σσ* transition in anti, cisoid alternating oligosilanes: Clear-cut evidence for suppression of conjugation effect by a cisoid turn

Article abstract of DOI:10.1021/ja034500e

On the basis of the UV absorption spectra of the anti,cisoid alternating oligosilanes, the first experimental clear-cut evidence was presented for the generally accepted idea that the σ conjugation in polysilanes does not effectively extend through a tetr

Full text of DOI:10.1021/ja034500e

Published on Web 05/28/2003
σσ* Transition in anti,cisoid Alternating Oligosilanes: Clear-Cut Evidence for
Suppression of Conjugation Effect by a cisoid Turn
Hayato Tsuji, Masayoshi Terada, Akio Toshimitsu, and Kohei Tamao*
Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan
Received February 5, 2003; E-mail: tamao@scl.kyoto-u.ac.jp
Polysilanes comprising catenated silicon-silicon bonds in their
backbones are representative σ-conjugated compounds and are
attractive in view of their unique optical properties, such as UV
absorption.1 The absorption maxima λmax of oligo- and polysilanes
were found to red-shift as the silicon chain becomes longer, similar
to those of the π-conjugated compounds such as polyacetylenes
and polyphenylenes. The UV absorption spectra of some polysilanes
are found to be thermochromic,2 which is explained in terms of
the silicon backbone conformation change. A recently introduced
explanation3 for the conformational dependence of the spectra of
tetrasilanes invoked an avoided crossing of the two lowest-energy
valence excited states of B symmetry, and this has been experi-
mentally confirmed by utilizing conformationally constrained
tetrasilanes.4 The UV absorption spectra of conformationally
constrained hexasilanes4b demonstrated that an anti turn effectively
extends the σ-conjugated system, while a syn turn at the termini
does not. For a better understanding of the electronic states in
polysilanes, it is highly desirable to extend the investigation to
longer silicon chain compounds. In the present paper, we will report
the preparation and UV absorption of anti(A),cisoid(C)5 alternating
hexa- to decasilanes to discuss the effect on the photophysical
properties when an incorporated syn turns into an anti segment.
The bicyclic disilane 1 with two tetramethylene tethers was used
as a monomer unit; the C(Ph)-Si-Si-C(Ph) dihedral angle was
found to be 30.7° (cisoid) by X-ray crystallography. Reductive
condensation of 2 was carried out with lithium metal in the presence
of chlorotrimethylsilane as the terminator (Scheme 1).6 This reaction
Figure 1. UV absorption spectra of anti,cisoid-alternating oligosilanes 3-6
(A) and absorption maximum wavelength of constrained and unconstrained
Scheme 1
oligosilanes (B).
nm, unlike all the other presently studied oligosilanes. This is the
only member of the series that contains no anti tetrasilane subunit.
(2) The CAC hexasilane 4 has an absorption maximum at 243 nm
derived from the σσ* transition at the central anti turn, as observed
before for the pentamethylene analogue.4b (3) In the series 4-6,
the absorption maximum wavelengths λmax remain constant near
240 nm, regardless of the silicon chain length. (4) The absorption
maximum of the decasilane 6 at 236 nm is accompanied by a
shoulder around 245 nm. (5) The absorption around 210 nm, for
which the cisoid fragments are responsible, also intensifies as the
silicon chain becomes longer.
In Figure 1B, the primary absorption maximum wavelengths of
the permethylated oligosilanes,1a,7 and the A,C alternating oligosi-
lanes are plotted against the number of silicon atoms. The absorption
maximum of the permethylated oligosilanes steadily shifts toward
longer wavelengths as the silicon chain becomes longer because
of the elongation of the successive anti (or transoid) segment length.
In contrast, that of the A,C alternating oligosilanes remains almost
constant around 240 nm, corresponding to absorption derived from
the anti fragment, irrespective of the increase in the number of
silicon atoms.
afforded the bis-silylated monomer to the tetramer (3-6) of the
disilane unit, which corresponds to a linear tetrasilane to decasilane.
These oligomers were separated by recycling preparative GPC and
1
identified by H, 13C, and 29Si NMR as well as mass spectra. The
