Hole-transporting periodic mesostructured organosilica

Article abstract of DOI:10.1021/ja9050263

(Figure Presented) Hole-transporting framework is formed by surfactant-templated sol-gel polycondensation of an electroactive phenylenevinylene-based organosilane precursor. Molecular geometry of the three-armed precursor contributes to both formation of

Full text of DOI:10.1021/ja9050263

Published on Web 09/22/2009
Hole-Transporting Periodic Mesostructured Organosilica
Norihiro Mizoshita, Masamichi Ikai, Takao Tani, and Shinji Inagaki*
Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan, and Core Research for EVolutional Science
and Technology (CREST), Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan
Received June 18, 2009; E-mail: inagaki@mosk.tytlabs.co.jp
Periodic mesoporous organosilicas (PMOs)1-3 are attracting
increasing attention due to their highly functional framework
structures. Various organic bridging groups (R) ranging from
aliphatic and aromatic hydrocarbons to heterocyclics and metal
complexes can be densely and covalently embedded within the
framework of mesoporous materials by surfactant-templated
polycondensation of well-designed organosilane precursors
(R[Si(OR′)]n, n g 2).2,3 Since PMOs possess large surface areas,
designable hybrid frameworks, and mesoscale periodic porous
structures such as channel arrays and 3D cages, they have great
potential for numerous applications, including optical4 and
catalytic systems.5 A number of PMOs with aromatic and
π-conjugated bridging groups have been developed,3 and these
PMOs have been shown to have useful optical properties, such
as strong light absorption, fluorescence, and light harvesting and
excitation energy transfer.3,4 However, there has been no report
on charge conduction in the frameworks of PMOs although other
approaches to the development of charge-conductive porous
materials have been proposed using inorganic semiconductors
and metals.6
achieve both the formation of periodic mesostructures and charge
transport in the organosilica pore walls.
Figure 1. Preparation of mesostructured phenylenevinylene-silica hybrids
from three-armed precursor 1.
In the present paper, we first report long-range hole transport in
the framework of mesostructured organosilica. For the preparation
of mesostructured materials comprising a hole-transporting orga-
nosilica framework (Figure 1), we designed a three-armed phe-
nylenevinylene compound with three triethoxysilyl groups (1), based
on the strategies of (i) increasing the interaction between the
precursor and the surfactant micelles; (ii) efficient formation of
siloxane cross-links, resulting in stabilization of periodic mesos-
tructured frameworks; and (iii) induction of charge hopping between
organic bridges due to dense accumulation of the two-dimensionally
expanded π-system directly connected to the silica backbone. In
the present study, mesostructured and electroactive organosilica
hybrid films were prepared on quartz substrates by evaporation-
induced self-assembly of nonionic template surfactant Brij76
(C18H37(OCH2CH2)10OH) and 1 without other pure silica precursors.
The hole-transporting behavior in the mesostructured organosilica
framework was examined by time-of-flight (TOF) measurements.
Organosilane precursor 1 was easily and efficiently synthesized
by similar synthetic procedures to those used for rod-like OPV
precursors.3i A three-armed aromatic iodide, 1,3,5-tris(4-iodostyryl)-
benzene, was obtained from 1,3,5-tris(bromomethyl)benzene and
4-iodobenzaldehyde in two steps (81% yield). Rh-catalyzed sily-
lation of 1,3,5-tris(4-iodostyryl)benzene afforded organosilane
precursor 1 in 89% yield.
Mesostructured organosilica films were successfully obtained by
acidic sol-gel polycondensation of 1 in the presence of template
surfactant Brij76. Evaporation of solvents (THF and ethanol) from
sol mixtures containing 1, Brij76, and a catalytic amount of
hydrochloric acid yielded transparent organosilica films (denoted
as 1-Brij76-F). Figure 2 shows the X-ray diffraction pattern of the
organosilica film (solid line). The film showed an intense diffraction
peak at d ) 5.26 nm corresponding to the formation of a periodic
mesostructure. Weak diffraction peaks were also observed at d )
3.05, 2.63, 2.01, and 1.72 nm. The reciprocals of the d-spacing
The goal of the present study is to achieve charge transport in
the pore walls of mesostructured organosilica hybrids with the aid
of electroactive organic bridges. Some nonstructured organosilica
hybrids have been demonstrated to exhibit charge-transporting
properties even in the presence of siloxane networks,7 indicating
the possibility of realizing charge-transporting PMOs. Periodic
mesoporous organosilicas with charge-transporting frameworks may
lead to the development of a new class of photovoltaic materials
and photocatalytic systems involving electron-transfer processes.
