A controlled, iterative synthesis and the electronic properties of oligo[(p-phenyleneethynylene)-alt-(2,5-siloleneethynylene)]s

Article abstract of DOI:10.1021/ja047983a

An introductory series of conjugated siloleneethynylene co-oligomers has been prepared from a key 2-chloro-5-iodosilole intermediate via site-specific cross-coupling reactions. The tetramer (9) and pentamer (10) both exhibit absorption maxima matching tho

Full text of DOI:10.1021/ja047983a

Published on Web 07/31/2004
A Controlled, Iterative Synthesis and the Electronic Properties
of Oligo[(p-phenyleneethynylene)-alt-(2,5-siloleneethynylene)]s
Andrew J. Boydston, Youshi Yin, and Brian L. Pagenkopf*
Contribution from the Department of Chemistry and Biochemistry,
The UniVersity of Texas at Austin, Austin, Texas 78712
Received April 7, 2004; E-mail: pagenkopf@mail.utexas.edu
Abstract: An introductory series of conjugated siloleneethynylene co-oligomers has been prepared from
a key 2-chloro-5-iodosilole intermediate via site-specific cross-coupling reactions. The tetramer (9) and
pentamer (10) both exhibit absorption maxima matching those of the corresponding silole copolymers.
Extinction coefficients for the oligomers in this series are large, and in the case of the pentamer (10) the
value exceeds 180 000 M-1 cm-1. The compounds all emit in the visible region with the greatest quantum
efficiencies being 8.97 × 10-2 (monomer) and 2.99 × 10-2 (pentamer).
band gaps for alkyne-rich systems.7 The study of oligomeric
Introduction
silole-acetylene systems will provide insight into the structural,
thermal, morphological, and electronic properties as a function
of chain length.
The synthesis and study of novel chromophores is essential
to the expansion and advancement of organic electronic and
sensor materials science. There are often synthetic advantages
to the preparation of polymeric π-conjugated systems, but
frequently existing technologies for their preparation lack the
precision required for securing exact chain lengths or controlling
site-specific functional group installation. In this regard, an
exciting approach to gaining fundamental information into the
properties of polymers is the investigation of a discrete set of
corresponding oligomeric chromophores of precise length and
composition.1 The challenges inherent to selective oligomer
preparation have attracted the attention of synthetic chemists
for decades, and important contributions have appeared in the
areas of thienylene,1a phenylene,2 and other arylene oligomers.1b,3
In contrast, procedures for the preparation of well-defined
oligomeric silole chromophores have remained elusive. This is
surprising given that conjugated silole networks have potential
applications in a wide range of areas including electrolumines-
cence,4 photoluminescence,5 and nitroaromatic sensor technolo-
gies.6 Silole polymers rich in acetylenic functionality are
particularly interesting because they display unusually narrow
Phenyleneethynylene oligomers and macrocycles have been
prepared by regiospecific alkynylations with aryl triazenes as
masked iodoarenes8 and through the use of aromatics bearing
combinations of halide and/or triflate functionality.9 The former
route using Moore’s MeI-induced triazene decomposition8 has
not been applied to substrates bearing silole rings in the
chromophore. Likewise, the latter approach had not been
developed for silole chemistry prior to our recent work on
donor-acceptor siloles.10
The availability of 2,5-dibromosiloles (e.g., 1) opened the
door to π-conjugated polymeric acetylenic siloles through
transition metal-catalyzed cross-coupling reactions.11 In 1997,
Barton and co-workers reported the synthesis of silole-acetylene
polymers 2 and 3 (Figure 1).12 In comparison with poly-
(phenyleneethynylene)s (λmax ) 425 nm)13 and poly(thiophene-
(6) (a) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc.
2003, 125, 3821-3830. (b) Sohn, H.; Calhoun, R. M.; Sailor, M. J.; Trogler,
W. C. Angew. Chem., Int. Ed. 2001, 40, 2104-2105.
(7) Hissler, M.; Dyer, P. W.; Reau, R. Coord. Chem. ReV. 2003, 244, 1-44.
(8) Moore, J. S.; Weinstein, E. J.; Wu, Z. Tetrahedron Lett. 1991, 32, 2465-
2466.
(9) For selected examples, see: (a) Tao, W.; Nesbitt, S.; Heck, R. J. Org.
