New type of polysilanes: Poly(1,1-silole)s

Article abstract of DOI:10.1016/S0022-328X(00)00539-8

Poly(1,1-silole)s were synthesized for the first time in 1999, by us and by West and his coworkers, as a new type of polysilane which is expected to have a low-lying LUMO due to the σ*-π*conjugation characteristic of the silole ring. Summarized herein are the development of new synthetic methodologies and the structural aspects, photophysical properties and a unique reactivity of the resulting oligo- and poly(1,1-silole)s.

Full text of DOI:10.1016/S0022-328X(00)00539-8

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 611 (2000) 5–11
Review
New type of polysilanes: poly(1,1-silole)s
Kohei Tamao *, Shigehiro Yamaguchi
Institute for Chemical Research, Kyoto Uni6ersity, Uji, Kyoto 611-0011, Japan
Received 10 April 2000; accepted 22 April 2000
Abstract
Poly(1,1-silole)s were synthesized for the first time in 1999, by us and by West and his coworkers, as a new type of polysilane
which is expected to have a low-lying LUMO due to the s*–p*conjugation characteristic of the silole ring. Summarized herein
are the development of new synthetic methodologies and the structural aspects, photophysical properties and a unique reactivity
of the resulting oligo- and poly(1,1-silole)s. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Polysilane; Silole; Polysilole; Wurtz-type coupling reaction; s*–p* conjugation; UV spectrum; Fluoresence spectum
1. Introduction
[13] polymers catenated through the ring silicon atoms,
are attractive target molecules [14]. In the silole skele-
ton, the orthogonally fixed arrangement of the two
exocyclic single bonds on the silicon atom to the buta-
diene plane effectively causes the s*–p* conjugation
between the s* orbital of the silicon moiety and the p*
orbital of the butadiene moiety, leading to its low-lying
LUMO, as shown in Fig. 1 [15,16]. The catenation of
the silole ring at the silicon atoms in the all-anti fashion
with respect to the polysilane chain is expected to
realize the orbital interaction between the s* orbital
delocalized over the polysilane main chain and the p*
orbital localized on every silole ring. Actually, a recent
semiempirical calculation on the poly(1,1-silole)s has
predicted their low-lying LUMO levels due to the s*–
p* conjugation [17]. In experimental, the syntheses of
the poly(1,1-siloles) and related oligomers and polymers
have been addressed by us and several other research
groups (Chart 1): Sakurai and coworkers have reported
the synthesis of silole-incorporated polysilane (3) [18].
Tanaka and coworkers have reported the dehydrogena-
tive polymerization to poly(dibenzosilole) (4) [19]. West
and coworkers have succeeded in the synthesis of
poly(1,1-silole) (5) and their application to the organic
EL devices [20]. The model studies on the oligo(1,1-
silole)s (6) up to pentamer have also been reported by
Kira and coworkers [21]. We have independently stud-
ied the oligo(1,1-silole)s (1) and poly(1,1-silole)s (2),
starting from the development of the new synthetic
method of the monomer, 1,1-difunctionalized siloles.
Polysilanes are one-dimensional polymers consisting
of the siliconꢀsilicon single bond, one of the representa-
tive interelement linkage [1]. Owing to the mobile elec-
trons of the SiꢀSi linkage with high orbital energies,
polysilanes have unique properties such as absorption
and fluorescence in the UV region [1a] and high hole-
transporting ability [2]. These distinguished properties
are rationalized due to the s-conjugation of the main
chain [1a,3] and raise themselves as promising candi-
dates for new optoelectronic materials including or-
ganic electroluminescence materials [4] and photo-
conducting materials [5], in addition to the materials for
photoresists [6]. Current researches in this field are
directed toward the elucidation of the s-conjugation
[7,8], the developments of efficient and effective syn-
thetic methods of high polymers [9], functionalization
for the further applications [10], and the creation of the
new type of polysilanes [11,12]. Especially in the last
subject, the control and tuning of the electronic struc-
tures of polysilanes is a fundamental goal. In general,
the electronic structures of polysilanes are significantly
perturbed by specific orbital interactions between
polysilane main chains and the substituents [1a]. In this
context, poly(1,1-silole)s, silole (silacyclopentadiene)
* Corresponding author. Tel.: +81-774-378-3180; fax: +81-774-
378-3186.
E-mail address: tamao@scl.kyoto-u.ac.jp (K. Tamao).
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00539-8

