Synthesis of poly(phenylene ethynylene)s containing trialkoxysilane side chains

Article abstract of DOI:10.2174/157017809789124911

New hybrid monomers and polymers consisting of rigid π-conjugated rods flanked with trialkoxylsilane side chains have been successfully synthesized. It has been shown that the monomer undergoes a sol-gel process to form transparent gels, from which the or

Full text of DOI:10.2174/157017809789124911

Letters in Organic Chemistry, 2009, 6, 485-490
485
Synthesis of Poly(Phenylene Ethynylene)s Containing Trialkoxysilane Side
Chains
Vikalp Thakor, Mahuya Bagui and Zhonghua Peng*
Department of Chemistry, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO 64110, USA
Received January 29, 2009: Revised July 02, 2009: Accepted July 02, 2009
Abstract: New hybrid monomers and polymers consisting of rigid ꢀ-conjugated rods flanked with trialkoxylsilane side
chains have been successfully synthesized. It has been shown that the monomer undergoes a sol-gel process to form
transparent gels, from which the organic rods can be subsequently removed to produce nanoporous materials.
Keywords: Sol-gel, conjugated polymer, poly(phenylene ethynylene), fluorescence, nanoporous.
The sol-gel process is one of the most important wet
chemical routes to afford organic-inorganic hybrid materials
as its mild processing conditions allow the covalent
incorporation of a variety of organic moieties into an
inorganic network [1-2]. The organic component is usually
introduced first to the precursor monomer which
subsequently undergoes the sol-gel process to form the
inorganic network. Trialkoxylsilanes, with a formula of
RSi(OR)3, are by far the most popular sol-gel precursors [3-
7]. Extensive and systematic studies have been carried out to
examine what effect the organic substituents (R) have on the
gelation behavior and how they affect the characteristics of
the resulting polymeric gels [7]. To render the gel
result in nanoporous materials with pore shapes and sizes
closely matching the removed rods (molecular imprints).
Here, we report the synthesis of three such hybrids whose
structures are shown in Fig. (2). Monomers 1 and 2 contain
short ꢀ-segments while P1-P3 are based on conjugated
polymers. It should be noted that while the conjugated
polymer/silica hybrids have been extensively explored, the
hybridization has so far been limited to conjugated polymers
containing ionic side chains [10,11,18-21]. Because of the
lack of covalent linkage between the conjugated polymer and
the inorganic network and the incompatability between the
hydrophobic organic polymer and the hydrophilic inorganic
matrices, the amount of the conjugated polymer which can
Si(OEt)3
Si(OEt)3
ꢀ-conjugated rod
(EtO)3Si
Si(OEt)3
(A)
(B)
Fig. (1). Model sketches of two types of bridged bistrialkoxysilanes.
electrically-active, organic conjugated systems can be
covalently embedded into the sol-gel matrix either as
pendants or as spacers [8-15]. It has been shown that
polysilsesquioxanes with rigid bridging groups/spacers form
more readily than simple polysilsesquioxanes [16]. The
bridging groups form highly condensed networks, with the
bridging group securely bound by two covalent bonds to the
inorganic components [17]. Interestingly, most if not all
organic ꢀꢁconjugated rod spacers are linked to the inorganic
network through its two ends (Model A in Fig. (1)). Another
approach, where the organic rod is anchored to the sol-gel
matrix through its waist (Model B in Fig. (1)), is rarely
explored. The rotational flexibility of the organic rod in
Model B may allow the preparation of more densely-packed
networks. Removing the organic rods from the xerogels may
be uniformly distributed is rather limited. The new hybrid
monomers and polymers, however, are potential sol-gel
precursors to conjugated polymer/silica hybrids where the
organic conjugated systems are covalently linked to the
inorganic network.
The syntheses to the targeted monomers and polymers
are shown in Scheme 1. The trialkoxysilylated sol-gel
precursor 1 was prepared by a carbamate coupling reaction
between
the
2,5-diiodohydroquinone
and
3-
(triethoxysilyl)propyl isocyanate [22]. The reaction was
carried out at room temperature in diethyl ether in the
presence of triethylamine. FTIR measurements show that
after 24 h, the isocyanate band at 2270 cm-1 has completely
disappeared and new strong bands at 1713 (carbonyl
stretching), 1078 (Si-OEt stretching), and 1541 cm-1 (NH-
CO stretching) have appeared, indicating the completion of
the reaction. The 1H NMR spectrum of 1 gives one aromatic
signal at 7.55 ppm and a triplet at 5.54 ppm (N-H). The Si-
binding methylene protons are strongly shielded and appear
*Address correspondence to this author at the Department of Chemistry,
University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO
64110, USA; E-mail: pengz@umkc.edu
1570-1786/09 $55.00+.00
© 2009 Bentham Science Publishers Ltd.

