Synthesis of silicon-containing unsaturated polymers by hydrosilylation reaction. Photophysical studies

Article abstract of DOI:10.1021/ma034618f

Silicon-containing unsaturated polymers have been synthesized using the Pt(acac)₂-catalyzed photoactivated hydrosilylation of alkynes. These polymers fluoresce at 360 nm when excited at 270 nm due to the π-conjugated vinylphenyl segment in the polymer chain. The polymers have excellent solubility in several organic solvents.

Full text of DOI:10.1021/ma034618f

Macromolecules 2003, 36, 8225-8230
8225
Articles
Synthesis of Silicon-Containing Unsaturated Polymers by
Hydrosilylation Reactions. Photophysical Studies
F ei Wa n g, Bila l R. Ka a fa r a n i, a n d Dou gla s, C. Neck er s*,†
Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403
Received May 12, 2003; Revised Manuscript Received August 21, 2003
ABSTRACT: Silicon-containing unsaturated polymers have been synthesized using the Pt(acac)2-catalyzed
photoactivated hydrosilylation of alkynes. These polymers fluoresce at 360 nm when excited at 270 nm
due to the π-conjugated vinylphenyl segment in the polymer chain. The polymers have excellent solubility
in several organic solvents.
In tr od u ction
fluorescence of the polymer over the model compound.
This red shift was proposed to be due to the weak σ-π
conjugation through the silicon atoms in the polymer
chain or due to interchain aggregation of oligomer
chains.
In this paper, conjugated polymers with organosili-
cons as the spacer were synthesized using platinum(II)
bis(acetylacetonato) [Pt(acac)2]-catalyzed photohydrosi-
lylation of alkynes. These polymers photoluminesce
when excited at 270 nm.
Conjugated polymers are of interest as emissive layers
in organic light-emitting devices.1,2 The emission can
be controlled by use of a spacer between chromophores
in the polymer chain; the extent of control depends on
the choice of chromophore. It is also possible to tune
the luminescence properties of such polymers at the
molecular level. Electroluminescence from a polymer
containing well-defined fluorophores linked by flexible
spacers has been studied.3 Polymers utilizing silicon-
based spacers have several advantages, and a wide
variety of such silicon-containing polymers have been
designed and synthesized.4 The solubility and process-
ability of the polymer can be improved due to the chain
flexibility arising from the silicon atom. Absorption and
emission data suggest only weak σ-π conjugation
coupling between chromophores by way of the silicon
linker.5,6
Kim7 studied the synthesis and optical properties of
poly(p-phenylenevinylene)-like polymers containing sili-
con atoms in the main chain. The photoluminescence
of these polymers between 440 and 480 nm indicates
that the regular π-conjugated system is effectively
interrupted by the organosilicon units, thus resulting
in a blue emission. These units improve processing and
limit the π-conjugation length.7
Exp er im en ta l Section
Ma t er ia ls a n d In st r u m en t s. THF was purchased from
Aldrich Chemical Co. and refluxed with sodium and benzophe-
none until the color turned blue to remove moisture. Triethy-
lamine was freshly distilled. All other reagents were purchased
from Acros Chemical Co. and used as received unless otherwise
noted. Irradiations were carried out in a Rayonet photochemi-
cal reactor (350 nm lamps) equipped with a jacketed beaker
(Pyrex). NMR spectra were obtained using a Varian Gemini
200 NMR spectrometer, and chemical shifts are in parts per
million with the corresponding deuterated solvents as the
internal standard. GC/MS spectra were obtained on a Shi-
madzu GC/MS-QP5050 mass spectrometer coupled to a GC-
17A (Restek ST1-5 column, 30 m × 0.25 mm × 0.25 µm).
((p-Br om op h en yl)eth yn yl)tr im eth ylsila n e (2) was syn-
thesized via the Sonogashira coupling reaction.17 To a solution
of 5.1 g (18 mmol) of 1-bromo-4-iodobenzene and 2.1 g (21
mmol) of ethynyltrimethylsilane in 80 mL of anhydrous
triethylamine were added 0.23 g (0.33 mmol) of bis(triph-
enylphosphine)palladium(II) chloride and 19 mg (0.10 mmol)
of cuprous iodide. The reaction mixture was stirred at room
temperature under argon overnight, then diluted with ben-
zene, washed twice with 200 mL of water, and dried over
MgSO4. Solvent was evaporated under vacuum and the residue
purified by chromatography on silica gel with hexanes as the
eluent. A colorless liquid was obtained (90% yield) and
characterized by 1H NMR and GC/MS. 1H NMR (200 MHz,
CDCl3): δ 0.25 (s, 9H), 7.34 (d, J ) 8.4 Hz, 2H), 7.42 (d, J )