Chem. 1990, 55, 63-69. (b) Tykwinksi, R. R. Angew. Chem., Int. Ed. 2003,
42, 1566-1568. (c) Sonogashira, K. J. Organomet. Chem. 2002, 653, 46-
49. For selected examples of applications utilizing regiospecific cross
couplings, see: (d) Sonoda, M.; Inaba, A.; Itahashi, K.; Tobe, Y. Org.
Lett. 2001, 3, 2419-2421. (e) Kamikawa, T.; Hayashi, T. J. Org. Chem.
1998, 63, 8922-8925. (f) Blanchette, H. S.; Brand, S. C.; Naruse, H.;
Weakley, T. J. R.; Haley, M. M. Tetrahedron 2000, 56, 9581-9588. (g)
Waybright, S. M.; McApline, K.; Laskoski, M.; Smith, M. D.; Bunz, U.
H. F. J. Am. Chem. Soc. 2002, 124, 8661-8666. (h) Tovar, J. D.; Swager,
T. M. J. Organomet. Chem. 2002, 653, 215-222. (i) Marsden, J. A.; Palmer,
G. J.; Haley, M. M. Eur. J. Org. Chem. 2003, 2355-2369 and references
therein. (j) Metal-Catalyzed Cross-coupling Reactions; Diederich, F., Stang,
P. J., Eds.; Wiley-VCH: Weinheim, 1998.
(1) (a) Tour, J. M. Chem. ReV. 1996, 96, 537-553. (b) Martin, R. E.; Diederich,
F. Angew. Chem., Int. Ed. 1999, 38, 1350-1377.
(2) Moore, J. S. Acc. Chem. Res. 1997, 30, 402-413.
(3) (a) Electronic Materials: The Oligomer Approach; Mu¨llen, K., Wegner,
G., Eds.; Wiley-VCH: Weinheim, 1998. (b) Pelter, A.; Jenkins, I.; Jones,
D. E. Tetrahedron 1997, 53, 10357-10400.
(4) For selected examples, see: (a) Chen, J.; Law, C. C. W.; Lam, J. W. Y.;
Dong, Y.; Lo, S. M. F.; Williams, I. D.; Zhu, D.; Tang, B. Z. Chem. Mater.
2003, 15, 1535-1546. (b) Kim, B.-H.; Woo, H.-G. Organometallics 2002,
21, 2796-2798. (c) Uchida, M.; Izumizawa, T.; Nakano, T.; Yamaguchi,
S.; Tamao, K.; Furukawa, K. Chem. Mater. 2001, 13, 2680-2683. (d)
Yamaguchi, S.; Endo, T.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Tamao,
K. Chem. Lett. 2001, 30, 98-99. (e) Tamao, K.; Uchida, M.; Izumizawa,
T.; Furukawa, K.; Yamaguchi, S. J. Am. Chem. Soc. 1996, 118, 11974-
11975. (f) Yamaguchi, S.; Endo, T.; Uchida, M.; Izumizawa, T.; Furukawa,
K.; Tamao, K. Chem.-Eur. J. 2000, 6, 1683-1692.
(10) Boydston, A. J.; Yin, Y.; Pagenkopf, B. L. J. Am. Chem. Soc. 2004, 126,
3724-3725.
(5) For selected examples, see: (a) Liu, M. S.; Luo, J.; Jen, A. K.-Y. Chem.
Mater. 2003, 15, 3496-3500. (b) Chen, J.; Peng, H.; Law, C. C. W.; Dong,
Y.; Lam, J. W. Y.; Williams, I. D.; Tang, B. Z. Macromolecules 2003, 36,
4319-4327. (c) Tang, B. Z.; Zhan, X.; Yu, G.; Lee, P. P. S.; Liu, Y.; Zhu,
D. J. Mater. Chem. 2001, 11, 2974-2978. (d) Zhao, W.; Liu, Y.; West, R.
Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 2000, 41, 780.
(11) (a) Yamaguchi, S.; Tamao, K. J. Organomet. Chem. 2002, 653, 223-228.
(b) Tamao, K.; Yamaguchi, S.; Shiro, M. J. Am. Chem. Soc. 1994, 116,
11715-11722.
(12) Chen, W.; Ijadi-Maghsoodi, S.; Barton, T. Polym. Prepr. (Am. Chem. Soc.,
DiV. Polym. Chem.) 1997, 38, 189-190.
9
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J. AM. CHEM. SOC. 2004, 126, 10350-10354
10.1021/ja047983a CCC: $27.50 © 2004 American Chemical Society

Synthesis of Oligomeric Silolene Chromophores
A R T I C L E S
Figure 1. Examples of Barton’s silole-containing polymers. Absorption
data for polymers in THF at room temperature.
