Synthesis of a spirobifluorene-bridged allylsilane precursor for periodic mesoporous organosilica

Article abstract of DOI:10.1039/c0cc05823h

A novel spirobifluorene-bridged allylsilane precursor, which can be easily purified by silica gel chromatography, was prepared by using a new molecular building block for allylsilane sol-gel precursors (MBAS) and successfully converted into a highly fluor

Full text of DOI:10.1039/c0cc05823h

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COMMUNICATION
Synthesis of a spirobifluorene-bridged allylsilane precursor for periodic
mesoporous organosilicaw
Natsuko Tanaka,a Norihiro Mizoshita,bc Yoshifumi Maegawa,bc Takao Tani,bc
Shinji Inagaki,*bc Yogesh R. Jorapurac and Toyoshi Shimada*ac
Received 28th December 2010, Accepted 5th March 2011
DOI: 10.1039/c0cc05823h
A novel spirobifluorene-bridged allylsilane precursor, which can
be easily purified by silica gel chromatography, was prepared by
using a new molecular building block for allylsilane sol–gel
precursors (MBAS) and successfully converted into a highly
fluorescent periodic mesoporous organosilica film.
Fig. 1 Examples of molecular building blocks for allylsilane sol–gel
precursors (MBAS).
Among the various types of organic–inorganic hybrid materials,
periodic mesoporous organosilicas (PMOs) have been established
because of the easiness in synthesis, sufficient stability in
as some of the most promising materials.1 Many PMOs
purification and applicability to sol–gel processes. These
have been synthesized from organic-bridged alkoxysilane
findings enable us to synthesize a variety of sol–gel precursors
precursors (R[Si(OR0)3]n; n Z 2, R: organic group, R0: Me,
with large organic groups and a high purity, which are
Et etc.),2–6 which were typically prepared by halogen–lithium
favorable for the synthesis of highly functional PMOs.
and –magnesium exchange reactions or rhodium-catalyzed
silylations.7–9 However, as alkoxysilane is difficult to purify
precursor for the synthesis of PMOs has so far been limited
However, the advanced utilization of the allylorganosilane
to only a benzene-bridged allylsilane precursor.10c,d
using silica gel chromatography owing to its high reactivity
with silica, it is limited in its applicability to large organic-
Here, we report the synthesis of a novel spirobifluorene-
bridged alkoxysilanes with high boiling points which are not
bridged allylsilane precursor 5 via the unique MBAS route
available for distillation. Besides, the conventional silylation
and its conversion into a highly fluorescent PMO film
approaches sometimes cause unfavorable side reactions.
using surfactant-directed self-assembly (Schemes 1 and 2).
Recently, we reported an alternative class of sol–gel precursors,
Spirobifluorene is a promising emitting phosphor owing to
the unique spatial conformation of two fluorene units.13–20
The emissive PMO can be applied to highly luminescent films
with tunable emission color21 because of its unique light-
harvesting function.22 We designed a new ethynyl-MBAS 3
for the synthesis of the spirobifluorene derivative 5 (Scheme 1).
The MBAS 3 can be widely utilized for the synthesis of
various extended p-conjugated sol–gel precursors using the
Sonogashira coupling reaction. The allylsilylated 5 has an
advantage of being easily purifiable by silica gel chromato-
graphy, but conventional alkoxysilylated spirobifluorene
cannot be purified by either chromatography or distillation.
