Facile synthesis of functional alkoxysilane precursor with short linkers toward organosilica hybrids with a high density of chromophores

Article abstract of DOI:10.1246/cl.2012.316

Tetraphenylpyrene (TPPy)-bridged alkoxysilane precursors with short linkers were synthesized by development of a facile synthetic route, and solgel polymerized to transparent organosilica films which showed higher absoption coefficient than the convention

Full text of DOI:10.1246/cl.2012.316

doi:10.1246/cl.2012.316
316
Published on the web February 25, 2012
Facile Synthesis of Functional Alkoxysilane Precursor with Short Linkers
toward Organosilica Hybrids with a High Density of Chromophores
Yoshifumi Maegawa,1,2 Norihiro Mizoshita,1,2 Takao Tani,1,2 and Shinji Inagaki*1,2
1Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192
2Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST),
Kawaguchi, Saitama 332-0012
(Received November 25, 2011; CL-111132; E-mail: inagaki@mosk.tytlabs.co.jp)
Tetraphenylpyrene (TPPy)-bridged alkoxysilane precursors
R1
O
with short linkers were synthesized by development of a facile
synthetic route, and sol-gel polymerized to transparent organo-
silica films which showed higher absoption coefficient than the
conventional route due to dense accumulation of TPPy units in
the films.
R1
=
O
N
H
Si(OEt)3
1-U3
R1
R1
=
Si(Oi-Pr)3
1-C3
R1
Organosilica hybrids, synthesized by sol-gel polymerization
of organic-bridged alkoxysilane precursors (R[Si(OR¤)3]n, n > 2,
R: functional organic group, R¤: Me, Et), are a new class of
functional organic-inorganic hybrid materials in which organic
groups are covalently and homogeneously embedded in the
robust silica networks.1,2 Recently, these materials have received
considerable attention in various areas such as optical materials,3
photocatalyst,4 and electronic devices due to their organic
functionalities and high stability.5 For the development of these
applications, efficient light absorption and/or strong intermo-
lecular interaction are essential factors. Consequently, the high
density accumulation of organic chromophores in the organo-
silica hybrids has become a currently interesting work in this
field.
The density of chromophores in organosilica hybrids should
strongly depend on the molecular architecture of the alkoxy-
silane precursor. Thus, precursors with chromophores directly
attached with silyl groups, which are typically prepared by
halogen-lithium and -magnesium exchange reactions2a or
rhodium-catalyzed silylation of aryl halides,6 are the most
desirable for dense accumulation. However, the synthesis of
desired functional alkoxysilane precursor is generally difficult,
because the high reactivity of trialkoxysilyl groups is likely to
cause side reactions such as polymerization and decomposition
during these silylation reactions and purification. Furthermore,
conventional routes are often limited in terms of difficulties in
the preparation of halogenated substrate and functional tolerance
in their reaction conditions. In addition, the Si-C bonds of this
type of precursors sometimes cleave during the acidic or basic
sol-gel polymerization.7 Meanwhile, coupling reaction of silane
coupling agents with hydroxy- or amino-substituted substrate
has been widely used for the synthesis of functional organo-
silane precursors due to easiness in the preparation of substrates
and mild reaction conditions.8 We recently reported the synthesis
of 1,3,6,8-tetraphenylpyrene (TPPy)-bridged alkoxysilane pre-
cursor 1-U3 by coupling of tetrahydroxy-substituted TPPy with
triethoxy(isocyanatopropyl)silane and its conversion to the
transparent periodic mesostructured organosilica film with
efficient blue fluorescence emissions (Figure 1).9 However, the
long linkers in 1-U3 were also found to limit light absorption
efficiency of the hybrid films. It is required to establish silylation
R1
=
Si(Oi-Pr)3
1-C1
R1
Figure 1. TPPy-bridged alkoxysilane precursors 1-U3, 1-C3,
and 1-C1.
H-Si
Si
(a)
Pt cat.
H
H
H-Si
Si
(b)
Pd cat.
Figure 2. Regioselective hydrosilylation of olefinic aromatics
catalyzed by platinum- or palladium-catalyst.
routes other than direct silylation for dense accumulation of the
organic-bridging groups (e.g., TPPy in 1-U3). There have been
few reports, however, on successful synthesis of high functional
organosilane precursors with short linker.10
Herein, we report facile synthetic routes of aromatic-bridged
alkoxysilanes with short linkers, their sol-gel conversions to
organosilica films, and optical properties of the films.