8.4 Hz, 2H). MS (m/z): 252.
Hydrosilylation is an effective method for preparing
silicon-containing polymers.8-13 Platinum and rhodium
complexes such as Karstedt’s catalyst,8 chloroplatinic
11-13
acid,9,10 and RhI(PPh3)3
are the common catalysts
for such reactions. Hyperbranched polycarbosilanes
were also synthesized by hydrosilylation addition
reactions.14-16
Kepler8 reported the synthesis of light-emitting oli-
gocarbosilanes with Karstedt’s catalyst. The model
compound that is the repeat unit in the oligocarbosilane
was also synthesized. Kepler observed a red shift of the
p-Br om op h en yla cetylen e (3) was synthesized in 91%
yield following a literature procedure.18 The light yellow
compound was characterized by 1H NMR and GC/MS. 1H NMR
(200 MHz, CDCl3): δ 3.13 (s, 1H), 7.37 (d, J ) 8.4 Hz, 2H),
7.45 (d, J ) 8.4 Hz, 2H). MS (m/z): 180.
* To whom correspondence should be addressed.
† Contribution No. 501 from the Center for Photochemical
Sciences.
10.1021/ma034618f CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/09/2003

8226 Wang et al.
Macromolecules, Vol. 36, No. 22, 2003
(p-(Dim eth ylsilyl)p h en yl)a cetylen e (4) was synthesized
following a literature procedure (63% yield) as a colorless liquid
that turned yellow on standing.5 The compound was charac-
terized by H NMR and GC/MS. H NMR (200 MHz, CDCl3):
δ 0.33 (d, J ) 4.0 Hz, 6H), 3.10 (s, 1H), 4.42 (m, 1H), 7.49 (s,
4H). MS (m/z): 160.
Sch em e 1. Syn th esis of 4a
1
1
1,4-Dieth yn ylben zen e (6) was obtained as the side prod-
uct of the synthesis of 4. It is a white solid with a melting
point of 91-93 °C. 1H NMR (200 MHz, CDCl3): δ 3.18 (s, 2H),
7.45 (s, 4H). MS (m/z): 126.
2-((Tr im eth ylsilyl)eth yn yl)th iop h en e (8) was synthe-
sized as 2, with 2-bromothiophene as the starting material.
The residue was purified by chromatography on silica gel using
hexanes as the eluent. A yellow liquid was obtained (50%).
The compound was characterized by 1H NMR and GC/MS. 1H
NMR (200 MHz, CDCl3): δ 0.26 (s, 9H), 6.96 (t, 1H), 7.25 (d,
2H). MS (m/z): 180.
a
Reagents and conditions: (a) Pd(PPh3)2Cl2, CuI, NEt3,
HCCSiMe3, argon, 15 h; (b) MeOH, K2CO3, 5 h; (c) (1) n-BuLi,
THF, -78 °C, argon; (2) ClSiMe2H.
Sch em e 2. Syn th esis of 11a
2-Iod o-5-eth yn ylth iop h en e (10) was synthesized in 84%
yield following a literature procedure.16 1H NMR (200 MHz,
CDCl3): δ 3.39 (s, 1H), 6.90 (d, J ) 3.6 Hz, 1H), 7.09 (d, J )
4 Hz, 1H). MS (m/z): 234.
2-((Dim eth ylsilyl)eth yn yl)th iop h en e (11) was synthe-
sized as 4, with 10 as the starting material. A yellow liquid
was obtained in a yield of 91%. 1H NMR (200 MHz, CDCl3): δ
0.31 (d, 6H), 4.27 (m, 1H), 6.93 (t, 1H), 7.26 (d, 2H). 13C NMR
(200 MHz, CDCl3): δ 132.88 (CH), 127.57 (CH), 126.86 (CH),
122.80 (CH), 98.84 (C), 95.69 (C), -3.0 (CH3), -3.10(CH3).
HRMS (m/z): calcd for C8H10SiS 166.3189, found 166.0279.