486 Letters in Organic Chemistry, 2009, Vol. 6, No. 6
Thakor et al.
Si(OEt)3
Si(OEt)3
Si(OEt)3
O
O
O
O
O
NH
O
NH
NH
O(CH2CH2O)3CH3
I
I
y
n
x
O
O
O
O
O
O
O(CH2CH2O)3CH3
HN
HN
HN
(EtO)3Si
(EtO)3Si
(EtO)3Si
1
P1-P3
2
Fig. (2). Structures of new hybrid monomers and polymers based on Model B.
Si(OEt)3
Si(OEt)3
O
O
O
NH
NH
NCO
HO
O
(EtO)3Si
I
I
I
I
Et3N, ether
64%
Pd(PPh3)2Cl2, CuI Et3N,
DMF
O
O
OH
O
O
2
HN
45%
HN
(EtO)3Si
NH
(EtO)3Si
1
Si(OEt)3
O
O
RO
RO
OR
I
I
OR
OR
3
4
y
n
Pd(PPh3)2Cl2, CuI Et3N,
DMF
x
O
OR
R = (CH2CH2O)3CH3
P1: x = 1, y = 1
HN
O
(EtO)3Si
P2: x = 1, y = 9
P3: x = 0, y = 1
Scheme 1. Synthesis of sol-gel precursor monomers (1,2) and polymers (P1-P3).
at 0.68 ppm, while the ethoxy protons furnish signals at 1.22
To prepare the conjugated polymers containing
and 3.82 ppm.
trialkoxysilane side chains, monomer 1 was polymerized
with monomer 4 using the Sonogashira coupling reaction
(see Scheme 1). The resulting polymer P1 has a number-
average molecular weight of 21 kDa and a polydispersity of
1.31. This polymer, however, has a poor solubility in all
organic solvents, which prevents its full structural and
property characterizations. Suspecting that the poor
solubility is due to the high trialkoxysilane content in the
polymer, we sought to decrease its content by
copolymerizing monomers 1 and 3 with monomer 4. The
polymerization was initially run in one pot (the ratio for
monomers 1, 3, and 4 was 1:4:5). While a polymer was
successfully prepared (Mw, 22 kDa; PD, 1.26), the 1H NMR
spectrum of this polymer indicates that only a portion of 1
Hybrid 2 was synthesized by the Sonogashira coupling
reaction of 1 with ethynylbenzene [23]. The 1H NMR
spectrum of 2, shown in Fig. (3a) displayed all the aliphatic
proton signals with the same chemical shifts as those of 1,
while the aromatic proton signals of the phenylene
ethynylene backbone overlapped thereby appearing as
multiplets in the 7.25-7.45 ppm region. The 13C NMR
spectrum of 2 shows two peaks at 96.4 and 84.2,
corresponding to the two internal alkyne carbons. The iodo-
binding carbon signal, appearing at 90.1 ppm in 1,
disappeared completely. A total of 15 13C signals are
observed, consistent with the desired structure of 2.