We synthesized novel TPPy-bridged alkoxysilane precur-
sors with short linkers, 1-C3 and 1-C1 (Figure 1). To develop
the synthetic route of these precursors, we employed two main
reactions, regioselective hydrosilylation and the Suzuki-Miyaura
coupling. It is well-known that the platinum-catalyzed hydro-
silylation of allyl-substituted compound gives the £-silylated
(linear) product (Figure 2a).11 On the other hand, palladium-
catalyzed hydrosilylation of vinyl-substituted compound fur-
nishes the ¡-silylated (branched) product (Figure 2b).12 There-
fore, organosilane precursors with short linkers could be
prepared from olefinic aromatics by regioselective hydrosilyla-
tion.13
The efficient synthetic route of olefinic aromatics has been
limited to the allylation of unstable lithiated species.10a Although
this method has been applied to the synthesis of various olefinic
aromatics, the reaction requires moisture-free and cryogenic
conditions and often results in low yields or undesired product.
Chem. Lett. 2012, 41, 316-318
© 2012 The Chemical Society of Japan
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317
R2
8
7
6
5
Br
R2
a
Br
Br
4
3
R2
Br
2
1
0
2a: R2
2b: R2
=
=
(95%)
(93%)
R2
500
250
300
350
400
450
Wavelength / nm
b, d
66% (2 steps)
c, d
2a
1-C3
2b
1-C1
88% (2 steps)
Figure 3. Dependence of the absorption coefficients for 1-U3-F
(white circles), 1-C3-F (gray circles), 1-C1-F (black circles),
and TPPy-F (solid line) on the wavelength. The absorption
coefficients were obtained by normalizing the absorbance of the
films at every wavelength according to the film thicknesses.
Scheme 1. Synthetic routes to newly designed TPPy-bridged
alkoxysilane precursors; 1-C3 and 1-C1. Reagents and con-
ditions: (a) [Pd(PPh3)4], K2CO3 (aq), 4-allylphenylboronic acid,
1,4-dioxane, 90 °C for 2a, [Pd(PPh3)4], K3PO4 (aq), 4-vinyl-
phenylboronic acid, 1,4-dioxane, 80 °C for 2b; (b) H2[PtCl6]¢
6H2O, HSiCl3, benzene, 40 °C; (c) [PdCl(³-C3H5)]2, rac-H-
MOP, HSiCl3, benzene, 40 °C; (d) i-PrOH, pyridine, CH2Cl2,
0 °C-rt.
to the suppression of heterogeneous shrinkage during solvent
evaporation.16 The thicknesses of 1-U3-F, 1-C3-F, and 1-C1-F
were 1450 « 50, 455 « 30, and 235 « 30 nm, respectively. The
change in the thickness can be attributed to the difference in the
viscosity of the sol solutions.
To overcome these limitations and difficulties, we employed a
useful approach, the Suzuki-Miyaura coupling of aryl halide with
olefin-substituted phenylboronic acid as shown in Scheme 1.14
The olefinic TPPy derivatives 2a and 2b were synthesized
by the palladium-catalyzed Suzuki-Miyaura coupling of 1,3,6,8-
tetrabromopyrene with 4-allylphenylboronic acid or 4-vinyl-
phenylboronic acid, respectively (Scheme 1). Each reaction
successfully proceeded in the presence of [Pd(PPh3)4] (12
mol %) as catalyst and gave the desired product 2a and 2b in
95% and 93% yield, respectively. Fortunately, undesired side
reactions such as isomerization and polymerization of olefinic
moiety were not observed. The obtained 2a and 2b were then
regioselectively hydrosilylated with trichlorosilane in the pres-
ence of platinum or palladium catalyst, respectively. The use of
H2[PtCl6]¢6H2O (Speier’s catalyst)11 was found to be effective
for 2a. The hydrosilylation proceeded regioselectively at £-
position of four allyl groups and gave the TPPy-bridged
trichlorosilane in almost quantitative yield. The 4-fold hydro-
silylation of 2b was carried out in the presence of [PdCl(³-
C3H5)]2/rac-H-MOP.12 As expected, the reaction regioselective-
ly proceeded at ¡-positions of four vinyl groups and furnished
the desired TPPy-bridged trichlorosilane quantitatively. How-
ever, the obtained chlorosilanes were too moisture-sensitive to
handle as a precursor for the organosilica hybrids. Thus, these
trichlorosilyl groups were transformed into relatively stable
triisopropoxysilyl groups by quenching with 2-propanol in the
presence of pyridine as a base, which afforded desired TPPy-
bridged alkoxysilane precursors with short linkers 1-C3 and
1-C1 in 66% and 88% yield, respectively (Figures S1-S4).15,18
From TPPy-bridged organosilanes 1-U3, 1-C3, and 1-C1,
organosilica hybrid films could be obtained by sol-gel polymer-
ization in the presence of hydrochloric acid and water in
propylene glycol propyl ether. Transparent yellow films without
cracks and inclusions were successfully obtained from all the
precursors (Figure S518). The use of the high boiling point
solvent facilitates the formation of uniform thickness of film due
We evaluated the optical properties of TPPy-bridged
alkoxysilane precursors and derived organosilica films to
elucidate the effects of the linker length on accumulation state
of TPPy moieties in these silica hybrids. The novel TPPy
precursors 1-C3 and 1-C1 showed primary similar absorption
and fluorescence properties compared with those of conventional
precursor 1-U3 and their parent TPPy molecule (Figure S618).