1,3,5-((Tr im eth ylsilyl)eth yn yl)ben zen e (14) was syn-
thesized as 2, with 1,3,5-tribromobenzene as the starting
material. The residue was purified by chromatography on silica
gel with hexanes as the eluent. A colorless liquid was obtained
(70% yield). The compound was characterized by 1H NMR and
a
Reagents and conditions: (a) Pd(PPh3)2Cl2, CuI, NEt3,
HCCSiMe3, argon, 15 h; (b) LDA, I2, -78 °C; (c) MeOH, K2CO3,
5 h; (d) (1) n-BuLi, THF, -78 °C, argon; (2) ClSiMe2H.
1
GC/MS. H NMR (200 MHz, CDCl3): δ 0.23 (s, 27 H), 7.49 (s,
Gen er a l P r oced u r e for F lu or escen ce Qu a n tu m Yield
Mea su r em en t.21 Fluorescence quantum yields were measured
in cyclohexane using naphthalene as the standard. Using the
same apparatus, the quantum yield of an unknown is related
to that of a standard by eq 1. The subscript u refers to the
3H). MS (m/z): 262.
1,3,5-Tr ieth yn ylben zen e (15)19 was synthesized via a
procedure similar to that of 3, with 14 as the starting material.
A light yellow solid was obtained in a yield of 91%. The melting
1
point is 100-101 °C. The compound was characterized by H
NMR and GC/MS. 1H NMR (200 MHz, CDCl3): δ 3.11 (s, 3H),
7.57 (s, 3H). MS (m/z): 150.
AsFu
2
η
φu )
φs
(1)
( )
AuFs η0
((p-(Dim eth ylsilyl)p h en yl)eth yn yl)tr im eth ysila n e (16)
was synthesized as 2, with 2 as the starting material. After
hydrolytic workup and rotary evaporation of the solvent, the
residue was purified by flash chromatography (silica gel,
hexanes) to provide a colorless liquid (3.2 g, 86%). The
compound was characterized by 1H NMR and GC/MS. 1H NMR
(200 MHz, CDCl3): δ 0.26 (s, 9H), 0.34 (d, 6H), 7.58 (d, 2H),
7.60 (d, 2H). Anal. Calcd: C, 67.17; H, 8.67. Found: C, 67.20;
H, 8.46.
unknown and s to the standard, and the other symbols have
the following meanings: φ is the quantum yield, A is the UV-
vis absorption at the excitation wavelength, F is the integrated
emission area across the band, and η is the index of refraction
of the solvent containing the unknown (η) and the standard
(η0) at the sodium D line and the temperature of the emission
measurement. In this case, naphthalene in cyclohexane was
used as the standard so φs ) 0.19, η0 ) 1.363 (cyclohexane),
and η ) 1.424 (dichloromethane).
1,3,5-Tr ivin ylben zen e (17) was synthesized in 60% yield
1
following a literature procedure.20 It was characterized by H
1
NMR and GC/MS. H NMR (200 MHz, CDCl3): δ 5.23 (d, J )
Resu lt a n d Discu ssion
10.6 Hz, 3H), 5.72 (d, J ) 17.6 Hz, 3H), 6.72 (dd, J ) 11.0 Hz,
J ) 17.6 Hz, 3H), 7.33 (s, 1H). MS (m/z): 156.
P h otop olym er iza tion of Mon om er s 4, 11, a n d 18.
Polymers 4a , 11a , and 18a were synthesized by Pt-
(acac)2-catalyzed photoactivated hydrosilylation reac-
tions of the appropriate silanes. The latter contained
both a Si-H bond and a carbon-carbon triple bond.
Monomers 4 and 11 were synthesized as shown in
Schemes 1 and 2. Oligomers 11a and 18a were obtained
after 30 min of irradiation from photopolymerization of
monomers 11 and 18 through Pt(acac)2-catalyzed pho-
toactivated hydrosilylation (Scheme 3). Efforts to pre-
cipitate oligomers with methanol were not successful.
Both oligomers are brown glassy solids. The polymeri-
zation result is shown in Table 1.
Th e F ir st Gen er a tion of a Sta r P olym er (16a ) was
synthesized by Pt(acac)2-catalyzed hydrosilylation of 15 and
16 with a molar ratio of 1:3. It was characterized by GC/MS
and elementary analysis. Anal. Calcd for C51H66Si6‚1/2H2O: C,
71.51; H, 7.88. Found: C, 71.17; H, 7.91.
Gen er a l P r oced u r e for P h otop olym er iza tion of Ca r -
bosila n e Mon om er s. All the polymerizations were conducted
in bulk at a molar ratio of Pt(acac)2 to the monomer of 10-3:1.
