Ladder distyrylbenzenes with silicon and chalcogen bridges: Synthesis, structures, and properties

Article abstract of DOI:10.1021/ol062615s

(Chemical Equation Presented) A cascade-type anionic double cyclization of (o-silylphenyl)(ohalophenyl)acetylenes via lithiation followed by treatment with elemental chalcogen produces silicon and chalcogen-bridged stilbenes. Based on this reaction, a ser

Full text of DOI:10.1021/ol062615s

ORGANIC
LETTERS
2007
Vol. 9, No. 1
93-96
Ladder Distyrylbenzenes with Silicon
and Chalcogen Bridges: Synthesis,
Structures, and Properties
Kazuhiro Mouri, Atsushi Wakamiya, Hiroshi Yamada, Takashi Kajiwara, and
Shigehiro Yamaguchi*
Department of Chemistry, Graduate School of Science, Nagoya UniVersity, and
SORST, Japan Science and Technology Agency (JST), Chikusa,
Nagoya 464-8602, Japan
yamaguchi@chem.nagoya-u.ac.jp
Received October 24, 2006
ABSTRACT
A cascade-type anionic double cyclization of (o-silylphenyl)(o-halophenyl)acetylenes via lithiation followed by treatment with elemental chalcogen
produces silicon and chalcogen-bridged stilbenes. Based on this reaction, a series of silicon and sulfur- or silicon and selenium-bridged
ladder distyrylbenzenes have been synthesized. Their chemical modification by oxidation, crystal structures, and photophysical properties are
described.
Ladder π-conjugated systems with fused polycyclic skeletons
are an important class of materials for organic electronics.1
Their flat and rigid π-conjugated frameworks promise the
effective extension of the π-conjugation without any con-
formational disorder, thus leading to a set of desirable
properties such as an intense luminescence and high carrier
mobility. Although ladder oligo- or poly(p-phenylene)s with
methylene2 or heteroatom bridges3 are a representative
example, whose significant potentials for applications have
been proven by recent extensive research, their vinylogous
homologues, ladder oligo- or poly(p-phenylenevinylene)s
(LOPVs or LPPVs), have attracted only limited attention to
date.4-7 As a new entry into this class of compounds, we
are now interested in a silylene and chalcogen (S or Se)-
bridged LPPV skeleton (Figure 1). The introduction of the
silylene bridges would make it possible to perturb the
(3) (a) Haryono, A.; Miyatake, K.; Natori, J.; Tsuchida, E. Macromol-
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10.1021/ol062615s CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/09/2006

