Organic-inorganic composites comprised of ordered stacks of amphiphilic molecular disks

Full text of DOI:10.1021/ja004034i

2438
J. Am. Chem. Soc. 2001, 123, 2438-2439
Organic-Inorganic Composites Comprised of
Ordered Stacks of Amphiphilic Molecular Disks
Mutsumi Kimura,*,† Kazumi Wada,† Kazuchika Ohta,†
Kenji Hanabusa,† Hirofusa Shirai,*,† and Nagao Kobayashi‡
Department of Functional Polymer Science
Faculty of Textile Science and Technology
Shinshu UniVersity, Ueda 386-8567, Japan
Department of Chemistry, Graduate School of Science
Tohoku UniVersity, Sendai 980-8587, Japan
Figure 1. Absorption spectra of CuPc(C10EO3)8 in CH2Cl2 and CH2Cl2-
ethanol of ratio of 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, and 9:1. [CuPc-
(C10EO3)8] ) 11.2 µM.
ReceiVed NoVember 21, 2000
solvents and the heating of these ordered stacks leads to
dissociation into single building units. Herein we describe the
preservation of columnar morphology by the deposition of
inorganic walls. Furthermore, wrapping by inorganic wall may
result in the isolation of a single column consisting of one-
dimensional ordered stacking of numerous molecular disks.
Since the discovery of mesoporous MCM-41 having a hex-
agonally ordered structure,1 manipulation of inorganic superstruc-
tures under a template effect of organic supramolecular aggregates
has been a highly active area of research.2 Sol-gel polymerization
of inorganic precursors at the surface of aggregates (templates)
allowed the creation of highly ordered organic-inorganic com-
posites by exploiting noncovalent interactions such as electrostatic,
hydrogen-bonding, and van der Waals interaction. The deposition
of inorganic walls around organic aggregates also preserved the
morphologies of flexible organic supramolecular structures.3
π-Conjugated disklike molecules such as triphenylene and
metallophthalocyanine are attractive building units for the forma-
tion of highly ordered columnar stacks.4 These ordered stacks
might enable an efficient electron or energy transport parallel to
the columnar axis. To obtain ordered stacks of disklike molecules,
a number of chemical and physical techniques involving liquid
crystal, vacuum deposition, and Langmuir-Blodgett film transfer
have been investigated.5 Nolte et al. reported the spontaneous
formation of ordered stacks of phthalocyanine derivatives through
three noncovalent interactions: core-core interaction, van der
Waals interaction among aliphatic side chains, and the sandwich-
type complex formation between crown ether voids and K+ ion.6
Recently, we reported the formation of fibrous assemblies made
of amphiphilic metallophthalocyanines stabilized by hydrogen
bonding among hydroxy groups.7 However, the dissolution in
† Shinshu University.
Novel amphiphilic disklike molecule CuPc(C10EO3)8 was
synthesized according to the conventional method developed for
octaalkoxyphthalocyanines (see Supporting Information). These
compounds contain a central phthalocyanine core, eight decoxy
spacers, and peripheral eight triethylene glycol chains. The
nonionic surfactants and block polymers containing oligo- and
poly(ethylene glycol) headgroups have been widely used as
templates for the preparation of mesoporous silica.8 The introduc-
tion of triethylene glycol chains into the disklike molecules can
induce the assembly of inorganic sources. The synthesized
phthalocyanine derivatives exhibited excellent solubility in polar
and nonpolar solvents except for hydrocarbons. Figure 1 shows
the UV-vis spectra of CuPc(C10EO3)8 in CH2Cl2 and a mixture
of CH2Cl2 and ethanol. UV-vis spectrum in CH2Cl2 had a strong
sharp peak at 677 nm, typical of nonaggregated phthalocyanine.9
When ethanol was admixed, the Q-band was broadened, and the
maximum was blue shifted. These spectral changes can be
ascribed to the formation of phthalocyanine stacks having a
coafacial arrangement. By increasing polarity of solvent, the
‡ Tohoku University.
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10.1021/ja004034i CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/16/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 10, 2001 2439
Figure 3. (a) TEM image of calcinated sample synthesized using CuPc-
(C10EO3)8. (b) Nitrogen adsorption (solid line)-desorption (dotted line)
isotherm and pore size distribution curve for the calcinated sample.
Figure 2. Powder XRD patterns of the organic-inorganic composite
containing CuPc(C10EO3)8 and silica and assigned Miller indices (hkl).
hydrophobic phthalocyanine cores come closer and form a
coafacial one-dimensional stack in polar solvents.
stacks of CuPc(C10EO3)8. Washing with organic solvents (CHCl3
and DMF) scarcely released the incorporated CuPc(C10EO3)8 from
the composite. After hydrolysis of TEOS in acidic media in the
presence of CuPc(C10EO3)8, the stacking of CuPc(C10EO3)8 was
stabilized by the association between peripheral triethylene glycol
chains and silica.