Electrically-Active Organic-Inorganic Hybrids
Letters in Organic Chemistry, 2009, Vol. 6, No. 6
487
Fig. (3). 1H NMR spectra of (a) sol-gel precursor 2; (b) P2 prepared in one pot in two steps; and (c) P2 prepared in one pot in one step.
was incorporated into the polymer. As shown in Fig. (3c), an
integration ratio of the ethoxy’s methyl protons (proton a)
over the oligoether’s ethylene protons (proton 1) is found to
be only 1:10, far lower than the expected 1:2 ratio based on
the monomer loadings. It appears that monomer 1 has a
lower reactivity in comparison to monomer 3.
fluorescence emissions. The half-intensity band width for P1
and P2 is 26 and 23 nm respectively, much narrower than
that of polymer 3, indicating a lower extent of aggregation of
the PPE backbone in polymers P1 and P2. The solution
fluorescence quantum yields for P1 and P2 are 0.34 and
0.42, respectively, again higher than that of P3 (0.22).
Hence, to ensure a polymer with the desired composition,
the copolymerization reaction was carried out in one pot but
in two steps. The less reactive monomer 1 was first allowed
to react with monomer 4. After the complete consumption of
1, monomer 3 was then added to continue the polymerization
While polymer P2 has lower trialkoxysilane side chains
and although the initial polymer did not precipitate out from
the polymerization solution, after precipitation from
methanol and subsequent drying in vacuo, the resulting
polymer showed only a partial solubility. The fact that
polymer P3 also became partially soluble after drying
excludes crosslinking of partially hydrolyzed trialkoxy-
silanes as the major cause for the loss of solubility, but rather
points to a factor which is intrinsic to the triglyme-
substituted PPEs. It is likely that the PPE backbones pack
closely when the solvent evaporates. To improve the hybrid
polymer’s solubility, co-monomers with bulky side chains
may be needed. We are currently synthesizing such
polymers.
1
process. The H NMR spectrum of the resulting polymer, as
shown in Fig. (3b), showed that the same two protons, i.e.
the Si-OEt methyl proton a and the oligoether’s ethylene
proton 1 appeared in an intensity ratio of 1:3, much closer to
the expected 1:2 ratio. The number- average molecular
weight of the polymer is found to be 39 kDa, which is also
higher than that of the polymer prepared in one pot but only
one step process.
Fig. (4) shows the absorption and fluorescence emission
spectra of the new hybrid molecules and polymers in dilution
chloroform solutions. Polymers P1 and P2 have an
absorption maximum of 418 and 438 nm, respectively,
significantly red-shifted compared to that of hybrid 2 (ꢀmax
for 2 is 328 nm). Both P1 and P2 give intense and sharp
To evaluate the sol gel process and the subsequent
removal of the organic rods from the resulting gels, hybrid 2
was subjected to sol gel conditions. The hydrolysis-
polycondensation reaction (sol-gel) was performed using
HCl as the catalyst, as shown in Scheme 2. When a THF

488 Letters in Organic Chemistry, 2009, Vol. 6, No. 6
Thakor et al.
Fig. (4). UV/Vis absorption (a) and fluorescence emission (b) spectra of 1, 2, and P1-P3.
Scheme 2. Synthesis of xerogel from precursor 2 and subsequent removal of the organic rod from the resulting xerogel. The enclosed table
shows the gelation time under various catalyst ratios.
solution of 2 was treated with water in the presence of 10.8
mol% HCl, a red-colored gel was obtained in merely 5 min
and 40 seconds. Decreasing the acid concentration extends
the gelation process. After being cured for 2 days at room
temperature, followed by washing with THF and then drying
in vacuo, xerogel XG2a was isolated as red powders.
To remove organic rods from the resulting xerogel, a
suspension of the crushed xerogel in THF was refluxed in
the presence of LiAlH4 for 36 h [24]. A light gray colored
xerogel XG2b was obtained, whose FTIR spectrum (Fig.
(5b)) shows no ethynyl absorption at 2228 cm-1, while the
carbonyl stretch was also absent, indicating the complete
removal of the organic rods. It is also noted that the Si-OEt
stretch at 1178 cm-1 disappeared completely while the Si-OH
stretch at 1382 cm-1 became very intense, denoting the
complete hydrolysis of the unreacted ethoxysilyl end groups.
FTIR spectra of 2 and its resulting xerogel XG2a clearly
show the structural changes after gelation. As shown in Fig.
(5a), XG2a gives an intense new absorption at 1059 cm-1
which is associated with the Si-O-Si stretching, an ultra
broad and intense absorption band around 3500 cm-1, which
is attributed to uncondensed silanols (Si-OH), and water
molecules bound to the xerogel. The observation of an
absorption band at 1177 cm-1 indicates the presence of some
unreacted ethoxysilyl end groups. The strong carbonyl
stretch at 1734 cm-1 and the enthynyl signal at 2228 cm-1 are
conspicuous.
In conclusion, a new type of trialkoxysilane-containing
hybrid monomers and polymers has been successfully
synthesized. Preliminary studies have shown that the hybrid
monomers can undergo the sol-gel process to form
transparent gels and the rigid organic rods can be completely
removed from the resulting xerogels to produce nanoporous
materials.