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K. Tamao, S. Yamaguchi / Journal of Organometallic Chemistry 611 (2000) 5–11
This account is focused on our recent results of this
chemistry.
2. Results and discussion
2.1. Monomer synthesis
For the synthesis of poly(1,1-silole)s, 1,1-difunction-
alized siloles such as 1,1-dichlorosilole are crucial pre-
cursors. However, traditional synthetic routes to them
have been limited to only three types of reactions, i.e.
the coupling reaction of 1,4-di(lithio)butadiene deriva-
tives with SiX4 [22], the flash vacuum pyrolysis of
1-allylsilacyclopent-3-ene [23], and the transmetalation
from zirconacyclopentadienes [24], followed by func-
tional group transformation [25]. We commenced with
the development of new general, facile synthetic method
of the 1,1-difunctionalized siloles. We have recently
reported the new silole cyclization reaction, that is the
intramolecular reductive cyclization of bis(phenyl-
ethynyl)silanes [26]. Based on this reaction, we have
succeeded in the synthesis of 1,1-diaminosiloles which
are key precursors for a series of 1,1-difunctionalized
siloles [27].
The synthesis of 1,1-diaminosiloles is shown in
Scheme 1. The reduction of bis(diethylamino)bis-
(phenylethynyl)silane (7) with an excess amount of
lithium naphthalenide (LiNaph, 4 mol amount) at low
temperature cleanly formed 2,5-dilithio-1,1-diamino-
silole (8), which was further trapped with trimethylsilyl
chloride and dimethyl sulfate to give the corresponding
1,1-diaminosiloles (9) in good yields.
Chart 1. A list of oligo- and poly(1,1-silole)s and related compounds
so far prepared.
A series of 1,1-difunctionalized siloles having alkoxy,
Cl, F, and OH functionalities were prepared by trans-
formation from 1,1-diaminosiloles as shown in Scheme
2. Alcoholysis of diaminosiloles in the presence of AlCl3
yielded 1,1-di(alkoxy)siloles (10). 1,1-Dichlorosiloles
(11) were prepared in good yields by treatment with dry
HCl gas. 1,1-Difluorosiloles (12) were also obtained
from diaminosiloles via dichlorosiloles without isola-
tion by subsequent treatment with Py(HF)x or ZnF2.
An unsymmetrically substituted 1,1-difunctionalized
silole (13) is also accessible by the selective monoamina-
tion of dichlorosiloles 11 using Et2NH–Et3N. Hydroly-
sis reactions of dichlorosilole and difluorosilole afford
different type of products, di(hydroxy)silole (14) and
(fluoro)(hydroxy)silole (15), respectively [28].
The present methodology is also applicable to the
synthesis of 1-monofunctionalized siloles. Thus, as a
representative example, 1-chloro-1-methyl-2,5-dimethyl-
silole (18) was prepared from the corresponding
(amino)(methyl)bis(phenylethynyl)silane (16) via 1-
amino-1-methyl-2,5-dimethylsilole (17) as shown in
Scheme 3.
Fig. 1. Schematic drawing of orbital correlation diagram of silole
(Ref. [15a]).
The stability of these 1,1-difunctionalized siloles is
highly dependent on the bulkiness of the 2,5-sub-
stituents. Especially, the 1-halogenated siloles with 2,5-
methyl groups are unstable in the air and should be
stored under an inert atmosphere. These demonstrate
Scheme 1. Reagents and conditions: (i) LiNaph (4 mol amounts),
THF, −78°C, 1 h; (ii) Me3SiCl (4 mol amounts), −78°C–r.t., 8 h;
(iii) (MeO)2SO2 (4 mol amounts), −78°C–r.t., 10 h.