The ordered inorganic structures after removal of CuPc(C10-
EO3)8 by calcination were also investigated by TEM, XRD, and
nitrogen BET adsorption measurements. Calcination of the
composite at 450 °C for 2 h gave a gray powder material. TEM
image of calcinated sample showed a hexagonal array of regularly
sized holes of ∼2.8 nm diameter separated by ca. 1.0 nm-thick
silica walls (Figure 3a). XRD pattern can be indexed to the
hexagonal structure with a ) 3.46 nm, consistent with the TEM
image. The calcinated sample had a BET surface area of 1300
m2/g and a narrow pore-size distribution around 2.35 nm (Figure
3b). These results indicate the formation of highly ordered
mesoporous silica through the preservation of long-range ordering
during the calcination process. The regulated array of holes in
silica provided evidence of the isolation of a single column of
CuPc(C10EO3)8 stacks by the deposition of silica walls.
In conclusion, the present paper demonstrates the creation of
organic-inorganic composites containing one-dimensional or-
dered stacking of molecular disks by liquid crystal template sol-
gel polymerization. The experimental results support the claim
of the creation of highly ordered organic-inorganic composite
material. Work is underway in our laboratory to measure the
electronic conductivity of the single column having ordered
stacking of copper phthalocyanines encapsulated within silica
walls.13
The mesomorphic behavior of CuPc(C10EO3)8 was examined
by temperature-controlled polarizing microscopy, differential
scanning calorimetry (DSC), and powder X-ray diffraction (XRD)
measurements. When the isotropic liquid of CuPc(C10EO3)8 was
cooled slowly, a mosaic texture appeared under the polarizing
microscope, a texture which is characteristic for the hexagonal
columnar mesophase Dh.10 Upon heating, the two phase transitions
were detected by DSC at 136 and 220 °C, corresponding to the
transition from crystalline solid (K) to Dh and the clearing point,
respectively. XRD studies of CuPc(C10EO3)8 gave three Bragg
x
reflections in the ratio 1:1/ 3:1/2, indicating that the mesophase
consists of a two-dimensional hexagonal lattice of columnar stacks
with a lattice constant a ) 4.25 nm (see Supporting Information).
In addition, CuPc(C10EO3)8 exhibited a sharp reflection at 0.34
nm due to the stacking distance between the phthalocyanine cores,
suggesting the presence of long-range periodicity along the
columnar axis.
Sol-gel polymerization of tetraethoxysilane (TEOS) in the
presence of CuPc(C10EO3)8 was carried out according to the
method reported by Stucky et al. (see Supporting Information).11
After stirring of the reaction mixture at 25 °C, green precipitate
was corrected, washed with water, and dried at 50 °C. The product
yield was above 90% on the basis of the initial amounts of CuPc-
(C10EO3)8 and silicon. The sol-gel polymerization process in the
presence of CuPc(C10EO3)8 was monitored by the optical polarized
microscopy of a mixed solution prepared by CuPc(C10EO3)8,
TEOS, and 2.0 M HCl aqueous solution (CuPc(C10EO3)8:TEOS:
H2O ) 1:2:1 (by weight)). This mixture appeared birefringent
under polarized microscope, indicating a formation of hexagonal
phase.12 When the solidified mixture was heated above the
clearing point of CuPc(C10EO3)8 at 220 °C, the optical texture
remained unaltered. The hexagonal mesostructure of the organic-
inorganic composite was also confirmed by XRD (Figure 2). In
the powder XRD pattern of the precipitate, three peaks observed
at 2.52, 2.18, and 1.65 nm can be indexed to a hexagonal structure
with a ) 5.04 nm. Moreover, the reflection at 0.34 nm
corresponding to the stacking distance was also detected. The
increase of the lattice constant compared to that of only CuPc-
(C10EO3)8 suggests the formation of silica wall around ordered
Acknowledgment. This research was partially supported by a Grant-
in-Aid for COE Research “Advanced Fiber/Textile Science and Technol-
ogy” (No. 10CE2003) and Scientific Research (No. 11450366 and No.
12129205) from the Ministry of Education, Science, Sports, and Culture
of Japan.
Supporting Information Available: Synthetic procedures, optical
polarized microscopy pictures, and XRD pattern of CuPc(C10EO3)8.
Synthetic procedure, SEM picture, FT-IR spectra, and TGA curve of silica
composite materials (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
(10) McKeown, N. B. Phthalocyanine Materials Synthesis, Structure and
Function; Cambridge University Press: New York, 1998.
(11) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am.
Chem. Soc. 1998, 120, 6024.
JA004034I
(12) Attard, G. S.; Glyde, J. C.; Go¨ltner, C. G. Nature 1995, 378, 366.
(13) Wu, C.-G.; Bein, T. Science 1994, 264, 1757.