Various inorganic and organic hosts both natural (biological and geological origins) and artificial ones are available and the inclusion chemistry of them have extensively been investigated for the applications and chemist’s curiosity. We have been interested in the host-guest chemistry of inorganic frameworks especially materials with low dimensional nanospace. The host-guest interactions have also been used for the molecular recognition (concentration/separation). The various functions (especially photofunctions) of host-guest complexes have been investigated. Here we show some examples of our previous studies on this topic.
Fig. 1 Structure of smectite.
Layered compounds are one class of host materials to accommodate guest species in the two dimensional interlayer space to form host-guest complexes named as “intercalation compound”. Smectite is one of the layered materials which is studied most extensively for the organization of guest species and characterized by their ability to accommodate guest species through various mechanisms such as electrostatic, ion-dipole, hydrogen bonding, and some time redox interactions. The interlayer distance depends on the intercalated cation and its solvation in the range from nanometer to non-limited swelling.
The periodic mesoporous silicas, which are synthesized by using surfactant as supramolecular templates and the mesostructures depend on the surfactants mesophases (Fig. 2). The nanopore shape varies from isolated sphere, one dimensional channel to three dimensionally interconnected pore network. The pore size is also variable from a few nm to a few tens of nm. Using particles as template, one can achieve pore with several hundred of nm.
As to the guest species, we have many options starting from ion/molecule, polymer and biopolymer to nanoparticle of metals and compounds. Resulting host-guest complexes are expected as catalyst and its support, adsorbent, drug carrier, sensor and photonics, so that a large number of scientific publications are available. Some of the host-guest complexes of zeolites and clay minerals have been practically applied in chemical industry as separation, reactions, rheology. Such advanced materials’ application (value added) as chiral separation column, cosmetic pigment, food additives has also been realized.
Fig. 2 Observed phases of silica-surfactant mesostructured materials.
Fig. 3 Schematic drawing to classify the typical intercalation reactions of layered solids.
Fig. 4 Introduction of the guest materials into (a) silica-surfactant mesostructured and into (b) mesoporous silica.
Various chemical interactions are useful as a driving force for the preparation of host-guest complexes, for example, the layered silicates, the cation exchange with interlayer exchangeable cations, the adsorption of polar molecules by ion–dipole interactions with interlayer cations and/or hydrogen bonding with the surface oxygen atom of the silicate sheet, and grafting with silane coupling reagent. Charge transfer can be also used as the driving for the intercalation and it is important for graphite intercalation compounds. (Fig. 3).
As to the mesoporous silicas, we can introduce guest species into silica-surfactant mesostructures and guest species into the mesoporous solids (Fig. 4a and 4b). One of the most important differences between the two systems is the possible remaining nanospace in the materials. In the silica-surfactant mesostructures containing guest species, the access and diffusion of the guest species are limited because the nanospace was filled with surfactants (Fig. 4a). On the other hand, in porous silica, the guest species can easily access and diffuse in the remaining nanospace (Fig. 4b).
We have been interested in the spatial distribution and mobility of the guest species in nanospaces. As shown in Fig. 5, we can control them to control the function. For the potential applications, the spatial distribution of guest species (or functional units) should be designed to initiate desired reactions in catalyst applications, to bind target species strongly in adsorbent applications, to fix/release drug molecules in drug delivery applications, to react with target molecules selectively in sensing applications, etc.
Fig. 5 Schematic drawings of the variation
(a) and the diffusion of the subsequent guest
(b) in the remaining nano space.