Mesostructured organosilica with a 9-mesityl-10-methylacridinium bridging unit: Photoinduced charge separation in the organosilica framework

Article abstract of DOI:10.1039/c0cc03354e

Polycondensation of 9-mesityl-10-methylacridinium-bridged organosilane in the presence of a nonionic surfactant yielded a mesostructured organosilica solid with a functional framework that exhibited long-lived photoinduced charge separation.

Full text of DOI:10.1039/c0cc03354e

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Mesostructured organosilica with a 9-mesityl-10-methylacridinium
bridging unit: photoinduced charge separation in the organosilica
frameworkw
Norihiro Mizoshita,ab Ken-ichi Yamanaka,ab Toyoshi Shimada,*bc Takao Taniab and
Shinji Inagaki*ab
Received 19th August 2010, Accepted 14th October 2010
DOI: 10.1039/c0cc03354e
Polycondensation of 9-mesityl-10-methylacridinium-bridged
organosilane in the presence of a nonionic surfactant yielded a
mesostructured organosilica solid with a functional framework
that exhibited long-lived photoinduced charge separation.
control of the meso-scale porous structure, which provides a
high surface area,7–9 and precise morphology control.10
Recently, unique and significant optical and electrical properties
have been demonstrated for systems in which p-conjugated
organic bridges are dispersed or densely accumulated within
the pore walls of PMOs, providing efficient photoluminescence,
hole transport within pore walls and light-harvesting antenna
properties.9 Moreover, systems capable of photocatalytic
hydrogen evolution and CO2 reduction have been constructed;
in the case of hydrogen evolution, charge transfer complexes
were included within the PMO framework,9h while in CO2
Photoexcitation of electroactive chromophores can induce
a variety of chemical reactions. Of particular interest for
photocatalysis or energy conversion applications is photo-
induced electron transfer in donor–acceptor systems.1,2
While photoinduced charge separation in charge transfer
complexes consisting of donor and acceptor molecules has
been widely studied, increasing attention has been focused on
covalently-linked donor–acceptor dyads for the construction
of more efficient photoreaction systems.2
Recently, 9-mesityl-10-methylacridinium (Mes–Acr+) salts,
in which the donor (Mes) and acceptor (Acr+) moieties are
directly connected, have been reported to exhibit prominent
photocatalytic behavior and electronic activities in solutions
or polymer matrices.3–6 The mechanism of Mes–Acr+-mediated
photoreaction has been examined intensively4,5 with the
Mes–Acr+ cation reported to serve as a dual sensitizer, with
the capacity for efficient singlet oxygen formation and electron-
transfer reaction.5d In practical terms, immobilization of the
Mes–Acr+ unit in a solid porous framework is important due
to the wide range of potential applications, including as solid-
state photocatalysts and in electrical devices. Furthermore,
dense packing of the Mes–Acr+ unit may also lead to the
induction of unique photochemical and electrical properties
through the specific intermolecular interactions.
reduction,
a rhenium complex was located among the
mesochannels.9i Expansion of the framework organics to
include photochemically and electronically active species is
thus of great importance for PMO-based optical, photo-
catalytic and electronic applications.
In this communication, we report on the preparation of
mesostructured organosilica materials consisting of
a
9-mesityl-10-methylacridinium (Mes–Acr+)–silica hybrid
framework, and describe their photoinduced charge separation
behavior. The mesostructured materials were successfully
obtained from the newly designed precursor 1 (Scheme 1) without
dilution of the scaffold silica sources (e.g., tetraethoxysilane),
Periodic mesoporous organosilicas (PMOs) prepared by
surfactant-directed self-assembly of bridged organosilane
precursors ((R0O)3Si–R–Si(OR0)3, R: organic bridging group)
are promising candidates as solid-state functional materials
because their function can be well tailored by appropriate
selection of organic bridges (–R–) within the framework,
a 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
b Core Research for Evolutional Science and Technology (CREST),
Japan Science and Technology Agency (JST), Kawaguchi,
Saitama 332-0012, Japan
c Department of Chemical Engineering,
Scheme 1 Synthetic route for preparation of Mes–Acr+ organosilane
precursor 1 and mesostructured organosilica. (a) NaH, MOMCl,
DMF; (b) MesMgBr, THF; (c) HCl aq.; (d) HSi(OEt)3, NEt3,
n-Bu4NI, [Rh(cod)(CH3CN)2]BF4, DMF; (e) MeOTf, CH2Cl2
(X = OTf); (f) Brij76, HCl aq., EtOH.
Nara National College of Technology, Yamatokoriyama,
Nara 639-1080, Japan. E-mail: shimada@chem.nara-k.ac.jp;
Fax: +81 743-55-6154; Tel: +81 743-55-6154
w Electronic supplementary information (ESI) available: Experimental
details, supplementary spectroscopic data and characterization of the
mesostructured organosilica materials. See DOI: 10.1039/c0cc03354e
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 9235–9237 9235

