chapter  8
44 Pages

Supramolecular Photochemistry of Cyclodextrin Materials

Cyclodextrins (CDs) are cyclic oligosaccharides produced from starch by the ac­ tion of the amylase of Bacillus marcerans. They possess a central cavity capable of accommodating hydrophobic organic molecules in aqueous solution. Although CDs consisting of 9,10,11,12,13,14,15,16, and 17 a-D-glucose members have been found [1-4], the most common CDs are commercially available CDs, a-, (3-, and 7 -CD, and the inclusion-complex formation of these three distinct CDs have been extensively studied [5-7]. These molecules have different cavity diameters, 4.9, 6 .2 , and 7.9 A for a-, P-, and 7 -CD, respectively, and are shaped like trun­ cated cones with a smaller and a larger opening at the primary hydroxyl and the secondary hydroxyl faces, respectively. The interior of the cavities are lined with ether oxygens and CiH and C4H groups and the wall is relatively hydrophobic. As a consequence of the nature of the cavities, the hydrophobic guests can be stabi­ lized in the cavities primarily by hydrophobic interactions. The stoichiometry of the complex formation is usually 1:1, but in some cases of 7 -CD, which have a larger cavity, 1:2 host-guest complexes are formed [8 ]. In studies of photo­ chemistry, it has been recognized that the restricted shape and size of the cavity geometrically constrain the included guest molecule and thereby stabilize the con­ formations that are less favored in free solution [9-11]. When a chromophore unit is covalently linked to CD, it is included in the CD cavity, forming a self-inclu-

sion complex. The photophysical property of the chromophophore part changes when the part is excluded from hydrophobic cavity to polar bulk water solution upon accommodation of exogenous guest species. This behavior may be used for the construction of molecule-detecting sensors. On the other hand, CDs can be used as scaffolds for constructing multichromophoric systems, in which stereose­ lective dimerization or effective energy migration may occur. In this chapter, we describe the topics on supramolecular photochemistry of CD-related systems, in­ cluding sensors [12-14], energy transfer, and unique photoreactions.