Preparation of novel photoluminescent oligocarbosilanes by hydrosilylation [4]

Article abstract of DOI:10.1021/ma011789g

The preparation of the photoluminescent oligocarbosilanes by hydrosilylation was discussed. Karstedt-catalyzed oligomerization of photoluminescent oligocarbosilanes was carried out in an argon atmosphere in a sealed pressure tube. The initially colorless monomer became dark brown within one minute after addition of catalysts. The photoluminescent polymers offer flexibilities in molecular design and good processability. Their luminescent properties could be tuned at the molecular level. The photoluminescent oligocarbosilanes are unique because they contain both conjugated and cross conjugated segments.

Full text of DOI:10.1021/ma011789g

Macromolecules 2002, 35, 2871-2872
2871
P r ep a r a tion of Novel P h otolu m in escen t
Oligoca r bosila n es by Hyd r osilyla tion
Sh eila E. Gr a d w ell a n d Cin d y L. Kep ler *
Department of Chemistry, Bloomsburg University of
Pennsylvania, Bloomsburg, Pennsylvania 17815
Received October 15, 2001
Revised Manuscript Received February 22, 2002
In tr od u ction . Photoluminescent polymers have been
of interest recently due to their potential application as
light-emitting diodes (LEDs).1 They offer flexibility in
molecular design and good processability and are inex-
pensive relative to their inorganic counterparts. Fur-
thermore, it is possible to tune their luminescent pro-
perties at the molecular level. Since the first report of
,2
3
poly(p-phenylenevinylene) (PPV)-based LEDs, many
PPV derivatives with improved solubility, processability,
F igu r e 1. Excitation scan and fluorescence of 1a .
4
and quantum efficiency have been prepared. Addition-
5
-7
ally, the incorporation of silicon into PPV side chains
and backbones has been reported.8-10 Silicon-containing
PPVs are attractive because of improved solubility and
processability due to flexibility around the silicon atoms.
Also, silicon atoms in the polymer backbone interrupt
the regular π-conjugation, resulting in unusual optical
properties.8
,11
We have investigated the use of hydrosilylation to
prepare silicon-containing PPV derivatives. Hydrosilyl-
ation has been employed in polymerizations since it was
first used to polymerize vinyldimethylsilane.12 While Pt-
catalyzed hydrosilylation is traditionally performed with
chloroplatinic acid, recently the 1,3-divinyltetrameth-
yldisiloxane-Pt complex (Karstedt’s catalyst) has been
1
3
utilized. Herein, we report a simple preparation of
light-emitting oligocarbosilanes by hydrosilylation with
Karstedt’s catalyst. These oligomers are some of the first
examples of photoluminescent poly(silylenevinylene)s
with pendant phenyl groups.14 They are unique in that
they contain both conjugated and cross-conjugated
segments.
F igu r e 2. Excitation scan and fluorescence of 2a .
Sch em e 1. Syn th esis a n d Oligom er iza tion of 1 a n d 2
Resu lts a n d Discu ssion . Two compounds, (phenyl-
1
5
ethynyl)dimethylsilane (1) and (phenylethynyl)meth-
ylphenylsilane16 (2), were prepared by reaction of lithium
phenylacetylide with dimethylchlorosilane or meth-
ylphenylchlorosilane, respectively (Scheme 1). Karstedt-
catalyzed oligomerization of 1 or 2 was carried out in
an argon atmosphere in a sealed pressure tube. The
initially colorless monomer became dark brown within
1
min after addition of catalyst. Variable time and
temperature studies of the reactions showed that condi-
tions of 80 °C for 48 h minimized time to completion
while maximizing chain length. Efforts to precipitate
the oligomers from methanol were not successful. The
catalyst was removed by flash column chromatography
on silica gel, giving oligomers 1a , Mw/Mn ) 1300/1000,
and 2a , Mw/Mn ) 1430/1320 (Scheme 1), that fluoresce
being dark brown. They are freely soluble in common
organic solvents and have not shown any degradation
1
1
13
in the UV region (Figures 1 and 2). H NMR end group
in ambient conditions as evidenced by H and C NMR.
Oligomers 1a and 2a are stable to about 150 °C in a
nitrogen atmosphere by TGA. At about 300 °C, cata-
strophic weight loss occurs until only 5% original mass
remains by 500 °C.
While Pt-catalyzed hydrosilylation proceeds stereospe-
cifically by cis addition, it is not necessarily regiospecific.
analysis corresponds reasonably well to the GPC data,
giving Mn ) 1100 and Mn ) 1500 for 1a and 2a ,
1
respectively. The H NMR integration ratio of aromatic,
vinylic, and SiMe resonances equals the expected ratios
in the oligomers. Both oligomers are glassy solids at
room temperature, with 1a being dark orange and 2a
1
0.1021/ma011789g CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/12/2002