K. Tamao, S. Yamaguchi / Journal of Organometallic Chemistry 611 (2000) 5–11
7
2.2. Oligo(1,1-silole)s
2.2.1. Synthesis
Oligo(1,1-silole)s are of interest as the models of
poly(1,1-silole)s to elucidate their fundamental proper-
ties. When we started this chemistry, only a few silole
dimers have been reported in the literature [29]. We have
conducted the synthesis of a series of oligo(1,1-silole)s
using the 1-monohalo or 1,1-dihalosilole as the crucial
precursors [30].
The syntheses of tersilole (1a) and quatersilole (1b) are
shown in Schemes 4 and 5, respectively. As has been well
documented [29c–f, 31], silole dianion (19) can be cleanly
prepared by the reaction of 1,1-dichlorosilole (11) with
excess Li metal in THF. The coupling reaction of the
dianion 19 with 1-chloro-1-methylsilole (18) gave tersilole
(1a) in 83% yield. Quatersilole (1b) has been obtained by
a similar coupling reaction. Our procedure was based on
the recent results reported independently by Boudjouk
and his coworkers [29d] and Tilley and his coworkers
[29e,f] on the formation of bisilole dianions. Thus, 11 was
reduced with 3 mol amounts of Na metal, resulting in the
formation of a mixture of the bisilole dianion (20) and
the silole dianion (19). The mixture was treated with
1-chloro-1-methylsilole (18) to afford quatersilole (1b) in
20% yield together with tersilole (1a) in 8% yield.
Scheme 2. Reagents and conditions: (i) R%OH, AlCl3 (0.25 mol
amount), r.t., 20 h; (ii) dry HCl gas, Et2O, −78°C; (iii) Py(HF)x (6
mol amounts), −78°C, 0.5 h; (iv) ZnF2 (3 mol amounts), r.t., 2 h; (v)
Et2NH (1.2 mol amounts), Et3N (1.2 mol amounts), r.t., 20 h; (vi)
THF–H2O, r.t., 1 h.
2.2.2. Structures of oligo(1,1-silole)s
Fig. 2 provides the ORTEP views of the crystal structures
of tersilole (1a) and quatersilole (1b). In the structure of
tersilole (1a), three silole rings are arranged as gauche-anti
conformations along with the Si1ꢀSi2ꢀSi3 trisilane skele-
ton. The dihedral angles BC1ꢀSi1ꢀSi2ꢀSi3 and
BSi1ꢀSi2ꢀSi3ꢀC2 are −64.8(4) and 172.0(4)°, respec-
tively. In the structure of quatersilole (20), there is a
centrosymmetric point at the center of the Si2ꢀSi3 bond.
Four silole rings have gauche-anti-gauche conformations
along with the Si1ꢀSi2ꢀSi3ꢀSi4 skeleton, having
BC1ꢀSi1ꢀSi2ꢀSi3,−56.7(2) and BSi1ꢀSi2ꢀSi3ꢀSi4,
180°. Thus, the partial geometry of tetrasilane skeleton,
Scheme 3. Reagents and conditions: (i) LiNaph (4 mol amounts),
THF, −78°C, 2 h; (ii) (MeO)2SO2 (4 mol amounts), −78–0°C, 0.5
h; (iii) dry HCl gas, Et2O, −78°C.
Scheme 4. Reagents and conditions: (i) Li (5 mol amounts), THF,
10°C, 10 h; (ii) 18 (2 mol amount), −78°C, 2 h.
the importance of the appropriate choice of 2,5-sub-
stituents for preparation and handling of the siloles
having functionalities on silicon. Notably, the present
methodology has a decisive merit in this respect that a
variety of substituents can be introduced onto the
2,5-positions of the silole rings, although the 3,4-phenyl
groups are inevitable for the present procedure [26].
Scheme 5. Reagents and conditions: (i) Na (3 mol amounts), THF,
r.t., 38 h; (ii) 18 (1.5 mol. amounts), −78°C, 3 h.

