Coplanar oligo(p-phenylenedisilenylene)s based on the octaethyl-substituted s-hydrindacenyl groups

Article abstract of DOI:10.1021/ja0764207

The silicon analogues of the oligo(p-phenylenevinylene)s (Si-OPVs) with highly planar structures have been synthesized using a newly developed ligand, the 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (Eind) group. Their X-ray crystal structures and spectr

Full text of DOI:10.1021/ja0764207

Published on Web 10/27/2007
Coplanar Oligo(p-phenylenedisilenylene)s Based on the Octaethyl-Substituted
s-Hydrindacenyl Groups
Aiko Fukazawa,†,‡ Yongming Li,† Shigehiro Yamaguchi,‡ Hayato Tsuji,*,†,¶ and Kohei Tamao*,†,§
International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto UniVersity,
Uji, Kyoto 611-0011, Japan, Department of Chemistry, Graduate School of Science, Nagoya UniVersity,
Furo, Chikusa, Nagoya 464-8602, Japan, and Frontier Research System, RIKEN, 2-1 Hirosawa,
Wako, Saitama 351-0198, Japan
Received August 27, 2007; E-mail: tsuji@chem.s.u-tokyo.ac.jp; tamao@riken.jp
Since the stable silene (SidC),1 disilene (SidSi),2 and diphos-
phene (PdP)3 were reported in 1981 by introducing a concept of
steric protection with bulky substituents, a variety of unsaturated
compounds of heavy main group elements have successfully been
isolated by many leading scientists by using their own, newly
developed bulky ligands.4 The incorporation of multiple bonds of
heavy main group elements into a π-conjugated framework would
provide access to new potential materials for organic electronics.
In this regard, several examples of oligomers and polymers
comprising heavy multiple bonds, such as PdC,5 PdP,6 SidSi,7-9
GedC,10and GedGe11 bonds in the main chain, have recently been
reported. However, this chemistry always suffers from a dilemma.
While the steric protection by bulky ligands is essential to stabilize
the highly reactive heavy multiple bonds, it causes the π-conjugated
framework to twist, which reduces the extension of the π-conjuga-
tion. A key for the further evolution of this chemistry is undoubtedly
to attain a well-defined ligand which can maintain the highly planar
π-conjugated framework, in addition to providing sufficient steric
protection of the reactive heavier multiple bonds.
We now present a new ligand, the 1,1,3,3,5,5,7,7-octaethyl-s-
hydrindacen-4-yl (Eind) group.12 By exploiting this ligand, we have
succeeded in synthesizing the disilene analogues of the oligo(p-
phenylenevinylene)s (Si-OPVs), disilastilbene 1 and tetrasila-
distyrylbenzene 2 with highly coplanar π-conjugated frameworks.
Their photophysical properties as well as X-ray single crystal
analyses provide clear evidence for the effective extension of the
π-conjugation. During the course of our investigation, Scheschke-
witz et al. reported a 2,4,6-triisopropylphenyl-substituted tetrasi-
ladistyrylbenzene derivative by a different route.9
with an exclusively E configuration as orange crystals in 76% yield
(eq 1), of which the structure was confirmed by X-ray crystal-
lography (Figure S1). Similarly, the tetrasiladistyrylbenzene 2 was
synthesized by the reductive coupling of the dibromosilane 3 and
bis(dibromosilane) 4 in a 5:1 ratio (eq 2). After separation by silica
gel column chromatography in a glovebox using degassed hexane
and toluene as the eluent, the tetrasiladistyrylbenzene 2 was
successfully isolated as purple-red crystals in 15% yield (based on
4), together with the disilastilbene 1 in 35% yield (based on 3) and
unidentified purple higher oligomers. These disilenes are air-stable
in the solid state (no decomposition in 1 was observed for at least
1 month), whereas they decomposed within 2 days in a dilute
solution at room temperature in air. Their high stability demon-
strated the effectiveness of the steric protection by the bulky Eind
groups.
Figure 1 shows the crystal structure of 2. Notably, the tetrasi-
ladistyrylbenzene skeleton is entirely coplanar with the dihedral
angle between the central and terminal benzene rings of 9.0°. It is
also noteworthy that the SidSi bonds adopt an almost planar
geometry with the twist angles of 0.3 and 3.8° as well as the trans-
bent angles of 0.7 and 2.7°.13 The disilastilbene 1 also has an entirely
planar geometry (Figure S1). The space-filling model (Figure 1c)
shows that the s-hydrindacene planes of the Eind groups are
orthogonal to the tetrasiladistyrylbenzene framework, and the
peripheral ethyl groups effectively protect the SidSi moieties
without severe steric repulsion between themselves.