We initially attempted to synthesize a spirobifluorene-based
precursor by rhodium-catalyzed triethoxysilylation of 2,20,7,70-
tetraiodo-9,90-spirobifluorene 4. However, the reaction
afforded a complex mixture containing no target compound
(Scheme 2). Eventually, the Sonogashira coupling reaction
with MBAS 3 successfully yielded the sol–gel precursor 5.
allylorganosilanes (R[Si(CH2CHQCH2)m(OEt)3–m]n; m = 1–3),
which behave as the synthetic equivalent of alkoxyorganosilane
and have sufficiently low reactivity to allow purification using
silica gel chromatography.10 Very recently, Jun et al. also
reported organofunctionalization on silica gel and glass with
methallylsilane in the presence of scandium triflate as a
catalyst at room temperature.11 Furthermore, we proposed a
general synthetic route for allylorganosilanes using a series of
molecular building blocks for allylsilane sol–gel precursors
(MBAS) and their palladium-catalyzed coupling reactions
(Fig. 1).12 The building blocks and derived precursors with
diallylethoxysilyl groups (m = 2) are especially versatile
a Department of Chemical Engineering, Nara National College of
Technology, 22 Yata-cho, Yamatokoriyama, Nara 639-1080, Japan.
E-mail: shimada@chem.nara-k.ac.jp; Fax: +81-743-55-6154;
Tel: +81-743-55-6154
b Toyota Central R&D Laboratories Inc., Nagakute, Aichi 480-1192,
Japan. E-mail: inagaki@mosk.tytlabs.co.jp; Fax: +81-561-63-6507;
Tel: +81-561-71-7393
c Core Research for Evolutional Science and Technology (CREST),
Japan Science and Technology Agency (JST), Kawaguchi,
Saitama 332-0012, Japan
w Electronic supplementary information (ESI) available: Experimental
details and spectroscopic data. See DOI: 10.1039/c0cc05823h
Scheme 1 Preparation of ethynyl-MBAS 3.
Chem. Commun., 2011, 47, 5025–5027 5025
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surfactant. The XRD pattern of the extracted film showed a
broad diffraction peak at d = 5.39 nm (Fig. 2a). Nitrogen
adsorption–desorption isotherms of the extracted film (peeling
off the substrate) after reinforcing the framework indicated the
formation of a mesoporous structure with a pore diameter of
ca. 4.1 nm although it seems to partially collapse (see ESIw).
These results show that 5 is suitable for the preparation of
mesostructured organosilica films. According to the molecular
geometry of 5, the hydrophobic spirobifluorene core is surrounded
by four silyl groups. These silyl groups can facilitate the
interaction of the precursor with hydrophilic surfaces of the
template surfactant micelles. Moreover, the four spatially
distributed silyl groups in 5 are more conducive to the formation
of a three-dimensionally crosslinked network, resulting in a
structurally stable organosilica framework after the removal
of the template. In contrast, the rod-like precursor 6 was not
suitable for the preparation of mesoporous organosilicas. The
polycondensed organosilica 6–Brij76 film showed XRD peaks
at d = 5.14, 2.60, 1.73, and 1.04 nm (Fig. 2b). These peaks
with a d-spacing ratio of 1 : 1/2 : 1/3 : 1/5 strongly suggested
the formation of a lamellar structure. After extraction of
Brij76, the XRD peaks completely disappeared, which
indicates that the 6-based organosilica does not form a
three-dimensionally crosslinked structure. In the sol–gel poly-
condensation process, the hydrophobic rigid rod-like core of 6
likely forms side-by-side aggregates, while the terminal silyl
groups of 6 can interact with the hydrophilic chain of Brij76,
which leads to the formation of the lamellar structure consisting
of organosilica layers and surfactant assemblies, rather than
a three-dimensional network. The side-by-side aggregation
of 6 is also supported by a 9 nm blue shift of the
absorption maximum wavelength of the 6–Brij76 film (343 nm)
in comparison with a solution of 6 in 2-propanol (352 nm)
(see ESIw).
Scheme 2 Synthesis of the spirobifluorene precursor 5 and meso-
structured organosilica. (a) [Rh(cod)(CH3CN)2]BF4, HSi(OEt)3, Et3N,
n-Bu4NI, DMF : toluene = 1 : 1; (b) 3, Pd2(dba)3, CuI, PPh3,
Et3N : toluene = 2 : 1, 100 1C, 24 h, 56%; (c) Brij76, HCl aq., THF.