Electrically-Active Organic-Inorganic Hybrids
Letters in Organic Chemistry, 2009, Vol. 6, No. 6
489
methanol afforded compound 1 in the form of white crystals
1
(9.1 g, 64%, mp 170-172 °C). H NMR (400 MHz, CDCl3):
ꢀ 7.55 (s, 2H), 5.54 (t, J = 5.8 Hz, 2H), 3.82 (q, J = 7.0 Hz,
12H), 3.28 (m, 4H), 1.69-1.74 (m, 4H), 1.22 (t, J = 7.0 Hz,
18H), 0.68 (t, J = 8.0 Hz, 4H); 13C NMR (100 MHz, CDCl3)
ꢀ 153.1, 149.0, 132.5, 90.1, 58.6, 43.7, 23.0, 18.3, 7.7; IR
(KBr) 3303, 2978, 2870, 1707, 1541, 1463, 1390, 1254,
1131, 1103, 1078, 943, 770, 677 cm-1. Anal. Calcd for
C26H46I2N2O10Si2: C 36.45%, H 5.41%; found C 36.13%, H
5.58%.
Compound 2
A mixture of Compound 1 (1.00 g, 1.167 mmol),
phenylacetylene (0.358 g, 3.502 mmol), Pd(PPh3)2Cl2 (0.033
g, 0.047 mmol), CuI (0.018 g, 0.093 mmol), triethylamine
(3.5 mL) and DMF (4 mL) was stirred at room temperature
for 30 min. After extraction and flash chromatography
eluting with hexane:ethyl acetate (2:1), the title compound
was obtained as
a brown fluffy solid, which was
recrystallized from methanol to obtain the pure product in
°
1
the form of a white solid (0.4 g, 45%, mp 188-190 C). H
NMR (400 MHz, CDCl3): ꢀ 7.47-7.42 (m, 4H), 7.29-7.25
(m, 8H), 5.53 (t, J = 5.8 Hz, 2H), 3.81 (q, J = 7.0 Hz, 12H),
3.29 (m, 4H), 1.74-1.66 (m, 4H), 1.21 (t, J = 7.0 Hz, 18H),
0.66 (t, J = 8.0 Hz, 4H); 13C NMR (100 MHz, CDCl3): ꢀ
154.2, 148.9, 131.8, 128.6, 126.9, 123.3, 118.7, 110.2, 96.4,
84.2, 58.7, 43.9, 23.3, 18.5, 7.9; IR (KBr): 3330, 2981, 2935,
2890, 2231, 1720, 1539, 1509, 1483, 1446, 1413, 1282,
1253, 1186, 1107, 1083, 960, 891, 760, 691 cm-1. Anal.
Calcd for C42H56N2O10Si2: C 62.66%, H 7.01%; found C
62.35%, H 7.17%.
Fig. (5). FTIR spectra of sol-gel precursor 2, xerogel XG2a and the
PPE-removed xerogel XG2b.
EXPERIMENTAL
Xerogel XG2a
Compounds 3 and 4 were prepared according to literature
procedures [25]. THF was purified by distillation over
sodium chips and benzophenone. Triethyl amine was
distilled over CaH2. All of the other chemicals were
purchased either from ACROS or Aldrich and were used as-
In four different reaction tubes containing compound 2
(0.05 g, 0.062 mmol), THF (0.155 mL) and water (6.7 μL),
were added, respectively, 13.4, 7.4, 2.5 and 0.6 μL of 1M
HCl solution. The acid/alkoxysilane mol ratio for the four
tubes is 10.8%, 6%, 2% and 0.5%, respectively. The reaction
tubes were then continuously shaken until the sol no longer
moved, indicating the completion of gelation. After curing
the wet gels for 2 days, they were crushed, washed with THF
and dried in vacuo overnight to obtain red colored xerogels.
FTIR (KBr) 3427, 3065, 2938, 2228, 1969, 1734, 1600,
1507, 1482, 1409, 1177, 1059, 918, 757, 690 cm-1.
1
received unless otherwise stated. The H NMR spectra were
collected on a Varian 400 MHz FT NMR spectrometer. GPC
measurements were performed at 30 °C on a Waters setup (a
Waters 210 pump, a Waters R401 differential refractometer,
and a styragel 4E column) with DMF as the eluent. FT-IR
spectra were obtained from samples dispersed in KBr pellets
and recorded on an IR100 FT-IR spectrometer (Thermo-
Nicolet Co.).
A Hewlett-Packard 8452A diode array
spectrophotometer was used to record the UV/Vis absorption
spectra. Photoluminescence properties were measured using
a Shimadzu RF-5301PC spectrofluorophotometer. Fluore-
scence quantum yields were determined by means of quinine
sulfate in 1N H2SO4 (ꢀfl ꢁ 0.55) as the standard.
PPE-Removed Xerogel XG2b
The mixture of xerogel XG2a (0.080 g), LiAlH4 (0.018
g), and THF(10 mL) was refluxed for 36 h. After filtration,
the resulting dark brown solid was sequentially washed with
THF (10 mL), 0.1M HCl (0.8 mL), 0.1M NH4OH (0.2 mL),
water (10 mL) and acetone (15 mL) over a period of 3 days
to yield a light gray colored insoluble, fluffy solid (0.057 g).
FTIR (KBr) 3469, 2936, 1648, 1634, 1545, 1507, 1382,
1002, 692, 535 cm-1.
Compound 1
To a colorless homogeneous solution of 2,5-dihydroxy-
1,4-diiodobenzene (6.0 g, 16.58 mmol) in anhydrous diethyl
ether (30.0 mL) and triethylamine (9.0 mL) was added
dropwise 3-(triethoxysilyl)propyl isocyanate (9.03 g, 36.48
mmol) during 30 min. The carbamate formation was
monitored by FTIR spectroscopy. The resulting solution was
stirred for 24 h, and then filtered. Recrystallization from
Polymer P2 in One Pot in One Step
A mixture of compound 4 (0.3132 g, 0.677 mmol),
compound 3 (0.3612 g, 0.542 mmol), compound 1 (0.1156 g,