The reaction mixture was stirred well before irradiation at 350
nm in the Rayonet reactor. In most of the cases, after a short
period of irradiation the reaction mixtures were kept in the
dark overnight at room temperature before workup and
analysis. Control experiments were conducted for the same
reaction mixtures under the same conditions without irradia-
tion. The polymers were isolated by dissolution in CH2Cl2 and
precipitated three times with MeOH. Polymers were charac-
terized by GPC, 1H NMR, 13C NMR, 29Si NMR, and FT-IR.
Cross-linked polymers were extracted in a Soxhlet extractor
with CH2Cl2 for 2 days and dried in an oven.
In the photopolymerization of monomer 4 via the Pt-
(acac)2-catalyzed photoactivated hydrosilylation, vigor-
ous and exothermic polymerization was observed after
less than 3 min of irradiation. The products are mostly
cross-linked polymers that cannot be dissolved in or-

Macromolecules, Vol. 36, No. 22, 2003
Synthesis of Si-Containing Unsaturated Polymers 8227
Sch em e 3. Syn th esis of Silicon -Con ta in in g P olym er s
fr om Mon om er s 11 a n d 18
Sch em e 5. Syn th esis of 15a
a
Reagents and conditions: (a) Pd(PPh3)2Cl2, CuI, NEt3,
HCCSiMe3, argon, 15 h; (b) MeOH, K2CO3, 5 h.
Sch em e 6. Syn th esis of a Silicon -Con ta in in g Sta r
P olym er w ith P t(a ca c)2-Ca ta lyzed P h otoa ctiva ted
Hyd r osilyla tion of 1,3,5-Tr ieth yn ylben zen e
Ta ble 1. P olym er iza tion Resu lts of Mon om er s 4, 11, a n d
18
polymer
Mw (g/mol)
PD
yield (%)
18a
11a
4a a
4a b
4a c
735
811
1.2
1.3
66
55
16890
8899
2.5
3.0
a
Cross-linked polymer from photopolymerization of 4 after
b
extraction in CH2Cl2 for 48 h. Soluble polymer from photopo-
lymerization of 4. c Control polymerization of 4 in the dark for 2
days.
Sch em e 4. P olym er iza tion of
(p-(Dim eth ylsilyl)p h en yl)a cetylen e
ganic solvents such as CH2Cl2, CHCl3, benzene, and
hexanes. The cross-linked polymers obtained have a
large swelling ratio (5.1 in CH2Cl2); therefore, the degree
of cross-linking cannot be high. A small amount of
soluble polymer was obtained following precipitation by
a large amount of methanol (Scheme 4). In the control
reaction, soluble polymer was obtained after 2 days of
dark reaction. The polymerization was smooth and slow.
The molecular weights of the polymers are listed in
Table 1.
Sch em e 7. Syn th esis of a Lin ea r Silicon -Con ta in in g
P olym er w ith P t(a ca c)2-Ca ta lyzed P h otoa ctiva ted
Hyd r osilyla tion of Alk yn e
The Pt(acac)2-catalyzed hydrosilylation reaction of
alkyne occurs via cis addition and forms three kinds of
products, with the â-trans isomer predominating over
the R isomer.22 The oligomers 11a and 18a as well as
polymer 4a contain both conjugated and cross-conju-
gated segments as a consequence of the formation of
both â-trans and R isomers. These polymers have good
solubility in common organic solvents.
P h otop olym er iza tion of Mon om er s 4 a n d 15 a n d
4 a n d 6. To synthesize longer conjugated polymers
containing both π conjugation and σ-σ conjugation,
compounds 6 and 15 were prepared (Schemes 5 and 1).
Photopolymerization of 15 and 4 at molar ratios of 1:10
and 1:20 gave soluble star polymer 15a (Scheme 6).
After 30 min of irradiation, the polymers were glassy
solids though they were of low molecular weight. After
postpolymerization in the dark at room temperature for
three weeks, hard solid polymers formed with much
higher molecular weights. Photopolymerization of 6 and
4 at a molar ratio of 1:10 also gave soluble polymer 6a
(Scheme 7). All these polymers were yellow powders
after purification and are soluble in common organic
solvents. See Table 2 for the results.