Scheme 2
Figure 1. Structural modification of poly(p-phenylenevinylene)s
to the silicon and chalcogen-bridged ladder derivatives.
electronic structure through the σ*-π* conjugation in the
silole substructure.8 In addition, the incorporation of the
thiophene or selenophene substructure would enhance the
chemical stability by increasing the aromatic character of
the π-conjugated frameworks. The possible electronic tuning
by the chemical oxidation from the thiophene substructure
to the thiophene-S,S-dioxide would also be of notable merit.9
As a model of this ladder system, we now disclose a synthetic
route to the silicon and chalcogen-bridged distyrylbenzenes.
Their chemical modification by oxidation, crystal structures,
and photophysical properties were also investigated.
On the basis of this cascade cyclization, a series of ladder
distyrylbenzenes 5 were successfully synthesized from the
diacetylenic starting materials, as shown in Scheme 2. Thus,
5a having the silylene and sulfur bridges at the inner and
outer positions, respectively, was obtained from 4a in a 40%
yield, while the reaction of 4b afforded 5b, which possesses
the silylene and sulfur bridges at the inverse positions
compared to 5a. Noticeably, this reaction also successfully
proceeds with selenium instead of sulfur, yielding sele-
nophene-derivative 5c in a 30% yield. The oxidation of the
produced ladder molecules 5a and 5b with 5 molar amounts
of mCPBA afforded the corresponding thiophene-S,S-dioxide
derivatives 6a and 6b in 18% and 45% yields, respectively.
All of the ladder compounds are soluble in common solvents
such as THF and CHCl3. The thermogravimetric analysis
revealed that not only the ladder compounds 5 but also the
thiophene-S,S-dioxide derivatives 6 have high thermal stabil-
ity with their decomposition temperatures Td5 for a 5% weight
loss beyond 300 °C (5a, 307 °C; 5b, 308 °C; 5c, 318 °C;
6a, 360 °C; 6b, 355 °C), indicative of possible fabrication
of vapor-deposited films using these compounds.
Our strategy to construct the Si,S-bridged ladder phenyl-
enevinylene skeletons is based on a new cascade-type anionic
cyclization from the (o-silylphenyl)(o-bromophenyl)acetyl-
ene precursors 1, as shown in Scheme 1. Thus, the lithium-
Scheme 1
halogen exchange reaction of 1 with t-BuLi followed by
treatment with elemental sulfur produces a thiolate anion 2,
which further undergoes the 5-endo-dig mode cyclization,
followed by a subsequent nucleophilic substitution at the
silicon center to give the double-cyclized Si,S-bridged
stilbene 3. In fact, compound 1 (R ) Me, X ) OEt) was
successfully converted into 3 in a 54% yield. It is worth
noting that the 5-endo-dig mode cyclization of (o-ethynyl-
benzene)thiolate is a general methodology for the construc-
tion of the benzothiophene skeleton.10 In our reaction, the
incorporation of the silyl group at the appropriate position
allows us to synthesize the doubly cyclized product.
An X-ray crystallographic analysis confirmed that all of
the ladder compounds 5 and 6 have highly coplanar
π-conjugated frameworks (see Supporting Information).
Among them, it is worth noting that 5a having the silylene
bridges at the inner positions forms a slipped face-to-face
packing structure, as shown in Figure 2. In general, the
incorporation of the tetra-coordinate silicon bridges is a
disadvantage for forming a densely packed structure, since
the substituents on the silicon atoms prevent the formation
of close π-stacking. In 5a, however, the substituents on the
silicon atoms play a role as a guide to regulate the
intermolecular interaction in the slipped face-to-face fashion.
This might be beneficial to achieving a high carrier mobility.
In this structure, the interfacial distance of the adjacent
molecules is about 3.6 Å.
(8) Yamaguchi, S.; Tamao, K. Bull. Chem. Soc. Jpn. 1996, 69,
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(9) (a) Tanaka, K.; Wang, S.; Yamabe, T. Synth. Met. 1989, 30, 57. (b)
Barbarella, G.; Favaretto, L.; Zambianchi, M.; Pudova, O.; Arbizzani,
C.; Bongini, A.; Mastragostino, M. AdV. Mater. 1998, 10, 551. (c)
Barbarella, G.; Favaretto, L.; Sotgiu, G.; Zambianchi, M.; Bongini, A.;
Arbizzani, C.; Mastragostino, M.; Anni, M.; Gigli, G.; Cingolani, R. J. Am.
Chem. Soc. 2000, 122, 11971. (d) Jiang, B.; Tilley, T. D. J. Am. Chem.
Soc. 1999, 121, 9744. (e) Suh, M. C.; Jiang, B.; Tilley, T. D. Angew. Chem.,
Int. Ed. 2000, 39, 2870. (f) Amir, E.; Rozen, S. Angew. Chem., Int. Ed.
2005, 44, 7374.
(10) For recent benzo[b]thiophene preparations, see: (a) Kitamura, T.;
Takachi, T.; Miyaji, M.; Kawasato, H.; Taniguchi, H. J. Chem. Soc., Perkin
Trans. 1 1994, 1907. (b) Flynn, B. L.; Verdier-Pinard, P.; Hamel, E. Org.
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6011 and references therein. (d) Hessian, K. O.; Flynn, B. L. Org. Lett.
2003, 5, 4377.
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Org. Lett., Vol. 9, No. 1, 2007

Table 1. Photophysical and Electrochemical Data for a Series
of Si,S- or Si,Se-Bridged Distyrylbenzenes and Related
Compounds
absorptiona
fluorescencea
redox potentialb
Eox,1/2 Ered,1/2
4.47 432(sh)f 0.56 0.71
d
cmpd λmax/nmc log ꢀ λmax/nmc ΦF
5a
5b
5c
6a
6b
7
409
401
408
442
e
e
e
4.59 408
4.63 420
4.43 470
0.43 0.63, 1.00
0.02 0.62
0.91
0.50
e
e
-1.71, -2.00
-1.49, -1.90
e
468(sh)f 4.35 507
394
4.73 408
0.75 0.70
a In THF. b Determined by cyclic voltammetry: 1 mM concentration of
sample, with n-Bu4NPF6 (0.1 M) in CH2Cl2 or THF for oxidation or
reduction, respectively. Potentials are given against ferrocene/ferrocenium
couple (Fc/Fc+). c Only the longest absorption and shortest emission
maximum wavelengths are given. d Absolute fluorescence quantum yield,
determined by a calibrated integrating sphere system. e Not observed. f The
highest emission band for 5a and highest absorption band for 6b are located
at 453 and 441 nm, respectively.
Figure 2. Crystal structure of Si,S-bridged distyrylbenzene 5a:
(a) top view and (b) side view (50% thermal probability for
ellipsoids).
For the absorption spectra in THF, the Si,S-bridged
distyrylbenzenes 5a and 5b have their longest absorption
maxima at 409 and 401 nm, respectively, while the fluores-
cence spectra of these compounds show an intense blue
emission with the shortest maxima at 432 and 408 nm,
respectively. The fluorescence quantum yields of 5a and 5b
are 0.56 and 0.43, respectively. The significantly small Stokes
shift of 5b (∆νmax 427 cm-1) is worthy of note, while that
of 5a is 1302 cm-1. These comparisons demonstrate that the
positions of the silylene and sulfur briges affect the electronic
structure more significantly in the excited state.
A comparison of 5b and 5c shows the effect of the
chalcogen elements. The selenophene derivative 5c also
shows a blue fluorescence at 420 nm, which is slightly longer
than that of 5b. As for the fluorescence quantum yield, 5c
has a significantly low value (ΦF 0.02) compared to that of
5b, probably due to the heavy element effect of the selenium.
On the other hand, the comparison between 5a and 7
indicates the effect of the silylene bridges. Comparing with
7, 5a shows a 24 nm red shift in the emission maxima,
proving the effect of the silylene bridge. The orbital
interaction between the silylene moiety and the π-conjugated
framework may be responsible for this difference (vide infra).
Figure 3. UV-vis absorption and fluorescence spectra of the
ladder distyrylbenzenes 5 and 6 in THF.
The thiophene-S,S-dioxide derivatives 6a and 6b exhibit
a greenish-yellow fluorescence. Their emission maxima red
shift by about 40 and 100 nm compared to 5a and 5b,
respectively. In addition, the oxidized derivatives 6a and 6b
tend to have fluorescence quantum yields higher than those
of 5a and 5b. The oxidation of the thiophene ring to the
thiophene-S,S-dioxide loses the aromatic character of the
The UV-vis absorption and fluorescence spectra of the
ladder distyrylbenzenes are shown in Figure 3, and their data
are summarized in Table 1, together with those of the C,S-
bridged congener 711 for comparison. These data are
discussed from the viewpoints of (1) effect of the positions
of the silylene and sulfur briges, (2) effect of the bridging
elements, and (3) effect of the chemical oxidation.
(11) Wong, K.-T.; Chao, T.-C.; Chi, L.-C.; Chu, Y.-Y.; Balaiah, A.; Chiu,
S.-F.; Liu, Y.-H.; Wang, Y. Org. Lett. 2006, 8, 5033.
Org. Lett., Vol. 9, No. 1, 2007
95

