Inclusion complexes and Z–E photoisomerization of β-cyclodextrin bearing an azobenzene pendant

Abstract

A new photoresponsive host (1), β-cyclodextrin (β-cd) bearing an azobenzene moiety, has been prepared for the purpose of regulating guest binding by light. The CD spectrum of the dark-adapted sample of (1) exhibits a positive band around 345 nm associated with the azobenzene π–π* transition, suggesting that (1) exists as an intramolecular complex, in which the E-azobenzene moiety is inserted into the β-cd cavity with its long axis parallel to the cd axis. After the azobenzene moiety of (1) has been isomerized from the E- to the Z-form by UV light irradiation, it exhibits a CD pattern which has strong positive and negative bands at 312 and 425 nm, respectively. The CD intensities of both E- and Z-forms of (1) decrease upon guest addition, and the analysis of the CD variations indicate that the intramolecular complexes of (1) are converted into 1 : 1 host-guest complexes. The E-isomer forms binding complexes with adamantane-1-carboxylic acid and adamantan-1-ol ca. five times more strongly than the Z-form. Less photocontrolled guest binding was observed with other guests. The effects of pH on molecular association behaviour as well as on photocontrol of guest binding of (1) are described in connection with the electronic charges that (1) possesses.