In particular, highly efficient electronic devices may be fabricated
if efficient generation and conduction of charge carriers can be
achieved in PMOs by forming p-n junctions with large surface
areas at the framework-mesopore interface.8
One of the challenges in realizing charge-transporting PMOs is the
formation of well-defined mesostructures from 100% electroactive
organosilane precursors with large π-conjugates. Generally, synthesiz-
ing PMOs from 100% organosilane precursors containing bulky
organic groups is difficult. This is attributable to the weak surfactant-
precursor interactions resulting from the low density of polar silanol
groups separated by bulky organic groups. Recently, we reported the
synthesis of visible-light-absorptive mesostructured films from bulky
oligo(phenylenevinylene) (OPV, -C6H4-C2H2-C6H2R′′2-C2H2-
C6H4-) rod-like organosilane precursors.3i However, a large amount
of pure silica precursor (tetraethoxysilane, >50 wt %) was required
to be mixed with the OPV precursors to obtain ordered mesostruc-
tured OPV films. The small ratio of the hydrophilic silyl groups to
the hydrophobic OPV moieties would suppress interactions between
hydrolyzed OPV precursors and surfactant micelles, resulting in
the formation of nonordered OPV materials from 100% OPV
precursors. Proper molecular design of precursors is required to
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values are in the ratio of 1:ꢀ3:2:ꢀ7:3, which is typical of a
hexagonal arrangement of mesochannels. After extraction of the
template,9 the periodic mesostructure of the organosilica hybrid was
retained, although X-ray diffraction peaks were broadened and the
main d-spacing value decreased to 4.91 nm (Figure 2, broken line).
For the samples both before and after extraction of Brij76, no sharp
peaks in X-ray diffraction were observed at a wide angle region
(2θ > 10°), indicating the absence of molecular-scale periodicity
in the framework. Note that mesostructured organosilica films were
obtained from 100% organosilane precursor 1 without adding other
alkoxysilane precursors. The three-armed geometry of the precursor
is effective for enhancing the interaction with the surfactant micelles
and the formation of three-dimensional siloxane cross-links.
Although direct observation of the mesoscale morphologies of
1-Brij76-F by electron microscopy was difficult due to the collapse
of the mesostructure by electron beam irradiation, the films likely
possess mesochannel arrays that are similar to a hexagonal packing.
Figure 3. (a) Double logarithmic plots of transient photocurrent of
1-Brij76-F (thickness: 5 µm) as a function of time at applied voltages of
150 and 350 V. (b) Plot of the hole mobility of 1-Brij76-F versus electric
field strength.
cm-1. The dependence of the hole mobility on the electric field strength
was not clearly observed in the present range of measurements (Figure
3b). On the other hand, the hole mobilities in amorphous films of
poly(p-phenylenevinylene) polymers10 have been reported to be on
the order of 10-5 cm2 V-1 s-1. The hole-transporting properties of the
mesostructured phenylenevinylene-silica hybrid are comparable to
those of π-conjugated organic amorphous polymers. The hole-
transporting behavior of a nonstructured homopolymer film prepared
from 1 (denoted as 1-F) and neat precursor 1 (polycrystalline solid)
was measured as a reference sample. Nonstructured organosilica film
1-F generated photocurrents, and the hole mobility was determined to
be ∼5.1 × 10-5 cm2 V-1 s-1, which is slightly higher than that of
1-Brij76-F. This result indicates that hole conduction in the dense
phenylenevinylene-silica network formed by polycondensation of 1
alone is not strongly hindered by the introduction of mesostructures.
No photocurrent was observed for neat precursor 1. Hole trapping in
grain boundaries of the crystalline solids or poor contact between the
sample and electrodes can persist in the polycrystalline material.
Molecular packing of uncondensed 1 in the crystal lattice may also
impede hole hopping between organic moieties.
