Light-Switchable Janus [2]Rotaxanes Based on alpha-Cyclodextrin Derivatives Bearing Two Recognition Sites Linked with Oligo(ethylene glycol)

Two Janus [2]rotaxanes, 5a and 5b, with a-cyclodextrin (alpha-CD) derivatives substituted on the 6-position with two recognition sites (azobenzene and heptamethylene (C7)) that were linked with linkers of different lengths (oligo(ethylene glycol) with a degree of polymerization equal to 2 or approximately 21) were synthesized and characterized. 20 ROESY NMR spectroscopy and circular dichroism (cd) spectra demonstrated that the recognition site of the alpha-CD moiety was switched by photoisomerization of the azobenzene moiety in 5a and 5b. The different size changes of 5a and 5b in hydrodynamic radius (R-H) owing to the different length of linker between two recognition sites were observed by pulse-field-gradient spin-echo NMR spectroscopy. The kinetic results indicated that the different length of linker had no or a weak effect for the photo-isomerization process of 5a and 5b.