hexasilane 4 was found to have a CAC conformation by X-ray
crystallography, which demonstrated that the cisoid configuration
of the disilane units has been retained during the oligo-condensation,
and therefore, it is quite reasonable to assume that the octasilane 5
and the decasilane 6 have the CACAC and the CACACAC
conformations, respectively.
The UV absorption spectra of 3-6 are shown in Figure 1A.
Several significant features are noted: (1) The C tetrasilane 3 shows
a spectrum reminiscent of that of the pentamethylene analogue.4b
It contains a shoulder-like absorption without any peak around 240
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10.1021/ja034500e CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. CIS Calculations for Oligosilanes 3-6
experimental evidence for the generally accepted idea that the
σ-conjugation in polysilanes does not effectively extend through a
tetrasilane fragment with a small dihedral angle such as a cisoid
turn,8-10 as suggested by the simple ladder C model10 of the σ
backbone structure. This result demonstrates that the UV absorption
spectral behavior of the all-transoid permethylated oligosilanes and
the present A,C alternating oligosilanes is similar to that of the
π-conjugated poly(p-phenylene)s and poly(m-phenylene)s,11,12 re-
spectively. Therefore, it can be concluded that the effect of the
conformation isomerism on the photophysical properties in the
σ-conjugation system corresponds to that of the position isomerism
in the π-conjugated system, which has been experimentally clarified
more than 60 years after the UV absorption measurements of poly-
(p-phenylene)s and poly(m-phenylene)s.11a
fc
oligosilanes state λmax
fc
a,b
a,b
oligosilanes state λmax
3
1B
2A
3A
2B
236
225
211
208
0.0444
0.0496
0.0121
0.5921
6
2A
1B
3A
2B
3B
248
0.0244
0.2006
0.0079
1.1346
0.2491
247
243
242
236
4
5
2A
1B
244
242
0.0045
0.3751
1B
2A
2B
247
244
241
0.1225
0.0844
0.6389
a Transitions appearing at longer than 230 nm are listed except for 3.
b The excitation energies were scaled by 0.76. c Oscillator strength. Transi-
tions with large oscillator strengths are shown in bold.
Acknowledgment. We express our grateful acknowledgment
to Professor Josef Michl and Professor Masahiro Ehara for their
helpful discussion. We thank the Ministry of Education, Culture,
Sports, Science, and Technology, Japan, for the Grants-in-Aid for
COE Research on Elements Science, 12CE2005 and 09239103.
Supporting Information Available: Experimental details, X-ray
crystallography of 1 and 3, and computational details (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
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Figure 2. Calculated high-energy occupied and low-energy unoccupied
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(B).
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CIS calculations (CIS/6-31G*) for 3-6 were carried out and are
summarized in Table 1. These results reproduce the absorption
spectra quite well although calculations were carried out on one of
the possible conformers of each oligosilane, which indicates that
their conformations are rigidly controlled by the tetramethylene
tethers that leaves little possibility to have the other conformers.
The peak of the CAC hexasilane 4 that appears at 243 nm is
assigned to the σσ* excitation localized at the central Si3-Si4 bond.
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tetrasilane of the anti conformation.4b Obviously, the additional
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most intense transition stays constant, and the corresponding peaks
appear around 240 nm regardless of the chain length. For 6, the
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Even the absorption maximum of octadecasilane 7, a virtual octamer
built on a computer with an A,C alternating conformation, was found
to stay around 245 nm (see Supporting Information), while that of
the n-permethylated octadecasilane appears at 291 nm.1a
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lane and octadecasilane is 1.27 eV in (A) and 0.42 eV in (B).
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A,C alternating oligosilanes, we have obtained the first clear-cut
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