This suggests that the present linkers hardly influenced the
electronic ground and excited state of these precursors in diluted
condition. In contrast, optical properties of derived organosilica
films were quite different from one another. Figure 3 shows the
wavelength dependence of the absorption coefficients (absorb-
ance normalized by the film thickness) for organosilica and
TPPy molecular films (TPPy-F). The absorption coefficient was
significantly increased in the order of 1-U3-F (17000 cm¹1) <
1-C3-F
(29400 cm¹1) < 1-C1-F
(39900 cm¹1) < TPPy-F
(59800 cm¹1). It is quite reasonable to observe the difference
in absorption coefficients because the ratios of TPPy moiety to
whole molecules are completely different in their precursors.
The maximum wavelengths of absorption spectra (-max) slightly
red-shifted with respected to that for corresponding precursor
solutions due to intermolecular interaction between TPPy
moieties at the ground state. However, the observed red shift
in the films is very small and shift values do not depend on the
linker length. This may be attributed to the bulky structure of
TPPy which inhibits dense packing of TPPy moiety in organo-
silica hybrids.
Figure 4 shows fluorescence emission spectra for the
organosilica and TPPy molecular films. The organosilica film
1-U3-F showed broad emission bands at 450 and 470 nm which
are assignable to monomer-like and excimer emissions, respec-
tively.9 The existence of the strong monomer-like band and the
relatively small red shift of the excimer band suggest relatively
weak intermolecular interaction between the TPPy moieties
in the excited state due to the long linker length in 1-U3. In
Chem. Lett. 2012, 41, 316-318
© 2012 The Chemical Society of Japan
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318
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350
400
450
500
550
600
650
Wavelength / nm
Figure 4. Fluorescence emission spectra of 1-U3-F (white
circles), 1-C3-F (gray circles), 1-C1-F (black circles), and
TPPy-F (solid line). The excitation wavelength was 380 nm for
all samples.
7
8
9
contrast, organosilica films 1-C3-F and 1-C1-F mainly showed
the excimer emission band at 478 nm, which is red-shifted from
that of 1-U3-F, together with a weak monomer-like emission at
440 nm. In addition, 1-C1-F showed a shoulder possibly due
to the emission from TPPy dimer at 510 nm.17 These results
strongly suggest that the shortening of the linker effectively
enhanced intermolecular interaction between the TPPy moieties
in the excited state. On the other hand, TPPy-F showed a
fluorescence spectrum similar to that of 1-U3-F despite the
densest packing of the TPPy moieties possibly due to their
different molecular arrangement from those of the organo-
silica films in which the TPPy moieties are fixed in the silica
network.
In conclusion, we demonstrated the facile synthetic route of
functional organic-bridged alkoxysilane with short linker by
regioselective hydrosilylation. Novel TPPy-bridged alkoxy-
silane precursors with short linker were successfully synthesized
using the present procedure with aim to increase the absorption
coefficient of TPPy-bridged organosilica film. A significant
improvement of the absorption coefficient of the film was
observed by shortening of linker length due to the high density
of TPPy moieties in the film.
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18 Supporting Information is available electronically on the
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5
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Chem. Lett. 2012, 41, 316-318
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/