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which resulted in an organosilica framework with a high
density of Mes–Acr+ units. Time-resolved absorption spectro-
scopy of the Mes–Acr+–silica hybrid films suggested that the
photoinduced charge separation state can be retained over
a microsecond time scale through ‘intermolecular’ charge
migration in the framework.
The Mes–Acr+-bridged precursor 1 was prepared starting
from commercially available 2,7-dibromoacridone 2 in 74%
overall yield (five steps) (Scheme 1). Protection of 2 with
methoxymethyl chloride (MOMCl) followed by treatment
with 2-mesitylmagnesium bromide and deprotection by
hydrochloric acid afforded the desired 2,7-dibromo-9-
mesitylacridine 4 in 75% yield (three steps). The resulting
dibromide 4 was successfully transformed to bis(triethoxysilyl)-
mesitylacridine 5 in 98% yield according to a previously
reported method, giving 3,6-bis(triethoxysilyl)carbazole.11
For subsequent methylation of 5, screening of several methylating
reagents (MeI, MeOTf and Me3OBF4) and solvents (PhMe,
n-C6H14, MeCN, Et2O and CH2Cl2) identified MeOTf
and CH2Cl2 as the optimal methylating reagent and solvent,
Fig. 1 XRD patterns of the organosilica film prepared from 1 and
Brij76 before (solid line) and after extraction (broken line) of the
template surfactant.
respectively, to give Mes–Acr+-bridged precursor
1 in
quantitative yield.
Mesostructured Mes–Acr+–silica hybrid materials were
successfully obtained by evaporation-induced self-assembly
from an acidic sol solution containing 1 and the template
nonionic surfactant Brij76 (C18H37(OCH2CH2)10OH) (molar
ratio of 1/Brij76/EtOH/H2O/HCl = 1.0/0.74/855/8.7/0.052).
Fig. 1 shows an X-ray diffraction (XRD) pattern of the cast
film prepared at room temperature. An intense peak corres-
ponding to d = 6.50 nm and weak diffraction peaks of
d = 3.81, 3.32, 2.50, 2.30 nm are observed, which can be
indexed as the (100), (110), (200), (210) and (300) planes of a
Fig.
2
(a) Transient absorption spectra of the mesostructured
organosilica film prepared from 1 after laser excitation at 355 nm.
(b) Time profiles of absorbance decay at 380 and 480 nm. (c) Time
profile of the reciprocal of the absorbance decay at 380 nm. Inset of
(a) shows transient absorption spectra of a transparent polycondensed
film of 1 without surfactant.
2D hexagonal mesostructure.z It is noteworthy that
a
mesostructured film containing a relatively large organic
bridge was successfully prepared from 100% organosilane
precursor 1 without the addition of pure silica precursors.
The mesostructure formation is expected to occur by the
interaction of the hydrophilic poly(ethylene oxide) chain of
Brij76 with the polar ionic organic bridge and the hydrophilic
silanol groups of the hydrolyzed 1. The periodic mesostructure
was maintained after extraction of the template surfactant by
immersing the organosilica hybrid film in ethanol (50 1C, 12 h).
After extraction, the XRD peak corresponding to meso-scale
periodicity was broadened, but its d-spacing value was almost the
same as that for the surfactant-templated film (Fig. 1).
absorption band at 380 nm is characteristic of Acrꢀ, which is
usually observed at 370 nm for Mes–Acr+ ions in acetonitrile.5
The present results indicate that photoinduced charge separation
+
of Acrꢀ and its counterpart Mesꢀ occurs in the organosilica
framework over a microsecond time scale. The half-life as
estimated from the absorbance decay at 380 nm is 3.5 ms
(Fig. 2b). For a Mes–Acr+ ion in benzonitrile solution, the
transient absorption spectrum was reported to exhibit decay of
microsecond3 or nanosecond order5b with first-order kinetics
consistent with an intramolecular process.
In order to examine the photochemical functionality of the
organosilica hybrid framework, transient absorption spectra
were measured for the mesostructured Mes–Acr+–silica hybrid
film prepared at a thickness of ca. 4 mm, for which sufficient
signal intensity was expected to be obtained. The transient
spectra, obtained by laser irradiation (l = 355 nm, 7 ns pulse),
showed absorption bands at ca. 380 nm and 420–550 nm
(Fig. 2a), although the spectral shapes were somewhat disturbed,
possibly due to a haze effect in the thick sample. These peaks
were observed over a microsecond time scale as shown in Fig. 2a
and b. The transient absorption band at 420–550 nm can be
attributed to one or more of various chemical species such as the
acridinyl radical (Acrꢀ), mesityl radical cation (Mesꢀ+) and
triplet state of Mes–Acr+.5 On the other hand, the sharp
The signal intensities of the organosilica film at 380 and
480 nm exhibited similar decay time profiles (Fig. 2b), which
+
suggests that the Mesꢀ and Acrꢀ species underwent charge
recombination. The decay curves of the change in absorbance,
DAbs, at 380 and 480 nm were not well-fitted by single
exponential functions, in contrast with those for a Mes–Acr+
solution, indicating that the mechanism for intramolecular
charge recombination is not as straightforward in the present
case. However, plotting the inverted DAbs values over time gives
a linear relation as shown in Fig. 2c. These results indicate
that the decay process obeys second-order kinetics, i.e.,
1/[Acrꢀ] = kt + 1/[Acrꢀ]0, where [Acrꢀ] is the concentration of
c
9236 Chem. Commun., 2010, 46, 9235–9237
This journal is The Royal Society of Chemistry 2010