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K. Tamao, S. Yamaguchi / Journal of Organometallic Chemistry 611 (2000) 5–11
2.3. Poly(1,1-silole)s
2.3.1. Synthesis
The most straightforward route to the poly(1,1-
silole)s is the Wurtz-type polycondensation of 1,1-di-
halosiloles. We have actually succeeded in the synthesis
of poly(1,1-silole) by a careful operation of the Wurtz-
type coupling of 1,1-dichlorosilole, as shown in Scheme
6 [32]. Thus, the 1,1-dichlorosilole (11) was allowed to
react with just 2 mol amounts of granular Li in THF at
−20°C for 20 h, followed by treatment with EtMgBr
to quench any reactive terminal groups. The GPC
analysis of the reaction products after reprecipitation
from MeOH revealed that the reaction afforded
poly(1,1-silole) (2), but together with cyclic hexamer
(21), as shown in Fig. 3. The oligomer was completely
removed by repeated reprecipitations from i-PrOH to
give pure poly(1,1-silole) (2) in 61% yield. Recrystalliza-
tion of the soluble part afforded the cyclic hexamer (21)
in 6% yield. The molecular weight of 2 estimated by
GPC using polystyrene standards is Mw=7200 and
Mn=6,300 (n:20) with a very narrow polydispersity
(Mw/Mn=1.14). The polymer 2 is slightly soluble in
THF and CHCl3 and stable enough to be handled in air
without special care.
The keys to achieving the synthesis of the poly(1,1-
silole) are the choice of starting materials and reaction
condition. We employed the dichlorosilole (11) which
has small methyl groups at the 2,5-positions and p-
ethyl groups on the 3,4-phenyl groups in order to
reduce the steric congestion and increase the solubility
of the resulting polymers. Also, the use of Li metal in
THF is crucial. The polycondensation of 11 under the
standard conditions for the Wurtz-type polysilane syn-
thesis using Na in refluxing toluene only gave a mixture
of oligomeric materials, which might contain the
SiꢀOꢀSi linkages in light of the broad band around
1000–1100 cm−1 in the IR spectrum.
Fig. 2. ORTEP drawings (50% thermal ellipsoids) of (a) tersilole (1a)
and (b) quatersilole (1b).
As an alternative route to the poly(1,1-silole)s, we
have also examined the coupling reaction of the silole
dianion (19) with chlorine-terminated tersilole (22)
which was prepared by the reaction of 19 with 13
Scheme 6. Reagents and conditions: (i) Li (2 mol amounts), THF,
−20°C, 10 h; (ii) EtMgBr (0.2 mol amount), THF, −20°C, 1 h; (iii)
R from MeOH, then i-PrOH.
from Si1 to Si3, is quite similar to that of tersilole.
These structures are contrasting to all-anti structures of
2,3,4,5-tetramethylsilole analogs and all-gauche struc-
tures of 2,3,4,5-tetraphenylsilole analogs, reported by
Kira et al. [21] and West et al. [20], respectively. Thus,
it is apparent that these conformational differences are
arising from the steric congestion due to the sub-
stituents at the 2,5-positions of silole rings.
Fig. 3. GPC traces after reprecipitation from (a) MeOH, then (b)
i-PrOH.