thiophene ring, and the resulting π system should be regarded
as the ladder distyrylbenzenes with the silylene bridges and
the strongly electron-withdrawing SO2 bridges. This differ-
ence is responsible for the red-shifted and enhanced fluo-
rescence.
orbital interaction between the SO2 moiety and π-conjugated
skeleton must also play an important role for decreasing the
LUMO levels.
We also evaluated the electrochemical properties by cyclic
voltammentry. Figure 5 shows the cyclic voltammograms
To obtain a deeper insight into the effect of the bridging
elements, we carried out density functional theory (DFT)
calculations (B3LYP/6-31G(d)) for 5-7. Figure 4a shows
Figure 5. Cyclic voltammograms of 5 and 6. Measurement
conditions: sample 1 mM in CH2Cl2 for 5 and THF for 6 with
n-Bu4NPF6 (0.1 M); scan rate 0.10 V s-1
.
for 5 and 6, and their data are listed in Table 1. Whereas 5
shows only reversible one-step or two-step oxidation waves,
in sharp contrast the S,S-dioxidized derivatives 6 exhibit only
reversible two-step reduction waves. These results confirm
the significant electronic perturbation from the p-type to
n-type by the oxidation of the sulfur atoms. In addition, the
reversible redox processes observed for 6 are worth noting.
The nonaromatic character of the thiophene-S,S-dioxide ring
does not diminish the electrochemical stability.
In summary, a cascade-type anionic double cyclization
allows us to synthesize a series of silicon and chalcogen (S
or Se)-bridged distyrylbenzenes. The sulfur derivatives are
further transformed into the S,S-dioxidized derivatives by
chemical oxidation. All of the produced ladder compounds
have coplanar π-conjugated frameworks, in which the
substituents on the silicon atoms play an important role in
regulating the packing structure. In addition, not only their
intense fluorescences but also their reversible redox behavior
indicate their potential use in organic electronic devices.
Further study along this line is now in progress.
Figure 4. DFT calculations of 5-7 (B3LYP/6-31G(d)): (a) plot
of the Kohn-Sham HOMO and LUMO energy levels and (b)
pictographical presentation of the LUMOs of 5a and 6b.
the relative Kohn-Sham HOMO and LUMO energy levels.
A comparison between 5a and 7 demonstrates that the orbital
interaction between the silylene bridges and π-conjugated
framework mainly affects the LUMO level through the σ*-
π* conjugation (Figure 4b). This type of orbital interaction
occurs more effectively in 5a having the disilaindacene
substructure compared to the case of 5b.12 Among the
compounds, the S,S-dioxidized derivatives 6a and 6b have
rather low-lying LUMOs, to which the SO2 moiety signifi-
cantly contributes, as shown in Figure 4b. In addition to the
inductive effect of the SO2 moiety, the effective σ*-π*
Acknowledgment. This work was supported by Grants-
in-Aid (No. 15205014 and No. 17069011) from the Ministry
of Education, Culture, Sports, Science, and Technology of
Japan and SORST, Japan Science and Technology Agency.
Supporting Information Available: Experimental pro-
cedures, spectral data for all new compounds, and crystal-
lographic data in CIF format and ORTEP drawings of 5 and
6. This material is available free of charge via the Internet
at http://pubs.acs.org.
(12) Yamaguchi, S.; Xu, C.; Yamada, H.; Wakamiya, A. J. Organomet.
Chem. 2005, 5365.
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