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Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
Oriented Nanocrystals for Host Materials
gen phthalate (KAP, 100 g dm¹3, 0.49 M, Kanto Chemical, 99.0%)
were prepared in purified water at room temperature. PAA
(concentration range; CPAA = 0 and 15-30 g dm¹3, 35 wt % aque-
ous solution, molecular weight 250000, Aldrich Chemical) was
then added to the stock solution. After these materials were
dissolved, 10 cm3 of the precursor solution was poured into a
polypropylene sample bottle (30 mm in diameter and 70 mm in
height) and the sample bottle was maintained at 25 °C for several
days without sealing. This sample was used for the subsequent
introduction of organic molecules.
Characterization. The morphology and nanoscopic structures
were analyzed by using a field emission scanning electron
microscope (FESEM, FEI Sirion) operated at 2.0 kV, a trans-
mission electron microscope (TEM, FEI Tecnai G2-12) operated at
120 kV, a field emission TEM (FETEM, FEI Tecnai F20) operated
at 200 kV, and powder X-ray diffraction (XRD, Bruker D8
Advance with Cu K¡ radiation). Dark-field (DF) and high-angle-
annular dark-field (HAADF) detectors were used in scanning
transmission electron microscopy (STEM). The powdered samples
were analyzed without any conductive treatment for FESEM and
FETEM analysis.
Inclusion of Guest Molecules. Azobenzene (AB, 10 mM,
Tokyo Kasei Kogyo) and coumarin (CM, 10 mM, Kanto Chemical,
98.0%) were dissolved in ethanol and then 20 mL of the ethanol
solution was poured into a sample bottle with the resultant
composite materials. The sample bottle was sealed and put in an
ultrasonic bath for 30 min. Then, the composite was immersed in
AB or CM ethanol solution at 25 °C for a day. The samples were
adequately washed by ethanol and then dried at room temperature.
The powdered samples were filled in a specimen holder to measure
the absorption spectra in ultraviolet-visible region by diffuse
reflectance mode (UV-vis, JASCO V-560). The irradiation part of
the holder consisted of a circular quartz glass window (16 mm in
diameter). The photochemical reaction was carried out by using
UV lights at 365 nm (6 W) and 254 nm (6 W) and visible light
(Olympus, LG-CL2). The specimen holder was placed near the
light source for the reaction and then the UV-vis spectra were
immediately recorded after irradiation.
oriented structures with the association of organic polymers, as
represented in Figure 1g. Therefore, the nanoscopic interspace,
as a host for organic molecules, is generated from the
nanocrystals and polymers (Figure 1h). The formation of a
mesocrystal is generally ascribed to the self-assembly of
modulated particles.4,5 Our results suggest that the bridges
lead to the similar structures through stepwise crystal growth,
including inhibition and restarting.6,7
Photoisomerization of Azobenzene. Figure 2 summarizes
the photoisomerization behavior of AB molecules in the
nanocrystalline mosaics of K2SO4/PAA and KAP/PAA. AB
molecules are never intercalated in the single crystals of K2SO4
and KAP without PAA. It is generally known that the
absorption bands around 325 and 435 nm correspond to ³-
³* and n-³* transitions of N=N in AB molecules, respec-
tively.19 The trans-to-cis transformation results in a remarkable
decrease and a slight increase in the absorption bands around
325 and 435 nm, respectively. In this study, the two peak
positions are different in the K2SO4/PAA- and KAP/PAA-
nanocrystalline mosaics (Figures 2a and 2c): the absorption
bands were located at 326 and 438 nm in the former composite
and at 328 and 443 nm in the latter case. When commercial AB
powder was mechanically mixed with fumed silica powder in a
certain ratio, the absorption bands appeared at 325 and 423 nm
(Figure S1 in the Supporting Information). In addition, the two
bands are observed at 318 and 442 nm in an ethanol solution.
These facts suggest that AB molecules are neither simply
attached nor mixed with the solid surfaces but rather they are
included in the interspaces of the composite structures around a
certain chemical environment.
The trans isomers of AB molecules are changed to cis after
irradiation with UV light at 365 nm, and then cis-to-trans
transition conversely takes place with the subsequent irradi-
ation with visible light (Figures 2a and 2c). The photoisomer-
ization reactions are achieved within 5 min of the irradiation of
light, and further irradiation does not result in spectral changes.
Furthermore, we demonstrated the reversible switching be-
tween trans and cis forms with the alternative irradiation of
UV and visible lights (Figures 2b and 2d). In this way, the
photoisomerization reaction is achieved in the nanoscopic
interspace of K2SO4/PAA and KAP/PAA.
Photodimerization of Coumarin. The photodimerization
of CM was also performed in the K2SO4/PAA and KAP/PAA
composites (Figure 3). The absorption bands of the included
CM are observed at 277 and 312 nm in the K2SO4/PAA
(Figure 3a), while the samples without PAA do not show any
peaks of CM molecules. Although the KAP crystal itself shows
absorption shorter than 310 nm, the shoulder resulting from
CM is recognized at approximately 320 nm (Figure 3c). As a
reference, the two peak positions are located at 281 and 315 nm
when commercial CM powder is simply mixed with a specified
amount of fumed silica (Figure S2 in the Supporting Informa-
tion). In another case, CM molecules in an ethanol solution
show the two absorption bands around 274 and 313 nm. The
differences in the peak positions indicate that CM molecules
are not simply attached but rather incorporated in the host
K2SO4/PAA and KAP/PAA. The photodimerization behavior
can be monitored by the absorbance around 315 nm. When UV
light at 365 nm is irradiated to the specimen, a gradual decrease
Results and Discussion
Structure of the Nanocrystalline Mosaics.
Figure 1
represents an overview of the mosaic architectures consisting of
oriented K2SO4 and KAP nanocrystals with the incorporation
of PAA. The aggregates of ca. 20 nm nanoparticles are
observed in the FESEM images (Figures 1a and 1d). The
spotted SAED pattern indicates that K2SO4 nanocrystals are
organized with a specific crystal orientation (Figure 1b). Since
peak broadening caused by the crystallite size is not observed
in the XRD pattern, the nanocrystals of KAP also make up the
oriented architecture (Figure 1e).27 The HAADF- and DF-
STEM images reveal that the nanoparticles are not isolated but
are connected to each other with the bridges (Figures 1c and
1f): the white regions indicate the presence of nanoparticles
and polymers. The oriented architecture is regarded as neither
a random aggregate nor a perfect single crystal. Since the
nanocrystals are partially bridged and fused with each other
(Figures 1c and 1f), the mosaic structures would behave as a
single crystalline structure in SAED and XRD patterns. The
formation of these nanoscopic structures is not influenced by a
change in the initial PAA concentration. The results suggest
that the bridged nanocrystals direct the formation of the
Oaki, Yuya
