6 Emission Enhancement of Dyes in/on Layered Silicates
(Surface-Fixation Induced Emission (S-FIE))
In general, observation and analysis of molecules on a solid surface is not easy
because they undergo various complicated phenomena such as aggregation [12]. On
using the size-matching effect, the photochemical behavior of adsorbed molecules
becomes simple. As monomeric species can be easily obtained even on solid
surfaces using the size-matching effect, the intrinsic photochemical properties of
monomer molecules on solid surfaces can be observed and studied. In recent years,
studies have shown that the fluorescence intensity is highly enhanced by complex
formation in layered silicates in general. One of the early examples of this effect is
the fluorescence enhancement of methyl viologen on clay reported by C. Detellier
et al. [47, 48]. While methyl viologen is not emissive in aqueous solution, it becomes
emissive when adsorbed on the clay surface. Although the fluorescence suffers
concentration quenching as the adsorption density increases, it is still enhanced by
more than hundred times. After this discovery, the size-matching effect was
established, and since then many examples of the fluorescence enhancement effect
in layered silicates have been found. In Fig. 11, the fluorescence enhancement
behavior of 1,3,5-tris[(N-pyridinium)aniline-4-yl]-benzene (TPAB) by complex formation with saponite is shown [49]. The fluorescence quantum yield (ϕ f ) increased
from 0.077 to 0.42 on complexation with clay. In this case, the radiative rate constant
(k fl ) increased from 0.081 Â 10
9 s
À1 to 0.13 Â 10
9 s
À1 , and the nonradiative rate
constant (k ic ) decreased from 0.97 Â 10
9 s
À1 to 0.19 Â 10
9 s
À1 , respectively. In this
case, the effects of saponite on both k fl and k ic enhance the fluorescence intensity.
Such fluorescence enhancement behavior was named “surface-fixation induced
emission (S-FIE)” [46]. Nowadays, another type of emission behavior named
“aggregation-induced emission (AIE)” receives a lot of attention from the viewpoint
of biological and chemical probes and optoelectronic systems [50–52]. In the case of
AIE, the suppression of vibrational motion leading to the radiationless dissipation of
the exciton energy in an aggregation state could play an important role in emission
enhancement. Although S-FIE has some similarities to AIE in terms of the actual
phenomenon and mechanism, each process possesses different advantages. In the
case of S-FIE, the photophysical observation and analysis is easy, because the
sample solution is transparent and the photochemical behavior is simple. It should
be noted that the fluorescence decay curve can be analyzed by single exponential
Fig. 10 (continued) ([Sb
V
(DPyP) (OH) 2 ]
1+ ), and dihydroxo[5,10,15,20-tetra(4-pyridyl)
porphyrinato]antimony(V) chloride ([Sb
V (TPyP) (OH) 2 ]
1+ ). Bottom: The absorption spectra of
[Sb
V (TPP)(OH) 2 ]
1+ in solution and on saponite. The absorption coefficient (log ɛ) is shown in
parentheses. Reprinted with permission from J. Phys. Chem. A, 2013, 117, 7823–7832 with a slight
modification. Copyright (2013) American Chemical Society. Reproduced with permission from
The Royal Society of Chemistry with a slight modification [41]
196
Y. Ishida and S. Takagi
(Surface-Fixation Induced Emission (S-FIE))
In general, observation and analysis of molecules on a solid surface is not easy
because they undergo various complicated phenomena such as aggregation [12]. On
using the size-matching effect, the photochemical behavior of adsorbed molecules
becomes simple. As monomeric species can be easily obtained even on solid
surfaces using the size-matching effect, the intrinsic photochemical properties of
monomer molecules on solid surfaces can be observed and studied. In recent years,
studies have shown that the fluorescence intensity is highly enhanced by complex
formation in layered silicates in general. One of the early examples of this effect is
the fluorescence enhancement of methyl viologen on clay reported by C. Detellier
et al. [47, 48]. While methyl viologen is not emissive in aqueous solution, it becomes
emissive when adsorbed on the clay surface. Although the fluorescence suffers
concentration quenching as the adsorption density increases, it is still enhanced by
more than hundred times. After this discovery, the size-matching effect was
established, and since then many examples of the fluorescence enhancement effect
in layered silicates have been found. In Fig. 11, the fluorescence enhancement
behavior of 1,3,5-tris[(N-pyridinium)aniline-4-yl]-benzene (TPAB) by complex formation with saponite is shown [49]. The fluorescence quantum yield (ϕ f ) increased
from 0.077 to 0.42 on complexation with clay. In this case, the radiative rate constant
(k fl ) increased from 0.081 Â 10
9 s
À1 to 0.13 Â 10
9 s
À1 , and the nonradiative rate
constant (k ic ) decreased from 0.97 Â 10
9 s
À1 to 0.19 Â 10
9 s
À1 , respectively. In this
case, the effects of saponite on both k fl and k ic enhance the fluorescence intensity.
Such fluorescence enhancement behavior was named “surface-fixation induced
emission (S-FIE)” [46]. Nowadays, another type of emission behavior named
“aggregation-induced emission (AIE)” receives a lot of attention from the viewpoint
of biological and chemical probes and optoelectronic systems [50–52]. In the case of
AIE, the suppression of vibrational motion leading to the radiationless dissipation of
the exciton energy in an aggregation state could play an important role in emission
enhancement. Although S-FIE has some similarities to AIE in terms of the actual
phenomenon and mechanism, each process possesses different advantages. In the
case of S-FIE, the photophysical observation and analysis is easy, because the
sample solution is transparent and the photochemical behavior is simple. It should
be noted that the fluorescence decay curve can be analyzed by single exponential
Fig. 10 (continued) ([Sb
V
(DPyP) (OH) 2 ]
1+ ), and dihydroxo[5,10,15,20-tetra(4-pyridyl)
porphyrinato]antimony(V) chloride ([Sb
V (TPyP) (OH) 2 ]
1+ ). Bottom: The absorption spectra of
[Sb
V (TPP)(OH) 2 ]
1+ in solution and on saponite. The absorption coefficient (log ɛ) is shown in
parentheses. Reprinted with permission from J. Phys. Chem. A, 2013, 117, 7823–7832 with a slight
modification. Copyright (2013) American Chemical Society. Reproduced with permission from
The Royal Society of Chemistry with a slight modification [41]
196
Y. Ishida and S. Takagi