The ethynyl-MBAS 3 was synthesized by the Sonogashira
coupling reaction of 1 with 2-methyl-3-butyn-2-ol followed
by deprotection with sodium hydroxide in 88% yield without
affecting the diallylethoxysilyl group (Scheme 1).23 The coupling
reaction of 3 with 2,20,7,70-tetraiodo-9,90-spirobifluorene 4 in
the presence of Pd2(dba)3 as catalyst in triethylamine/toluene =
2/1 at 100 1C for 24 h followed by purification with silica gel
column chromatography afforded the desired 2,20,7,70-
tetrakis(diallylethoxysilylphenylethynyl)-9,90-spirobifluorene
5
in 56% yield (Scheme 2). 2,7-Bis(diallylethoxysilylphenylethynyl)-
fluorene 6 without the spirocarbon was also synthesized by the
coupling reaction of 3 with 2,7-diiodofluorene for comparison
(Scheme 3).23
Acidic sol–gel polycondensation of 5 in the presence of
the nonionic template surfactant Brij76 (C18H37(OCH2CH2)10OH)
gave organosilica hybrid films with a meso-scale periodic
structure.23 Fig. 2a shows an X-ray diffraction (XRD)
pattern of the organosilica (5–Brij76) film. The film showed
a single intense peak at d = 5.52 nm and no higher-order
diffraction peaks were observed, which implies the formation
of a wormhole-like array of mesochannels. The periodic
mesostructure was retained after extraction of the template
The introduction of a spirobifluorene group into meso-
structured organosilica films has positive effects on the
fluorescence properties. Fig. 3 shows fluorescence spectra of
the organosilica films (5–Brij76 and 6–Brij76). The 5–Brij76
film showed a strong fluorescence emission with a quantum
yield (QY) of 0.59 upon excitation at l = 360 nm (QY of a
solution of 5 in 2-propanol: 0.92). The fluorescence spectrum
has a vibronic structure, which suggests that the organic
chromophore is fixed by the silica-based robust framework
Scheme 3 Synthesis of the bis(phenylethynyl)fluorene derivative 6.
Fig. 2 XRD patterns of (a) 5–Brij76 and (b) 6–Brij76 films before
Fig. 3 Fluorescence emission spectra of 5–Brij76 (solid line) and
6–Brij76 (dashed line) films.
(black) and after (gray) extraction of the template surfactant.
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5026 Chem. Commun., 2011, 47, 5025–5027
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without strong aggregation. Compared with spirobifluorene-
based molecular solid films,24 the 5–Brij76 film showed a shift
of the emission band to a longer wavelength exhibiting blue
owing to the extension of the p-conjugation length with
phenylethynyl substituents, which can be appropriate for
luminescence applications. On the other hand, the 6–Brij76
film showed a broad fluorescence spectrum attributable to
excimer-band emission with a QY of 0.38 (QY of a solution
of 6 in 2-propanol: 0.96). The lower QY of the 6-based
organosilica is probably due to self-quenching by the side-
by-side aggregation of the chromophore. Although solid-state
quenching is often observed in fluorescent polymer materials
including polyfluorene-based films,17–20,25 the bulky molecular
scaffold of 5 was effective in suppressing a large decrease in
QY during the polycondensation. The highly emissive PMO
film has potential for various luminescence applications
including white-LEDs and fluorescence sensing owing to its
unique light-harvesting properties.
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In conclusion, a novel spirobifluorene-bridged allylsilane
precursor, which could be purified by silica gel chromatography,
was successfully synthesized via a recently developed new
route using MBAS. Acidic sol–gel polycondensation of the
allylsilane precursors yielded mesostructured organosilica
films. The spirocarbon was effective in forming a structurally
stable organosilica framework and exhibiting efficient
fluorescence. These results demonstrate the suitability of MBAS
for use in designing various functional organosilane precursors
intended for the preparation of highly functional PMOs.
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