１．Development and Application of Cyclic Oligosaccharide Derivatives Bearing High Guest Inclusion Ability in Organic Media

Cyclodextrins (CDs), which are produced from starch by a enzyme ‘cyclodextrin glucanotransferase’, are a class of cyclic oligosaccharides consisting of several α-(1,4)-linked D-glucopyranose units. CDs composed of 6, 7, and 8 glucosidic units, called α-, β-, and γ-CD, respectively, are commercially available, and have been studied extensively. They have a doughnut-shaped cavity into which a guest molecule of an appropriate size and shape can be included. The ability of CDs to form inclusion complexes with organic molecules has been studied academically, and, in addition, it has found applications in many areas, including the food, cosmetic, and pharmaceutical industries.

 

 

 

 

Figure 1. (A) Chemical structures and schematic illustration of of cyclodextrins (CDs). (B) Schematic illustration of the inclusion and release of guest molecules into and from the CD cavity.

 

 

 

 

 

 

 

CDs can form inclusion complexes with a variety of molecules in aqueous media or in several kinds of polar organic media via the incorporation of the guests into the CD cavities. On the other hand, it has been believed that inclusion complex formation between CDs and guest molecules in nonpolar media would be extremely difficult, because the main driving force for the inclusion of guest molecules within the CD cavity is hydrophobic interactions and/or van der Waals interactions between the guests and the CD cavity, and the enormous amount of nonpolar solvents become a strong competitor for inclusion within the CD cavity. We found that 6-O-modified CDs effectively form inclusion complexes with guest molecules in nonpolar solvents, and demonstrated that these CD hosts functioned as a selective adsorbent for polychlorinated biphenyls (PCBs) contaminated in insulating oils. We also succeeded in the efficient separation and concentration of PCBs from oils by passing the PCBs-contaminated oils through a column loaded with the CD hosts.
More than 500,000 tons of PCB-contaminated oils are still stored in Japan, and thus the development of appropriate treatment methods for these oils is highly required. Our technique with a CD-loaded column can be expected to contribute to a large reduction in the stockpile of PCBs-contaminated insulating oils in Japan. We are now studying to put this technique into practical use.

 

 

 

 

Figure 2. Inclusion complex formation between CD derivatives and PCBs in oils.

 

 

 

 

 

 

 

２．Creation and Application of CD Supramolecular Structures

In crystalline structures, CDs adopt three types of assembly modes: ‘cage’ type, ‘channel’ type, and ‘layer’ type. We found that channel-type assemblies of γ-CD (γ-CD_channel), which were prepared according to Tonelli’s method (A.E. Tonelli et al., Langmuir 2002, 18, 10016.), formed unique cubic microstructures by scanning electron microscopic (SEM) observation. We also succeeded in the preparation of CD structures with various morphologies by varying the fabrication conditions, such as the γ-CD concentration in the aqueous solution and the type of poor solvent. It was found that the channel-type γ-CD assemblies possess excellent oil dispersion ability, and can form organogels in a variety of oils and organic solvents at ambient temperature. Those assemblies effectively captured harmful compounds in oil through the inclusion into their cavities.

 

 

 

 

Figure 3. Preparation of CD supramolecular structures with various molphologies.

 

 

 

 

 

 

 

３．Development and Application of Skeleton-Modified CDs

Since it has been believed that flexible design of CD hosts to fit the shape and size of the desired guest molecules would be difficult, the types of guest molecules applied for conventional CD hosts are relatively limited. In order to overcome this drawback and to extend the utility of CDs, we designed ‘spacer-inserted CDs’ whose cavity size and shape are freely controllable, and succeeded in synthesizing them in two steps starting from perrmethylated CDs. These spacer-inserted CDs were found to show unique inclusion ability. ‘Bond-converted CDs’ bearing a β-1,4-glucosidic bond were also synthesized from permethylated CDs. The bond-converted CD derived from perrmethylated α-CD formed an inclusion complex with not only conventional polymeric guests such as polytetrahydrofuran, poly(ε-caprolactone), and poly(propylene glycol) but also with poly(acrylic acid) to generate novel pseudopolyrotaxanes.

 

 

 

 

Figure 4. Synthesis of skeleton-modified CDs that are designed to fit the shape and size of guest molecules.

 

 

 

 

 

 

 

４．Creation of Polymer Tubes through One-Dimensional Fusion of Polymer Capsules

Nanotubes have attracted much attention as potential nanocontainers, nanoreactors, and drug carriers. However, the facile fabrication of organic nanotubes with controllable film thickness and diameters has remained a challenging subject. We succeeded in fabricating the hollow nanocapules of poly(lactic acid)s (PLAs) stereocomplex films by a combination of the layer-by-layer assembly technique and the silica template method, and found that these nanocapsules one-dimensionally fused together to generate novel PLA nanotubes. The formation of these nanotubes was affected by the molecular weights of the PLAs comprising the hollow capsules. We believe that this nanotube formation may provide a new strategy for creating a variety of nanotubes including ones with a defined diameter and film thickness.

 

 

 

 

Figure 5. Schematic illustration of preparation of PLA hollow nanocapsules and formation of PLA nanotubes through one-dimensional fusion of the PLA.

 

 

 

 

 

 

 

５．Chiral Photochemistry through the Charge-Transfer Band Excitation

Upon photoexcitation, molecules are excited in their electronically excited states and gain additional reactivity. Accordingly, electronically excited molecules readily undergo unique photochemical transformation that does not usually observed in the ground state. Electron donor and acceptor molecules interact each other in their ground state to form charge-transfer complex, in which new absorption band (i.e., CT band) is observed. Upon selective excitation at the CT band, the complex is excited to form excited CT complex. This excited species behaves rather differently than the conventional exciplex, generally formed in the direct photoreaction. We are perusing the new methodology for chiral photoreactions based on the selective CT excitation.

 

 

 

 

Figure 6. Chiral Photoreactions through the Selective Charge-Transfer Band Excitation (Top: Conceptual View, Bottom: Representative Example of Photoreaction).