the produced excited state is thermally activated (ν ≧ 1), thermal relaxation to ν ¼ 0
occurs. According to Kasha’s rule [1], relaxation from S n (n ≧ 2) to S 1 is fast. In
general, the lowest excited state (S 1 (ν ¼ 0)) is the active electronically excited state.
Several processes are possible from the active electronically excited state (S 1
(ν ¼ 0)). These processes include thermal deactivation to the electronically ground
state (S 0 (ν ¼ 0, 1, 2, . . .)), radiative deactivation to the electronically ground state
(S 0 (ν ¼ 0, 1, 2, . . .)), and an intersystem crossing to the triplet state, as shown in
Fig. 1. The rate constants are expressed as k ic , k fl , and k isc , respectively. The rate
constants are governed by the rules in transition probability [2, 3] such as the FranckCondon factor. Each molecule has its own individual rate constants. The quantum
yield (Φ) is an important parameter for these processes. The emission from S 1 is
called fluorescence. In the case of fluorescence quantum yield (Φ f ), Φ f is the ratio m/
n where n is the absorbed photon number and m is the emitted photon number. Φ fl is
expressed as k fl /(k fl + k ic + k isc ). As is the case with the S 1 state, the T 1 state
deactivates through thermal and radiative processes. According to the energy gap
law [4], T 1 state tends to suffer thermal deactivation; thus, to observe emission from
T 1 , a low temperature condition is necessary in general. Emission between states
with the same spin quantum number is called fluorescence, whereas that with a
different spin quantum number is called phosphorescence. In this chapter, the effects
of layered materials on the processes shown in Fig. 1, especially on emission
behavior, are described. Change in the energy state affects the emission wavelength
that is the emission color. On the contrary, change in transition probabilities affects
the emission quantum yield.
2 Layered Silicates
Layered materials [5–9] are host materials with a two-dimensional structure. Clay
minerals, graphene, layered double hydroxide, and so on are typical layered materials. Clay minerals are aluminum phyllosilicates, sometimes with iron, magnesium,
and alkali metals in the structure. In this chapter, we focused on synthetic saponites
because they are commonly used and well characterized. Their characteristic features include (1) nanostructured flat sheets that can be formed by elements with a
high Clarke number, (2) negatively or positively charged surfaces, (3) exfoliation or
stacking ability of individual nanosheets depending on the surrounding conditions,
(4) an interlayer space whose volume can be reversibly changed, and (5) optical
transparency in solution when the particle size is small (<50 nm) and the concentration is not too high. The molecules on the surface can change their structure,
intramolecular vibrational motion, and so on, which could then alter their photochemical properties.
Particularly in the case of artificially synthesized clay, the composition is clear
and purity is quite high. Sumecton SA (Kunimine, Japan) and Laponite (BYK,
Germany) are commercially available clays. The composition of Sumecton SA is
[(Si
4+
7.2 Al
3+
0.8 )(Mg 6 )O 20 (OH) 4 ]
À0.8 0.8Na
+
, and its structure is shown as Fig. 2a.
Tuning Emission Properties by Dye Encapsulation into Layered Silicates
187
occurs. According to Kasha’s rule [1], relaxation from S n (n ≧ 2) to S 1 is fast. In
general, the lowest excited state (S 1 (ν ¼ 0)) is the active electronically excited state.
Several processes are possible from the active electronically excited state (S 1
(ν ¼ 0)). These processes include thermal deactivation to the electronically ground
state (S 0 (ν ¼ 0, 1, 2, . . .)), radiative deactivation to the electronically ground state
(S 0 (ν ¼ 0, 1, 2, . . .)), and an intersystem crossing to the triplet state, as shown in
Fig. 1. The rate constants are expressed as k ic , k fl , and k isc , respectively. The rate
constants are governed by the rules in transition probability [2, 3] such as the FranckCondon factor. Each molecule has its own individual rate constants. The quantum
yield (Φ) is an important parameter for these processes. The emission from S 1 is
called fluorescence. In the case of fluorescence quantum yield (Φ f ), Φ f is the ratio m/
n where n is the absorbed photon number and m is the emitted photon number. Φ fl is
expressed as k fl /(k fl + k ic + k isc ). As is the case with the S 1 state, the T 1 state
deactivates through thermal and radiative processes. According to the energy gap
law [4], T 1 state tends to suffer thermal deactivation; thus, to observe emission from
T 1 , a low temperature condition is necessary in general. Emission between states
with the same spin quantum number is called fluorescence, whereas that with a
different spin quantum number is called phosphorescence. In this chapter, the effects
of layered materials on the processes shown in Fig. 1, especially on emission
behavior, are described. Change in the energy state affects the emission wavelength
that is the emission color. On the contrary, change in transition probabilities affects
the emission quantum yield.
2 Layered Silicates
Layered materials [5–9] are host materials with a two-dimensional structure. Clay
minerals, graphene, layered double hydroxide, and so on are typical layered materials. Clay minerals are aluminum phyllosilicates, sometimes with iron, magnesium,
and alkali metals in the structure. In this chapter, we focused on synthetic saponites
because they are commonly used and well characterized. Their characteristic features include (1) nanostructured flat sheets that can be formed by elements with a
high Clarke number, (2) negatively or positively charged surfaces, (3) exfoliation or
stacking ability of individual nanosheets depending on the surrounding conditions,
(4) an interlayer space whose volume can be reversibly changed, and (5) optical
transparency in solution when the particle size is small (<50 nm) and the concentration is not too high. The molecules on the surface can change their structure,
intramolecular vibrational motion, and so on, which could then alter their photochemical properties.
Particularly in the case of artificially synthesized clay, the composition is clear
and purity is quite high. Sumecton SA (Kunimine, Japan) and Laponite (BYK,
Germany) are commercially available clays. The composition of Sumecton SA is
[(Si
4+
7.2 Al
3+
0.8 )(Mg 6 )O 20 (OH) 4 ]
À0.8 0.8Na
+
, and its structure is shown as Fig. 2a.
Tuning Emission Properties by Dye Encapsulation into Layered Silicates
187