1 Introduction
Photochemical reactions in heterogeneous systems may differ significantly from
analogous reactions in homogeneous liquids/solutions or gas phases [1–6]. Important
roles of the media/supports to control such parameters as the reaction rates/yields
and product selectivity have been recognized so far, so that various photofunctional
hybrids have been designed by organizing molecular species in/on solid surfaces.
The location (proximity), orientation, association/aggregation, as well as the freedom (rotation and diffusion) of molecules on the surfaces or in the solids vary
depending on the host-guest interactions at the interface to affect the characteristics
and functions. The molecular and supramolecular designs (by the selection of host
and guest and their composition and the additives) have been done using nanospace
materials such as zeolites, mesoporous silicas, MOFs, COFs, and layered materials
for organizing molecular and polymeric photofunctional species, and unique/useful
photofunctions of the resulting hybrids have been reported [7–11]. Materials with
defined (ordered) nanospaces have advantages as supports to accommodate guest
species because their structure-property relationships will provide indispensable
information on designing materials with controlled properties [12]. Spectroscopic
properties, which are very sensitive to the environment, of the immobilized species,
have given insights to the nanoscopic structures of the host-guest systems where
conventional instrumental analysis does not have access [11–16]. By utilizing
photoprocesses, one can obtain such information as distribution [17–20], orientation
[21, 22], and mobility [23] of the guest species on/in nanospaces.
In this chapter, among possible host-guest systems, the studies on the organization of photofunctional species on/in clays (more accurately clay minerals and their
synthetic analogs) will be summarized with the emphasis on the developments in the
last two decades. The attention will mainly be focused on the role of the
nanostructures, which directly and indirectly correlate the photofunctions of the
guest species and the host-guest systems (Fig. 1).
2 Characteristics of Dye-Clay Hybrids for Photochemical
Studies and Photofunctional Materials
Smectites have been used most extensively for a wide range of application including
environmental and biomedical ones [24–28]. Smectites are a type of swellable 2:1
type layered clay minerals and consist of negatively charged silicate layer and charge
compensating interlayer cations which are exchangeable [29–31]. The negative
charge in the layers is generated by isomorphous substitution of framework metal
cations with similar size and lower valency, and to compensate this negative charge,
metal cations such as sodium and calcium occupy the interlayer space. The terms of
bentonite and montmorillonite have often confused, bentonite is a term of a natural
resource, and montmorillonite is the name of a clay mineral. The amount as well as
Photofunctions of Dye-Clay Hybrids: Recent Developments
253
Photochemical reactions in heterogeneous systems may differ significantly from
analogous reactions in homogeneous liquids/solutions or gas phases [1–6]. Important
roles of the media/supports to control such parameters as the reaction rates/yields
and product selectivity have been recognized so far, so that various photofunctional
hybrids have been designed by organizing molecular species in/on solid surfaces.
The location (proximity), orientation, association/aggregation, as well as the freedom (rotation and diffusion) of molecules on the surfaces or in the solids vary
depending on the host-guest interactions at the interface to affect the characteristics
and functions. The molecular and supramolecular designs (by the selection of host
and guest and their composition and the additives) have been done using nanospace
materials such as zeolites, mesoporous silicas, MOFs, COFs, and layered materials
for organizing molecular and polymeric photofunctional species, and unique/useful
photofunctions of the resulting hybrids have been reported [7–11]. Materials with
defined (ordered) nanospaces have advantages as supports to accommodate guest
species because their structure-property relationships will provide indispensable
information on designing materials with controlled properties [12]. Spectroscopic
properties, which are very sensitive to the environment, of the immobilized species,
have given insights to the nanoscopic structures of the host-guest systems where
conventional instrumental analysis does not have access [11–16]. By utilizing
photoprocesses, one can obtain such information as distribution [17–20], orientation
[21, 22], and mobility [23] of the guest species on/in nanospaces.
In this chapter, among possible host-guest systems, the studies on the organization of photofunctional species on/in clays (more accurately clay minerals and their
synthetic analogs) will be summarized with the emphasis on the developments in the
last two decades. The attention will mainly be focused on the role of the
nanostructures, which directly and indirectly correlate the photofunctions of the
guest species and the host-guest systems (Fig. 1).
2 Characteristics of Dye-Clay Hybrids for Photochemical
Studies and Photofunctional Materials
Smectites have been used most extensively for a wide range of application including
environmental and biomedical ones [24–28]. Smectites are a type of swellable 2:1
type layered clay minerals and consist of negatively charged silicate layer and charge
compensating interlayer cations which are exchangeable [29–31]. The negative
charge in the layers is generated by isomorphous substitution of framework metal
cations with similar size and lower valency, and to compensate this negative charge,
metal cations such as sodium and calcium occupy the interlayer space. The terms of
bentonite and montmorillonite have often confused, bentonite is a term of a natural
resource, and montmorillonite is the name of a clay mineral. The amount as well as
Photofunctions of Dye-Clay Hybrids: Recent Developments
253