Figure 3. Novel silole oligomers 7-10, and parent silole 6.
Scheme 2
Figure 2. Examples of Tamao’s silole-containing polymers. Absorption
data for polymers in CHCl3 at room temperature.
Scheme 1
asymmetrical siloles and the limited number of silole-compatible
alkyne protecting groups. These were overcome by a direct and
efficient synthesis of a mixed halo silole, optimized in situ
regiospecific cross couplings, and the employment of terminal
alkyne coupling partners when necessary.
We recently described a new process for the efficient
formation of and selective cross-coupling reactions with the
novel asymmetric chloroiodosilole 12.10 In this paper, we
illustrate the importance and utility of silole 12 as a powerful
intermediate for oligomeric silole synthesis through its deploy-
ment as a precursor for all of the asymmetrical siloles described
herein. The first crucial building block 13 was obtained in 58%
overall yield from diethynylsilane 11 by selective palladium-
catalyzed cross coupling between phenylacetylene and chloro-
iodosilole 12 (Scheme 2).
It is worth noting that cross-coupling reactions performed
under Stille20 or traditional Negishi21 conditions (i.e., use of
preformed alkynylstannane or zinc acetylide, respectively)
provided the desired compound in comparable yield. However,
Sonogashira22 conditions gave fickle results, and successful
cross-coupling attempts were only observed when Et3N or
PhCH3/Et3N solvent systems were used. THF/Et3N solvent
systems led to complex reaction mixtures from which no
identifiable products or starting materials could be isolated.23
Having secured an efficient and scaleable route to the silole
“end cap” 13, we continued the oligomer syntheses by prepara-
tion of symmetric infill pieces for the central domain. For
oligomers with even numbers of siloles, 1,4-diethynylbenzene
was an obvious key building block, and dimeric silole 7 was
prepared by standard cross-coupling techniques (Scheme 3).
ethynylene)s (λmax ) 438 nm),14 the silole moiety was strongly
implicated as responsible for the dramatically red-shifted
absorptions observed with these polymers.
In 1998, Tamao and co-workers reported silole polymers of
similar composition (Figure 2).15,16 Additionally, Tamao exam-
ined diethynylthiophene linkages (4) and observed an additional
red-shift in the absorption spectrum. These remarkable materials
are novel examples of alternating copolymers incorporating the
electronically unique silole ring.
The synthesis of length-specific oligomers requires the use
of an asymmetric building block that can serve either as an end
cap or as a starting point for iterative chain extension. Cross-
coupling-based strategies for the synthesis of acetylene contain-
ing oligomers frequently employ intermediates fitted with
i
orthogonal alkyne protecting groups (e.g., Me3Si vs Pr3Si vs
Me2COH).17 However, the use of these venerable protecting
groups in the presence of siloles is prohibited due to the greater
reactivity of the silole ring under the reaction conditions
currently developed for alkyne liberation.4f,15 In pioneering
investigations, Tamao circumvented the obstacle of silole
desymmetrization and successfully prepared asymmetric siloles
and silole-containing oligomers by manipulation of pendant
thiophenes18 and pyrroles (Scheme 1).19
(17) For selected examples, see: (a) Nielsen, M. B.; Utesch, N. F.; Moonen, N.
N. P.; Boudon, C.; Gisselbrecht, J.-P.; Concilio, S.; Piotto, S. P.; Seiler,
P.; Gunter, P.; Gross, M.; Diederich, F. Chem.-Eur. J. 2002, 8, 3601-
3613. (b) Blanchette, H. S.; Brand, S. C.; Naruse, H.; Weakley, T. J. R.;
Haley, M. M. Tetrahedron 2000, 56, 9581-9588. (c) Cuilei, S. C.;
Tykwinski, R. R. Org. Lett. 2000, 2, 3607-3610. (d) Wan, W. B.; Brand,
S. C. Pak, J. J.; Haley, M. M. Chem.-Eur. J. 2000, 6, 2044-2052. (e) De
Nicola, A.; Ringenbach, C.; Ziessel, R. Tetrahedron Lett. 2003, 44, 183-
187.
(18) (a) Tamao, K.; Yamaguchi, S.; Shiozaki, M.; Nakagawa, Y.; Ito, Y. J. Am.
Chem. Soc. 1992, 114, 5867-5869. (b) Tamao, K.; Yamaguchi, S.; Ito,
Y.; Matsuzaki, Y.; Yamabe, T.; Fukushima, M.; Shigeru, M. Macromol-
ecules 1996, 28, 8668-8675.