K. Tamao, S. Yamaguchi / Journal of Organometallic Chemistry 611 (2000) 5–11
9
Scheme 7. Reagents and conditions: (i) 19 (0.5 mol amount, prepared
from 11 with Li), THF, −78°C, 3 h; (ii) dry HCl gas, Et2O, −78°C;
(iii) 19 (1.1 mol amount, prepared from 11 with Li), THF, −78°C, 2
h; (iv) i-PrOH (2 mol amounts), Et3N (2 mol amounts), r.t.
Fig. 4. UV–Vis absorption (solid line) and fluorescence spectra
(dashed line) of polymer 2 in CHCl3.
absorption and fluorescence spectra of polymer 2 are
shown in Fig. 4.
followed by deaminochlorination, as shown in Scheme
7. The polymer 2% having essentially the same spectro-
scopic data as that of 2 was obtained in 48% yield,
although the molecular weight was relatively low;
Mw=3000 and Mn=2,400 (n:8) with Mw/Mn=1.25.
In this reaction, the cyclic hexamer was no longer
obtained.
The monosilole 23 has two absorption bands around
250 and 310 nm [33], assignable to the p–p* transitions
of the phenyl and silole moieties, respectively. In com-
parison with the monosilole (23), the oligosiloles 24, 1a
and 1b show distinct spectra, where the absorption of
the silole moieties is hidden by broad bands. Remark-
ably, tersilole (1a) and quatersilole (1b) have unique
2.3.2. Photophysical properties
The oligo(1,1-silole)s and poly(1,1-silole) showed in-
teresting photophysical properties. The data are sum-
marized in Table 1, together with the data for
monosilole (23) [33], bisilole (24) [30] and permethy-
lated [34] and perphenylated oligosilanes [35] for com-
parison. As
a
representative example, the UV
Table 1
UV absorption and fluorescence spectral data for oligo(1,1-silole)s, poly(1,1-silole) and related oligosilanes
a
Compound
UV absorption a
Fluorescence
b
umax (nm)
log m
umax (nm)
ƒf×103 c
Monosilole
Bisilole
Tersilole
Quarersilole
Cyclo-sexisilole
23
24
1a
1b
21
307
255
279
288
285
3.22
4.42
4.60
4.59
5.07
393
415
439
441
467
0.372
1.94
1.27
1.39
0.353
Polysilole
2
320(sh)
4.02 d
460
0.314
Ph(Ph2Si)3Ph e
Ph(Ph2Si)4Ph f
Me(Me2Si)3Me g
Me(Me2Si)4Meg
255
288
216
235
4.51
4.36
3.96
4.17
a In chloroform.
b Excited at the absorption maximum wavelength.
c Quantum yield determined with reference to quinine sulfate (ƒ310=0.55 in 0.1 M H2SO4).
d Per silole ring unit.
e See Ref. [35a].
f See Ref. [35b].
g See Ref. [34].

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K. Tamao, S. Yamaguchi / Journal of Organometallic Chemistry 611 (2000) 5–11
poly(1,1-silole) has been found to be quite unique; thus
all SiꢀSi bonds are readily cleaved by Li reduction to
form the 1,1-dilithiosilole.
Other theoretically predicted photophysical proper-
ties such as nonlinear optical properties may be ex-
plored by further experiments. A variety of new
chemistries are also anticipated in this new field of
polysilanes, into which we have just entered.
Scheme 8. Reagents and conditions: (i) Li (excess), THF, 10°C, 1 h;
(ii) Me3SiCl, 10°C.
absorptions at around 280 and 290 nm, respectively.
These absorption maxima are 50–60 nm longer than
those of the permethyl oligosilanes, and in comparison
with the perphenyl counterparts, 1a has a 20 nm longer
Acknowledgements
We are grateful to our coworkers who contributed to
this study and whose names are described in the refer-
ences listed below. We also thank the Ministry of
Education, Science, Sports and Culture, Japan, for the
Grant-in-Aids for Scientific Research on Priority Area
(A), ‘The Chemistry of Interelement Linkage’ (No.
09239103).
umax and 1b has a comparable umax
.
Polymer 2 has broad absorption bands with a shoul-
der at 320 nm, which is about 30–40 nm red-shifted
relative to those of the trimer 1a and tetramer 1b, while
an absorption maximum of the cyclic hexamer 21 is
comparable to those of the oligomers. In the fluores-
cence spectra, the poly(1,1-silole) (2) has a broad emis-
sion band around 460 nm, although the quantum yield
is rather low. The emission maximum wavelengths tend
to shift to a longer region going from the monosilole to
polymer, probably due to the mixing of the orbitals
between the silole moieties and polysilane moiety.
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silole) (2) may be the complete degradation of the
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orbital and the aromatic p orbitals. The experimental
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