490 Letters in Organic Chemistry, 2009, Vol. 6, No. 6
Thakor et al.
0.135 mmol), Pd(PPh3)2Cl2 (0.019 g, 0.027 mmol), CuI
(0.010 g, 0.054 mmol), triethylamine (1.9 mL) and DMF
(3.5 mL) was stirred at room temperature for 5 h and was
then filtered through celite. The filtrate was poured into
methanol. The precipitates were collected by filtration and
dried in vacuo to give polymer P2 as a dark brown solid
(0.541 g; Mn, 22 kDa; PD, 1.26). 1H NMR (400 MHz,
CDCl3): ꢀ 7.04-6.99 (m), 4.23-4.14 (br), 3.91-3.85 (br), 3.81-
3.73 (m), 3.64-3.57 (m), 3.54-3.46 (m), 3.36 (s), 3.34-3.31
(m), 1.26-1.15 (m). Anal. Calcd for C22.6H33.4N0.2O8.2Si0.2: C
61.04%, H 7.57%; found C 62.12%, H 7.59%.
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solubility for 1H NMR characterization. Anal. Calcd for
C50H78N2O18Si2: C 57.12%, H 7.48%; found C 56.32%, H
6.93%.
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Polymer P3. 0.1479 g of compound 3 reacted with
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1
solid. P3 does not have a sufficient solubility for H NMR
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Polymer P2 in One Pot in Two Steps
A mixture of compound 4 (0.3081 g, 0.666 mmol),
compound 1 (0.1139 g, 0.133 mmol), Pd(PPh3)2Cl2 (0.019 g,
0.027 mmol), CuI (0.010 g, 0.054 mmol), triethylamine (1.8
mL) and DMF (3.2 mL) was allowed to stir at room
temperature. After 2 hours, when TLC analysis indicated that
the carbamate monomer was completely consumed,
compound 3 (0.3552 g, 0.533 mmol) was added and the
reaction mixture was stirred for a further 2.5 h. Polymer P2
was isolated as a dark brown solid using the same procedure
[19]
[20]
[21]
[22]
[23]
[24]
[25]
1
as above (0.577 g; Mn, 39 kDa; PD: 1.4). H NMR (400
Xu, B.; Pan, Y.; Zhang, J.; Peng, Z. Synth. Met., 2000, 114, 337.
MHz, CDCl3): ꢀ 7.03-6.97 (m), 4.23-4.12 (br), 3.88 (s), 3.81-
3.73 (m), 3.70-3.66 (br), 3.64-3.57 (m), 3.55-3.47 (m), 3.36-
3.31 (m), 1.18 (t, J = 3.3 Hz), 0.63-0.55 (br). Anal. Calcd for
C22.6H33.4N0.2O8.2Si0.2: C 61.04%, H 7.57%; found C 60.36%,
H 7.42%.