(19) Tamao, K.; Ohno, S.; Yamaguchi, S. Chem. Commun. 1996, 1873-1874.
(20) Stille, J. K.; Simpson, J. H. J. Am. Chem. Soc. 1987, 109, 2138-2152.
(21) King, A. O.; Negishi, E.-I.; Villani, F. J., Jr.; Silveira, A., Jr. J. Org. Chem.
1978, 43, 358-360. For a modified procedure that does not require
anhydrous conditions, see: Anastasia, L.; Negishi, E. Org. Lett. 2001, 3,
3111-3113.
Results and Discussion
Construction of the Oligomer Series. In this paper, we
report the synthesis of a structurally homologous family of novel
oligomeric silolene chromophores (Figure 3), and we describe
their electronic and physical properties as a function of
conjugation length. Challenges confronting this endeavor were
the lack of an efficient and practical method for preparing
(13) Moroni, M.; Le Moigne, J.; Luzzati, S. Macromolecules 1994, 27, 562-571.
(14) Pang, Y.; Wang, Z.; Barton, T. Polym. Prepr. (Am. Chem. Soc., DiV. Polym.
Chem.) 1996, 37, 333-334.
(15) Yamaguchi, S.; Iimura, K.; Tamao, K. Chem. Lett. 1998, 27, 89-90.
(16) Polymer 2 was found in this case to have an absorption maximum at 505
nm, in contrast to 494 nm in Barton’s studies. This slight discrepancy can
likely be ascribed to different solvents.
(22) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16,
4467-4470.
(23) (a) For similar observations in silole cross couplings, see: Ohshita, J.;
Mimura, N.; Arase, H.; Nodono, M.; Kunai, A.; Komaguchi, K.; Shiotani,
M.; Ishikawa, M. Macromolecules 1998, 31, 7985-7987. (b) Reference
15.
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A R T I C L E S
Boydston et al.
Scheme 3
Scheme 6
Scheme 4
Scheme 5
Scheme 7
For the synthesis of the odd numbered oligomers, we initially
set out to use the simple diyne 15 made from cross coupling
with dibromide 1410,11b (Scheme 4). However, compound 15
was only stable in solution, and concentration resulted in the
formation of an insoluble brown powder. We note that other
siloles bearing terminal alkynes directly attached to the silole
ring also exhibited poor stability.24 We therefore shifted our
focus to the preparation of a more robust building block
amenable to long-term storage.
Cross-coupling of dibromide 14 with aryl alkyne 1625
followed by MeI-induced triazene decomposition8 provided bis-
(iodoarene) 17 in 84% yield for the two steps after purification
by recrystallization (Scheme 5). Alkyne extension was achieved
by cross-coupling with ethynylmagnesium bromide in the
presence of Pd catalyst to give 18, which was also purified by
recrystallization. Gratifyingly, compound 18 has proven to be
bench-stable in the solid state for months, and it participates
efficiently in cross-coupling reactions (vide infra).
With the end cap 13 and internal silole building block 18 at
hand, the trimer 8 was assembled by cross-coupling and purified
by recrystallization (Scheme 6). Both dimer 7 and trimer 8 are
orange-red powders displaying good solubility in most common
organic solvents (e.g., THF, Et2O, CH2Cl2, CHCl3, PhH, DME).
The tetrameric (9) and pentameric (10) oligomers were
constructed using larger end caps that were assembled by
synthetic methods similar to those described above, with only
minor sequence modifications (Scheme 7). Reaction of chloro-
silole 13 with aryl triazene 16 followed by MeI gave iodo-
arene 19 in 83% yield for the two steps after recrystallization.
Cross-coupling with ethynylmagnesium bromide provided ter-
minal alkyne 20 in 91% yield. Using 20 as the in situ cross-
coupling partner with chloroiodosilole 12 provided the extended
end cap 21.