Photopolymerization of 15 and 4 at a molar ratio of
1:50 gave partially cross-linked polymer. The high
concentration of 4 is responsible for the formation of
cross-linked polymer. This is probably due to the hy-

8228 Wang et al.
Macromolecules, Vol. 36, No. 22, 2003
Sch em e 9. Syn th esis of 16a
Ta ble 2. P h otop olym er iza tion Resu lts of 15 a n d 4 a n d 6
a n d 4
monomers
molar ratio
Mn (g/mol)
Mw (g/mol)
PD
6 and 4
1:10
1:20a
1:10
1:10b
1:20
1:20b
1:50
3788
1555
1080
2616
2003
4660
5941
6421
3121
1738
4652
3703
8790
14480
1.69
1.71
1.61
1.78
1.85
1.89
2.4
a
Reagents and conditions: (a) Pd(PPh3)2Cl2, CuI, NEt3,
15 and 4
HCCSiMe3, argon, 15 h; (b) (1) n-BuLi, THF, -78 °C, argon,
2. ClSiMe2H.
Sch em e 10. Syn th esis of th e F ir st Gen er a tion of a
Sta r P olym er (Tw o P ossible Isom er s)
a
b
Thermal polymerization (room temperature, dark). After
postpolymerization for 3 weeks (room temperature, dark).
Sch em e 8. Mech a n ism of th e F or m a tion of a
Cr oss-Lin k ed P olym er
because of its high molecular weight. In the 13C NMR
spectrum, the star polymers have a new signal at 131.25
ppm belonging to the carbon of the -CH group on the
core benzene ring. Other peaks have the same chemical
shifts as those found in the spectrum of the linear
polymer from 4. In the FT-IR spectrum, the star
polymer has a much stronger absorption of the terminal
alkyne group at 3316 and 2108 (weak) cm-1 than does
the linear polymer formed from monomer 4. Further-
more, there is almost no absorption of the Si-H bond
at 2116 cm-1 in the star polymer, while there is an
absorption at 2116 cm-1 in the thermal polymer formed
from monomer 4. This clearly shows that there is no
terminal Si-H bond in the star polymer while there is
a terminal Si-H bond in the linear polymer formed from
monomer 4. No difference is observed in the 29Si NMR
spectra of the star polymers and the linear polymer
formed from monomer 4.
drosilylation of the terminal Si-H bond of H across the
C-C double bond in the polymer chain (Scheme 8). In
the photoreaction of 15 with 4 at a lower molar ratio
the terminal Si-H bond reacts first with the C-C triple
bond of compound 15. There is no terminal Si-H bond
remaining as the polymerization becomes complete. No
cross-linked polymer was formed. On the other hand,
hydrosilylation of the internal C-C double bond is
difficult due to steric considerations. In the photopo-
lymerization of monomer 4, the exothermic hydrosily-
lation of the terminal alkynes along the polymer chain
is very rapid. A large amount of heat is released
following a short induction period, promoting the hy-
drosilylation reaction of some of the internal C-C
All these polymers and oligomers fluoresce when
excited at wavelengths from 250 to 300 nm. The
fluorescence of the oligomers and linear polymers is
double bonds and resulting in cross-linked polymers.
1
All the polymers were characterized by H NMR, 13
C
NMR, 29Si NMR, and FT-IR. For the linear polymer
formed from monomer 4, a doublet of doublets at
chemical shifts of 6.65 and 6.88 ppm with a large
coupling constant (J ) 19 Hz) is characteristic of the
â-trans isomer. The two broad singlet peaks belonging
to the R isomer at chemical shifts of 5.67 and 6.01 ppm
weak compared with that of the star polymer (Φfl
≈
10-3). The fluorescence maxima of the star polymers are
observed at ∼360 nm. The fluorescence quantum yield
decreases as the length of the star polymer increases.
To clarify the fluorescence chromophore, the first gen-
eration of a star polymer (16a ) was synthesized by
hydrosilylation of 15 and 16 (Schemes 9 and 10). The
core molecule 1,3,5-trivinylbenzene (17) was also syn-
thesized (Scheme 11). The UV-vis absorption spectrum
of 17 has an absorption maximum at 248 nm. The
maxima of the fluorescence are at 346 and 362 nm. The
normalized fluorescence spectra (λmax ) 346 and 362
nm) of compound 17 and the star polymers are shown
in Figure 1. A red shift of 10 nm was observed between
17 and 16a , spectrum a. There is no red shift in the
fluorescence spectra of 16a or the star polymers formed
from 1:10 and 1:20 molar ratios of 15 and 4, spectrum
b. However, the fluorescence spectrum of the star
1
were also observed in the H NMR spectrum. The two
vinyl carbons of the â-trans isomer are clearly shown
at 146.5 and 125.1 ppm in the 13C NMR. In the 29Si
NMR, two signals from the â-trans and R isomers in
the polymer chain are at chemical shifts of -8.417 and
-10.579 ppm, respectively.
