chemical reaction field around the molecule change the photochemical properties of
the molecule such as absorption wavelength, fluorescence wavelength, absorption
coefficient, and fluorescence quantum yield. For example, when ΔE (¼ |E(S 0 ) – E
(S 1 )|) is decreased, the absorption and fluorescence wavelength are shorted, and
fluorescence quantum yield tends to decrease because of the energy gap law [12–
17]. On the other hand, when Δr (¼ | r eq. À r ’eq. |) is decreased, the fluorescence
quantum yield tends to be large, because the overlap of wave functions between (S 1 ,
ν 0 ) and (S 0 , ν 0 ) and that between (S 1 , ν 0 ) and (S 0 , ν high ) become large and small,
respectively.
How do layered silicates as a chemical reaction field affect the photochemical
property of a molecule? The major effects of layered silicates are as follows. (1) The
molecule itself suffers changes in molecular structure and its molecular motion. It is
expected that the molecular structure becomes more planar and its motion is
suppressed on the surface of the layered silicate, because of the flat surface of the
layered silicates at the atomic level. (2) The molecule is in a high local concentration
and tends to form aggregates. Molecular aggregation also affects photochemical
properties because of the interaction between transition dipole moments. The details
of these effects will be described in the next section.
4 Complex of Dyes and Layered Silicates
The dye molecule and layered silicates form complexes through attractive interactions such as electrostatic and hydrophobic interactions. In the case of saponite that
is a typical anionic layered silicate, cationic molecules can be adsorbed on the
surface of layered silicates mainly by the electrostatic interactions. For example, it
is well known that methylene blue (Fig. 5) is easily adsorbed on layered silicates in
an aqueous solution or dispersion [18–22]. Complex formation behavior can be
monitored by UV-vis absorption measurements. The absorption spectra of methylene blue change with the time after mixing each solution [19, 22] as can be seen in
Fig. 5. In this system, there is an interaction between the layered silicate and
methylene blue as well as between methylene blue molecules, leading to the
formation of aggregates. In the case of organic molecules, aggregates are easily
formed on the layered silicate surface in aqueous suspension, mainly because of
hydrophobic interactions. Typical aggregates are H- and J-type aggregates, which
are non-emissive and emissive, respectively [18]. Although an assembly including
H- and J-type aggregates on layered silicates is unique, even for J-aggregates, the
excited lifetime of the molecule on layered silicates tends to be short; therefore,
molecules on layered silicates are less photochemically active in general. Many
researchers have felt that it is difficult to use layered silicates as a platform to
construct photochemical reaction systems and photo-functional materials.
In recent years, techniques to control the assembly of structures on layered
silicates have been developed [7, 23–32]. In the suitable combination of anionic
saponite and tetrakis(N-methylpyridinium-4-yl)porphyrin ( p-TMPyP), aggregation
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the molecule such as absorption wavelength, fluorescence wavelength, absorption
coefficient, and fluorescence quantum yield. For example, when ΔE (¼ |E(S 0 ) – E
(S 1 )|) is decreased, the absorption and fluorescence wavelength are shorted, and
fluorescence quantum yield tends to decrease because of the energy gap law [12–
17]. On the other hand, when Δr (¼ | r eq. À r ’eq. |) is decreased, the fluorescence
quantum yield tends to be large, because the overlap of wave functions between (S 1 ,
ν 0 ) and (S 0 , ν 0 ) and that between (S 1 , ν 0 ) and (S 0 , ν high ) become large and small,
respectively.
How do layered silicates as a chemical reaction field affect the photochemical
property of a molecule? The major effects of layered silicates are as follows. (1) The
molecule itself suffers changes in molecular structure and its molecular motion. It is
expected that the molecular structure becomes more planar and its motion is
suppressed on the surface of the layered silicate, because of the flat surface of the
layered silicates at the atomic level. (2) The molecule is in a high local concentration
and tends to form aggregates. Molecular aggregation also affects photochemical
properties because of the interaction between transition dipole moments. The details
of these effects will be described in the next section.
4 Complex of Dyes and Layered Silicates
The dye molecule and layered silicates form complexes through attractive interactions such as electrostatic and hydrophobic interactions. In the case of saponite that
is a typical anionic layered silicate, cationic molecules can be adsorbed on the
surface of layered silicates mainly by the electrostatic interactions. For example, it
is well known that methylene blue (Fig. 5) is easily adsorbed on layered silicates in
an aqueous solution or dispersion [18–22]. Complex formation behavior can be
monitored by UV-vis absorption measurements. The absorption spectra of methylene blue change with the time after mixing each solution [19, 22] as can be seen in
Fig. 5. In this system, there is an interaction between the layered silicate and
methylene blue as well as between methylene blue molecules, leading to the
formation of aggregates. In the case of organic molecules, aggregates are easily
formed on the layered silicate surface in aqueous suspension, mainly because of
hydrophobic interactions. Typical aggregates are H- and J-type aggregates, which
are non-emissive and emissive, respectively [18]. Although an assembly including
H- and J-type aggregates on layered silicates is unique, even for J-aggregates, the
excited lifetime of the molecule on layered silicates tends to be short; therefore,
molecules on layered silicates are less photochemically active in general. Many
researchers have felt that it is difficult to use layered silicates as a platform to
construct photochemical reaction systems and photo-functional materials.
In recent years, techniques to control the assembly of structures on layered
silicates have been developed [7, 23–32]. In the suitable combination of anionic
saponite and tetrakis(N-methylpyridinium-4-yl)porphyrin ( p-TMPyP), aggregation
190
Y. Ishida and S. Takagi