The tetrameric (9) and pentameric (10) siloles were prepared
in 76% and 64% yield, respectively, from end-cap 21 and the
appropriate internal silole building blocks (Scheme 8). These
oligomers are each red powders exhibiting poor solubility in
most organic solvents.26
Electronic Absorption and Photoluminescence. The elec-
tronic absorption spectra of the oligomer family are shown in
Figure 4 and are summarized in Table 1. The effect of chain
elongation on absorption wavelength diminishes as expected:
dimer 7 shows a 38 nm red-shift in comparison with monomer
6,27 from dimer 7 to trimer 8 the ∆λmax contracts to 12 nm,
and, finally, there is no change in absorption maxima on going
from tetramer to pentamer. Tetramer 9 and pentamer 10 each
display an absorption maximum within 2 nm of those reported
for the analogous high molecular weight silole-containing
polymers (i.e., Figures 1 and 2).12 That is, this work demon-
strates for the first time that the effective conjugation length
within the corresponding silole polymers, at least to the degree
of accuracy that can be estimated by absorption maxima, is
approximately equal to the tetramer 9. Based on the absorption
onset, the band gaps (Table 1) also show a convergence to a
low-voltage limit as the chain length approaches that of the
pentamer, which matches that reported for the polymer 2 (2.07
eV).15 The extinction coefficients for the oligomer series are
also of interest (Table 1). The trimer, tetramer, and pentamer
have extinction coefficients greater than 120 000 M-1 cm-1
,
which makes these compounds promising candidates for photo-
voltaic applications. To the best of our knowledge, these are
the highest molar absorptivities reported for any nonpolymeric
(24) 2-Ethynyl-1,1-dimethyl-3,4-diphenyl-5-(phenylethynyl)silole and 2-chloro-
5-ethynyl-1,1-dimethyl-3,4-diphenylsilole were also found to be unstable
under the cross-coupling reaction conditions and decomposed upon
prolonged storage.
(25) Zhang, J.; Pesak, D. J.; Ludwick, J. L.; Moore, J. S. J. Am. Chem. Soc.
1994, 116, 4227-4239.
(26) This is not a general limitation: solubility can be modulated by the use of
larger 1,1-dialkyl siloles.
(27) Compounds exactly analogous to
dibromosiloles; see refs 11b and 15.
6 have been prepared from 2,5-
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Synthesis of Oligomeric Silolene Chromophores
A R T I C L E S
Scheme 8
Table 1. Summary of Electronic Absorption, Emission, and
Calculated Bandgap Data for Oligomers
nature of the synthetic strategy presented herein will facilitate
the optimization of quantum efficiencies through tuning steric
interactions.28 The emission λmax values sequentially red-shift
through the series from monomer to trimer (Figure 6, Table 1),
at which point increasing conjugation length causes spectrum
absorption
emission
oligomer
λmax
log(ꢀ)
λmax
ΦF
∆λmax
∆E (eV)c
a
b
monomer (6)
dimer (7)
trimer (8)
tetramer (9)
pentamer (10) 492
429
467
479
492
4.74
4.85
5.09
5.17
5.27
520 8.97 × 10-2
526 0.26 × 10-2
543 0.37 × 10-2
550 0.22 × 10-2
538 2.99 × 10-2
2.43
2.30
2.11
2.09
2.08
38
12
13
0
broadening, but no increase in λmax
.
A comparison of these oligomers with the reported properties
of non-silole-containing oligo(aryleneethynylene)s reveals that
those incorporating siloles display absorption maxima at
significantly longer wavelengths (Figure 7). Specifically, at
maximum effective conjugation length, the siloleneethynylene
oligomers reported herein are more than 50 nm red-shifted with
reference to oligo(thienyleneethynylene)s,29 and greater than 100
nm red-shifted relative to oligo(phenyleneethynylene)s.30 Note
that the basis used in the juxtaposition of these materials’
properties was the total number of rings in the chromophore,
not the number of repeat units.31
a Determined with reference to fluorescein. b Relative to preceding
oligomer. c Estimated from the onset of absorption.
The red-shifts observed for the oligomer series reported herein
cannot be safely ascribed to the influence of the 3,4-diphenyl
functionality as these are known to generally exert a small
electronic effect. For example, comparison of silole chromo-
phores 22-26 (Figure 8) reveals that for 2,5-dithienyl siloles32
the inclusion of phenyl rings at the 3,4-position (24) results in
only a 3 and 9 nm red-shift in comparison with the 3,4-
unsubstituted (23) and 3,4-dialkyl silole (22), respectively. In
addition, for 2,5-diphenyl siloles33 25 and 26, substitution at
the 3,4-positions (26) resulted in a blue-shifted absorption
maximum in comparison with the 3,4-unsubstituted silole 25.
To gain insight into the influence exerted by a single silole
ring on the electronic and photoluminescent behavior of an
extended chromophore, we synthesized 27 (Scheme 9) for
direct comparison with its silole counterpart, trimer 8. The
electronic absorption spectrum of oligomer 27 shows a 33 nm
blue-shift of the absorption maximum and a slight decrease in
the extinction coefficient (27 λmax ) 446 nm, log ꢀ ) 4.96)
resulting from replacement of the center silole with a
phenyl ring (Figure 9). The emission λmax is also blue-shifted
relative to trimer 8 (Figure 10), but interestingly the quantum
efficiency is 20.11 × 10-2 in the case of 2734 versus 0.37 ×
10-2 for trimer 8.