Spectroscopic measurements of the organosilica films indicate
that the three-armed phenylenevinylene bridges are densely
embedded within the organosilica framework. Figure 4 shows
UV-vis and fluorescence spectra of 1-Brij76-F, 1-F, neat
precursor 1 (polycrystalline solid), and a dilute 2-propanol
solution of 1. Compared to the solution of 1, organosilica films
1-Brij76-F and 1-F showed slight blue shifts of the absorption
bands from 319 to 312 nm, suggesting partial π-stacking of
phenylenevinylene bridges in the frameworks. The fluorescence
spectra of the films showed red-shifted broad emissions that are
attributable to the excimer band at 423 nm. These results indi-
cate that phenylenevinylene bridges fixed within the frameworks
are densely packed and interactive in the excited state. The
arrangement of the phenylenevinylene units, which results from
Figure 2. X-ray diffraction patterns of 1-Brij76-F before (solid line) and
after (broken line) extraction of Brij76.
The porosity of the extracted sample was examined by nitrogen
adsorption-desorption isotherm measurements (see Supporting
Information). The isotherm was classified as type I (micropore)
rather than type IV (mesopore), likely due to the shrinkage of the
periodic mesopores indicated by the X-ray diffraction measure-
ments. However, the DFT pore diameter was calculated to be 2.4
nm, indicating small mesopores. The Brunauer-Emmett-Teller
(BET) surface area and pore volume were 941 m2 g-1 and 0.41
cm3 g-1, respectively. The large surface area is thought to result
from the formation of a surfactant-templated porous structure.
Assuming a hexagonal channel array in the extracted material, the
thickness of the pore wall is estimated to be ∼3.3 nm, suggesting
that the pore wall consists of three or four layers of the nonaligned
phenylenevinylene-silica repeating unit. The organosilica hybrid
prepared from 1 is available as a porous host material made of the
photo- and electroactive phenylenevinylene-silica framework with
a large surface area.
The hole-transporting behavior of the mesostructured film
1-Brij76-F was examined by TOF measurements. Under application
of DC electric fields, photocurrent generation and dispersive charge
transport were observed for 1-Brij76-F upon laser pulse irradiation
(λ ) 337 nm) on the side of the positive electrode of the cells (Figure
3a), indicating conduction of photogenerated holes within the orga-
nosilica framework of the mesostructured 1-Brij76-F film. Figure 3b
shows the hole mobilities calculated using the transit times of the charge
carriers as determined from double logarithmic plots of the transient
photocurrent as a function of time. The hole mobility in 1-Brij76-F
was ∼3.2 × 10-5 cm2 V-1 s-1 at an electric field strength of 0.6 MV
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C O M M U N I C A T I O N S
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between the organic bridging groups, leading to photocurrent
generation observed using the TOF method. In contrast, an
absorption band of the polycrystalline solid of 1 exhibited a red
shift in the UV-vis spectrum, and the fluorescence spectrum
was similar to that of the 2-propanol solution of 1. In the crystal
consisting of the uncondensed precursor, the π-conjugation
length of the precursor molecule may be extended due to the
increased planarity, whereas intermolecular interactions and
parallel π-stacking appear to be suppressed in the crystal lattice,
which may be the reason for the lack of charge conduction.
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In conclusion, mesostructured organosilica hybrids with elec-
troactive phenylenevinylene bridges were successfully obtained by
surfactant-templated polycondensation of a three-armed organosi-
lane precursor. Hole transport in the pore walls of the mesostruc-
tured organosilica was achieved, and the hole mobilities were on
the order of 10-5 cm2 V-1 s-1. The present results broaden the
potential application of mesostructured organosilica materials in
novel photocatalytic and photovoltaic systems, where the meso-
porous structure with large surface and interfacial areas is greatly
advantageous to promote the transfer of mass, charge, and energy
in the mesopores or at the framework-mesopore interface. In the
future, we hope to increase the hole mobilities in the pore walls by
enhancing the molecular-scale ordering of the densely embedded
π-conjugated bridges, because well-stacked OPV molecules are
reported to exhibit quite high carrier mobilities on the order of 10-1
cm2 V-1 s-1 in organic field effect transistors.11
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Supporting Information Available: Complete ref 10a, experimental
details, and additional data on infrared spectra, nitrogen adsorption-
desorption isotherm, transient photocurrents, X-ray diffraction, solid
state NMR, and TGA analysis. This material is available free of charge
via the Internet at http://pubs.acs.org.
(9) Extraction of the template was performed after exposure to vapor of
ammonia aqueous solution. See Supporting Information.
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