, 69, 2327. (b)
Tamao, K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi, S. J.
Am. Chem. Soc. 1996, 118, 11974. (c) Yamaguchi, S.; Endo, T.; Uchida,
M.; Izumizawa, T.; Furukawa, K.; Tamao, K. Chem.-Eur. J. 2000, 6, 1684.
(12) Serby, M.; Ijadi-Maghsoodi, S.; Barton, T. J. 33rd Symposium on
Organosilicon Chemistry; Saginaw, MI, 2000; PA-35.
(13) Calculations at the HF/6-31G(d) level showed that the LUMO of 2a is
about 0.1 eV lower than that of 4a.
(14) Hellwinkel, D.; Hasselbach, H.-J.; La¨mmerzahl, F. Angew. Chem., Int.
Ed. Engl. 1984, 23, 705.
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