Figure 4. Electronic absorption spectra for the series of first generation
silole oligomers in CH2Cl2 at room temperature.
Figure 5. Molar absorptivity as a function of MW × 10-3 (9, R2 ) 0.98)
and number of silole rings in the chromophore (2, R2 ) 0.98).
silole chromophore. As can be seen in Figure 5, the molar
absorptivity increases linearly with the number of silole rings
incorporated into the oligomer chain, and with the molecular
weights of the oligomers. Thus, this relationship may be useful
in estimating the molar absorptivities of silole polymers and
higher order oligomers.
(28) These results will be reported elsewhere.
(29) Pearson, D. L.; Schumm, J. S.; Tour, J. M. Macromolecules 1994, 27,
2348-2350.
(30) Jones, L., II; Schumm, J. S.; Tour, J. M. J. Org. Chem. 1997, 62, 1388-
1410.
The oligomers show modest quantum efficiencies with the
monomer being most efficient at 9%, the pentamer next at 3%,
and the other oligomers clustered around 0.3%. The modular
(31) For example, pentamer 10 is counted as an 11-mer.
(32) Yamaguchi, S.; Tamao, K. J. Chem. Soc., Dalton Trans. 1998, 3693-3702.
(33) Katkevics, M.; Yamaguchi, S.; Toshimitsu, A.; Tamao, K. Organometallics
1998, 17, 5796-5800 and references therein.
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A R T I C L E S
Boydston et al.
Figure 9. Electronic absorption spectra of trimer 8 (- - -) and 27 (-) in
CH2Cl2 at room temperature.
Figure 6. Photoluminescence spectra for the series of first generation silole
oligomers in CH2Cl2 at room temperature.
Figure 10. Photoluminescence spectra of trimer 8 (- - -) and 27 (-) in
CH2Cl2 at room temperature.
Figure 7. Absorption maxima as a function of rings in the backbone for
the alternating silole-containing co-oligomers presented herein ([), oligo-
(thienyleneethynylene)s29 (9), and oligo(phenyleneethynylene)s30 (2).
overall, requires few synthetic steps, and is highly atom
economical. The overall sequence efficiency makes the routes
described here amenable to considerable scale-up. Specifically,
the oligomers were prepared in excellent overall yield from
commercially available diethynyl silane 11: dimer 7 (44%),
trimer 8 (29%), tetramer 9 (23%), and pentamer 10 (13%). The
intermediates are generally solids and can be purified by
recrystallization alone (only three compounds required chro-
matographic purification). The fully optimized reaction condi-
tions described in the Supporting Information require no special
equipment, strict thermal control, or technical skills beyond
experience with anaerobic synthesis.
Figure 8. Absorption maxima (nm) in parentheses for the comparison of
the effects of differing 3,4-substituents.
It is gratifying to observe that the long wavelength absorptions
exhibited by these relatively short oligomers imply electronic
behavior similar to that of silole polymers reaching molecular
weights of 64 000.12,15 This work establishes for the first
time the effective conjugation length in oligo[(p-phenylene-
ethynylene)-alt-(2,5-siloleneethynylene)]s.
Scheme 9
We are currently conducting studies on the electrolumines-
cent, electro-generated chemiluminescent, and nonlinear optical
properties of these and related compounds.
Acknowledgment. We thank the Robert A. Welch Foundation
and the donors of the American Chemical Society Petroleum
Research fund for partial financial support of this research.
A.J.B. thanks the Robert A. Welch Foundation for a Welch
Summer Research Fellowship. Y.Y. thanks Pfizer for an
Undergraduate Research Fellowship.
Conclusions
Herein, we have reported the first homologous series of well-
defined oligomeric ethynylsilole chromophores and demon-
strated the power of synthetic strategies based on 2-chloro-5-
iodosilole 12. The construction of the oligomers centered around
direct desymmetrization and derivatization of a silole ring,
whereas previous strategies relied on monolithiation or mono-
bromination of pendant aryl groups, or monolithiation of
dibromosiloles. This new oligomer synthesis is very practical
Supporting Information Available: Detailed experimental
procedures and copies of NMR spectra of all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(34) ΦF was determined with reference to fluorescein.
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