1
Few differences in H NMR, 13C NMR, and FT-IR are
observed when one compares the structure of the star
polymer with that of the linear polymer formed from 4.
The 1H NMR clearly shows the star polymer has a
terminal acetylenic proton at 3.1 ppm while the linear
polymer of monomer 4 has no signals around 3.1 ppm

Macromolecules, Vol. 36, No. 22, 2003
Synthesis of Si-Containing Unsaturated Polymers 8229
Ta ble 4. F lu or escen ce Qu a n tu m Yield s of P olym er sa
Sch em e 11. Syn th esis of 17a
compoundb
a
b
c
d
e
f
φ
0.90
0.68
0.18
0.12
0.06
0.0090
a
The excitation wavelength is 270 nm, and naphthalene was
b
used as the standard (φ ) 0.19 in cyclohexane). Compounds: a
) 1,3,5-trivinylbenzene; b ) first generation of a star polymer; c
) star polymer from a molar ratio of 15 and 4 of 1:10; d ) star
polymer from a molar ratio of 15 and 4 of 1:20; e ) star polymer
from a molar ratio of 15 and 4 of 1:50; f ) 1,3,5-triethynylbenzene.
a
Reagents and conditions: (a) LiCl, Pd(PPh3)4, vinyltribu-
tylstannane, anhydrous dioxane, argon, 100 °C, 15 h.
F igu r e 2. Relationship of the UV absorption and intensity
of the fluorescence of the star polymer (1:10 molar ratio).
fluorescing chromophore is the core molecule 17. There
is no intermolecular self-quenching in that there is a
linear relationship between the fluorescence intensity
and concentration of the star polymer (Figure 2).
The longer the arm of the star polymer, the lower the
fluorescence quantum yield. This might be associated
with an intramolecular self-quenching of the star poly-
mer. Because of the flexibility of the polymer chain
containing a silicon atom, the arms of the star polymer
can be highly folded and, thus, interact with other arms
in the polymer. Intrachain chromophore interaction due
to folding of the polymer has been reported for silylene-
divinylbenzene copolymers.23,24 This idea, however,
must be tested with further experiments.
F igu r e 1. Fluorescence spectra of 1,3,5-trivinylbenzene and
star polymers: (a) 1,3,5-trivinylbenzene; (b) first generation
of a star polymer; (c-e) star polymers from a molar ratio of
6.15 and 6.4 of 1:10, 1:20, and 1:50, respectively.
Ta ble 3. Absor p tion Ma xim a of th e UV-Vis Sp ectr a of
Con clu sion
th e Cor e Molecu le a n d Sta r P olym er s
Photoluminescent silicon-containing polymers have
been efficiently synthesized using the Pt(acac)2-cata-
lyzed photoactivated hydrosilylation of alkynes. The
fluorescent chromophore derived is mainly due to the
π-conjugated vinylphenyl segment. The polymers have
excellent solubility in organic solvents.
compound
λmax (nm)
1,3,5-triethynylbenzene
1,3,5-trivinylbenzene
first generation of a star polymer
star polymers
235, 253
248
258, 270
270 (br)
polymer obtained from a 1:50 molar ratio of 15 and 4
has a red shift compared with those of other star
polymers prepared using lower ratios of starting mate-
rials (1:10, 1:20). Furthermore, the red shift of 10 nm
was also observed in the UV-vis spectrum of 17 when
compared with spectra from the star polymers (Table
3). These results suggest a very weak σ-π conjugation
in the polymer chain. The silicon atom actually inhibits
π conjugation, and the conjugation is restricted to a
single repeating unit.
1,3,5-Triethynylbenzene has a very small fluorescence
quantum yield, whereas the fluorescence quantum yield
of 17 is 0.90 when it is excited at 270 nm. This
fluorescence quantum yield is larger than those of all
the star polymers, Table 4, and suggests that the
Ack n ow led gm en t. We thank the U.S. Soybean
Board for financial support of this work. We also thank
Dr. Huiying Li for helpful discussions.
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