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Acrꢀ, supporting a collision model for two diffusive radical
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species with the same concentrations (i.e., [Acrꢀ] = [Mesꢀ+]).
+
Thus it is likely that the two radical species Mesꢀ and Acrꢀ
generated by photoexcitation of the Mes–Acr+ unit separately
migrate in the organosilica framework, resulting in a charge
separation state with a microsecond-order lifetime and subse-
quent charge recombination as a second-order process. Weak
electrostatic attractive forces between the cationic (Mesꢀ+) and
neutral (Acrꢀ) species may also play an important role in the
induction of such long-lived charge separation states in this
condensed matter system. Photoinduced charge separation
behavior was also evaluated for a haze-free non-structured
Mes–Acr+–silica film prepared without surfactants by time-
resolved spectroscopic analysis (Fig. 2a, inset), which showed
clear absorption spectral profiles and similar static and dynamic
optical behavior to that of the mesostructured film (see ESIw).
The formation of mesoporous structure in the
Mes–Acr+–silica has a great merit for application to solid
catalyst because reactant molecules can easily access the
charge-separated sites generated on the high-surface area
organosilica framework. Photocatalytic ability of the mesoporous
Mes–Acr+–silica was evaluated in photooxidation reactions,
which were previously reported in homogeneous Mes–Acr+
systems.4d,e Briefly, after extraction of Brij76, the mesostructured
Mes–Acr+–silica hybrid was ground and dispersed in a
dichloromethane solution of benzyl alcohol or triphenyl-
phosphine. After visible light irradiation (xenon lamp,
>385 nm, 12 h) of the suspensions under O2 bubbling,
benzaldehyde or triphenylphosphine oxide, respectively, was
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1
produced as confirmed by H NMR spectroscopy (see ESIw).
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¨
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had taken place during reaction.
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In conclusion, mesostructured organosilica hybrids with a
Mes–Acr+ bridging unit were prepared from a newly designed
organosilane precursor 1 without dilution. Photoinduced
charge separation within the Mes–Acr+–silica hybrid frame-
work was demonstrated over a microsecond time scale. PMOs
containing a high density of photochemically active organic
bridges have great potential for photovoltaic materials and
recyclable photocatalytic systems.
We gratefully acknowledge Prof. Hiroshi Miyasaka of
Osaka University for helpful discussion and comments.
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Notes and references
z Electron microscopy observation of the mesostructures was difficult
because the organosilica hybrids were rapidly damaged by electron
beam irradiation.
11 Y. Maegawa, Y. Goto, S. Inagaki and T. Shimada, Tetrahedron
Lett., 2006, 47, 6957–6960.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 9235–9237 9237