The high coplanarity, observed in 1 and 2, is of interest,
considering the fact that a disilene moiety usually has a bent and/
or twisted geometry depending on the substituents.14 In our case,
within the disilene unit, the ethyl side chains on the rigid
s-hydrindacene skeletons interlock with one another above and
below the SidSi moiety to enforce the planar geometry. The
resulting cavity surrounded by the ethyl groups fixes the framework
phenyl groups in a coplanar structure with the SidSi plane, which
is ideal for extension of the π-conjugation.
The photophysical data for 1 and 2 are summarized in Table 1,
together with those of the recently reported 5,6 trans-stilbene 6,15
and trans,trans-1,4-distyrylbenzene 716 for comparison. Their
spectra show several features to be noted as follows: (1) The
absorption maximum of 1 corresponding to the π-π* transition
appears at 461 nm (ꢀ 2.4 × 104), which is the longest value among
those of the already known tetraaryldisilenes (λmax 400-440 nm).
(2) The tetrasiladistyrylbenzene 2 has an absorption maximum at
543 nm with a larger molar extinction coefficient (ꢀ 3.0 × 104)
compared to the disilastilbene 1. (3) The λmax of 2 is about 190 nm
longer than that of the carbon analogue 7, demonstrating that the
incorporation of the SidSi bonds into the π-conjugated system
narrows the band gap. However, in terms of the degree of the
extension of the π-conjugation, the shift of the absorption maximum
The Eind-substituted Si-OPVs 1 and 2 were synthesized by the
reductive coupling of the corresponding dibromosilanes 3 and 4
with lithium naphthalenide (LiNaph) in THF. Thus, the homocou-
pling reaction of the dibromosilane 3 afforded the disilastilbene 1
† Kyoto University.
‡ Nagoya University.
§ RIKEN.
¶ Present address: Department of Chemistry, School of Science, The University
of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
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10.1021/ja0764207 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
responsible for the fluorescence. The radiative rate constant (kr)
and nonradiative rate constant (knr) of 2 were 5.6 × 107 and 5.0 ×
108 s-1, respectively, according to the calculation with the
experimental ΦF and τs values. The experimental kr value is
comparable to the theoretical value of 8.5 × 107 calculated from
the area of the lowest-energy absorption band.19 These observations
also support the assumption that the fluorescence is due to the
predominant conformer in solution.
In summary, we have synthesized planar Si-OPVs 1 and 2 by
introduction of a newly developed Eind ligand, which not only
efficiently protects the reactive SidSi bridge but also controls the
Si-OPV framework to coplanar structure. Further investigation in
synthesizing higher homologues of Si-OPVs and various π-extended
systems is currently in progress.
Acknowledgment. Dedicated to the memory of Prof. Makoto
Kumada. Financial support from Creative Scientific Research
(17GS0207) for H.T. is gratefully acknowledged. We thank Dr.
Motoo Shiro (Rigaku Co.) and Dr. Atsushi Wakamiya (Nagoya
Univ.) for their kind help with X-ray crystallographic analysis, and
Prof. Norihiro Tokitoh and Ms. Toshiko Hirano (ICR, Kyoto Univ.)
for elemental analysis. We are also grateful to Dr. Tsukasa Matsuo
(RIKEN) for valuable discussions.
Figure 1. Crystal structures of 2. ORTEP drawing (50% probability for
thermal ellipsoids): (a) top view, (b) front view. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and bond angles (°): Si1-Si2, 2.156-
(2); Si1-C1, 1.860(6); Si1-C4, 1.911(5); Si2-C32, 1.876(6); Si2-C38,
1.902(5); C1-Si1-C4, 114.5(3); C1-Si1-Si2, 120.4(2); C4-Si1-Si2,
125.12(19); C32-Si2-Si1, 117.51(19); C32-Si2-C38, 119.6(3); C38-
Si2-Si1, 122.79(19); (c) space filling model: red, silicon; gray, carbon;
white, hydrogen. Carbon atoms in the disilastyrylbenzene framework are
colored light-blue for clarity.
Supporting Information Available: Experimental details, spectral
data, crystallographic data of 1 and 2 (PDF and CIF), and MO
calculation results on 1 and 2. This material is available free of charge
via the Internet at http://pubs.acs.org.
Table 1. Photophysical Data of Disilenes 1 and 2 and Related
Compoundsa
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1
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6e
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335 (29900)
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(τs) of 0.10 and 1.8 ns, respectively, in a hexane solution. To the
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