2
872 Communications to the Editor
Macromolecules, Vol. 35, No. 8, 2002
Sch em e 2. Syn th esis of Mod el Com p ou n d s 3 a n d 4
of 1a and 2a show a sharp, less intense absorbance at
-
1
2
158 cm due to Si-H end groups and a band at 2113
1
-
cm due to C-C triple-bond end groups.
Preliminary studies in our group show that oligomer-
ization of 1 using Wilkinson’s catalyst, Rh(PPh3)3Cl,
gives a variation of 1a that fluoresces at 400 nm,
consistent with a greater degree of conjugation in the
oligomer backbone. Currently, we are working to ana-
lyze the microstructure of 1a to attempt to understand
why Rh catalysis produces a longer wavelength emitter
than Pt catalysis.
Oligomers 1a and 2a are unique in that they contain
both conjugated and cross-conjugated segments, as a
consequence of R- and â-addition. A Pt-catalyzed hy-
drosilylation reaction between (phenylethynyl)trimeth-
ylsilane and dimethylphenylsilane was carried out to
produce monomeric models for the R- and â-addition
processes which occur in the Pt-catalyzed oligomeriza-
Ack n ow led gm en t. This work was supported by the
State System of Higher Education Faculty Professional
Development Council and by a Bloomsburg University
faculty research award. We thank Dr. Mark Tapsak and
Mr. Gregory Long for assistance with this project.
1
13
tions (Scheme 2). Analysis of the reaction by H and
C
NMR, IR, and GC/MS and comparison with the litera-
ture17 revealed 1-phenyldimethylsilyl-2-(trimethylsilyl)-
phenylethene (3) as the major product (83%). Evidence
for another product, 2-phenyldimethylsilyl-2-(trimeth-
ylsilyl)phenylethene (4) (14%), comes from the presence
of additional signals in the aromatic and methyl regions
Su p p or tin g In for m a tion Ava ila ble: Materials, instru-
mentation, and procedures for preparation and spectral data
for 1, 2, 1a , 2a , and 3. This material is available free of charge
via the Internet at http://pubs.acs.org.
Refer en ces a n d Notes
1
3
of the C NMR spectrum and from GC/MS. In ac-
cordance with the model reaction, Scheme 1 illustrates
the two types of repeat units, m and n, within the
oligomers. The m-type repeat units are a result of
hydrosilylation occurring by â-addition and represent
the majority of the units, whereas the n-type repeat
units are cross-conjugated units formed by R-addition.
Since overlapping vinylic hydrogen resonances in the
m- and n-type units of the oligomers prohibit their
(
(
(
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1
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The fluorescence spectrum obtained for 3 differs
significantly from that of the oligomers. When irradiated
at 240 nm, 3 fluoresces weakly at 297 nm. When
irradiated at 240 nm, 1a fluoresces with maxima at 358
and 377 nm (Figure 1). When irradiated at 240 nm, 2a
fluoresces with a maximum at 355 nm (Figure 2). The
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cence of the oligomers is not due to isolated styrene-
type chromophores. The difference in emission wave-
length may be due to the occurrence of conjugation
through the silicon atoms in the oligomer backbones or
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(
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1
2
a that fluoresce from 370 to 420 nm.18 The compara-
(
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tively shorter emission wavelengths of 1a and 2a are
consistent with their shorter conjugated segments. We
are currently investigating the concentration depen-
dence of the emissions of the oligomer solutions to better
understand the photophysics of the oligomers.
(
(
17) Naka, A.; Okazaki, S.; Hayashi, M.; Ishikawa, M. J . Orga-
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The infrared spectra of 1 and 2 show an intense
-
1
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absorbance at 2161 cm due to the Si-H stretch. The
signal due to the C-C triple bond stretch is likely
masked by the broad Si-H absorbance. The IR spectra
1
MA011789G